MPlayer- 电影播放器

7.1. Making a high quality MPEG-4 ("DivX") rip of a DVD movie One frequently asked question is "How do I make the highest quality rip for a given size?". Another question is "How do I make t

7.1. Making a high quality MPEG-4 ("DivX")

rip of a DVD movie

One frequently asked question is "How do I make the highest quality rip

for a given size?". Another question is "How do I make the highest

quality DVD rip possible? I do not care about file size, I just want the best

quality."

The latter question is perhaps at least somewhat wrongly posed. After all, if

you do not care about file size, why not simply copy the entire MPEG-2 video

stream from the the DVD? Sure, your AVI will end up being 5GB, give

or take, but if you want the best quality and do not care about size,

this is certainly your best option.

In fact, the reason you want to transcode a DVD into MPEG-4 is

specifically because you do care about

file size.

It is difficult to offer a cookbook recipe on how to create a very high

quality DVD rip. There are several factors to consider, and you should

understand these details or else you are likely to end up disappointed

with your results. Below we will investigate some of these issues, and

then have a look at an example. We assume you are using

libavcodec to encode the video,

although the theory applies to other codecs as well.

If this seems to be too much for you, you should probably use one of the

many fine frontends that are listed in the

MEncoder section

of our related projects page.

That way, you should be able to achieve high quality rips without too much

thinking, because most of those tools are designed to take clever decisions

for you.

7.1.1. Preparing to encode: Identifying source material and framerate

Before you even think about encoding a movie, you need to take

several preliminary steps.

The first and most important step before you encode should be

determining what type of content you are dealing with.

If your source material comes from DVD or broadcast/cable/satellite

TV, it will be stored in one of two formats: NTSC for North

America and Japan, PAL for Europe, etc.

It is important to realize, however, that this is just the formatting for

presentation on a television, and often does

not correspond to the

original format of the movie.

Experience shows that NTSC material is a lot more difficult to encode,

because there more elements to identify in the source.

In order to produce a suitable encode, you need to know the original

format.

Failure to take this into account will result in various flaws in your

encode, including ugly combing (interlacing) artifacts and duplicated

or even lost frames.

Besides being ugly, the artifacts also harm coding efficiency:

You will get worse quality per unit bitrate.

7.1.1.1. Identifying source framerate

Here is a list of common types of source material, where you are

likely to find them, and their properties:

Standard Film: Produced for

theatrical display at 24fps.

PAL video: Recorded with a PAL

video camera at 50 fields per second.

A field consists of just the odd- or even-numbered lines of a

frame.

Television was designed to refresh these in alternation as a

cheap form of analog compression.

The human eye supposedly compensates for this, but once you

understand interlacing you will learn to see it on TV too and

never enjoy TV again.

Two fields do not make a

complete frame, because they are captured 1/50 of a second apart

in time, and thus they do not line up unless there is no motion.

NTSC Video: Recorded with an

NTSC video camera at 60000/1001 fields per second, or 60 fields per

second in the pre-color era.

Otherwise similar to PAL.

Animation: Usually drawn at

24fps, but also comes in mixed-framerate varieties.

Computer Graphics (CG): Can be

any framerate, but some are more common than others; 24 and

30 frames per second are typical for NTSC, and 25fps is typical

for PAL.

Old Film: Various lower

framerates.

7.1.1.2. Identifying source material

Movies consisting of frames are referred to as progressive,

while those consisting of independent fields are called

either interlaced or video - though this latter term is

ambiguous.

To further complicate matters, some movies will be a mix of

several of the above.

The most important distinction to make between all of these

formats is that some are frame-based, while others are

field-based.

Whenever a movie is prepared

for display on television (including DVD), it is converted to a

field-based format.

The various methods by which this can be done are collectively

referred to as "telecine", of which the infamous NTSC

"3:2 pulldown" is one variety.

Unless the original material was also field-based (and the same

fieldrate), you are getting the movie in a format other than the

original.

There are several common types of pulldown:

PAL 2:2 pulldown: The nicest of

them all.

Each frame is shown for the duration of two fields, by extracting the

even and odd lines and showing them in alternation.

If the original material is 24fps, this process speeds up the

movie by 4%.

PAL 2:2:2:2:2:2:2:2:2:2:2:3 pulldown:

Every 12th frame is shown for the duration of three fields, instead of

just two.

This avoids the 4% speedup issue, but makes the process much

more difficult to reverse.

It is usually seen in musical productions where adjusting the

speed by 4% would seriously damage the musical score.

NTSC 3:2 telecine: Frames are

shown alternately for the duration of 3 fields or 2 fields.

This gives a fieldrate 2.5 times the original framerate.

The result is also slowed down very slightly from 60 fields per

second to 60000/1001 fields per second to maintain NTSC fieldrate.

NTSC 2:2 pulldown: Used for

showing 30fps material on NTSC.

Nice, just like 2:2 PAL pulldown.

There are also methods for converting between NTSC and PAL video,

but such topics are beyond the scope of this guide.

If you encounter such a movie and want to encode it, your best

bet is to find a copy in the original format.

Conversion between these two formats is highly destructive and

cannot be reversed cleanly, so your encode will greatly suffer

if it is made from a converted source.

When video is stored on DVD, consecutive pairs of fields are

grouped as a frame, even though they are not intended to be shown

at the same moment in time.

The MPEG-2 standard used on DVD and digital TV provides a

way both to encode the original progressive frames and to store

the number of fields for which a frame should be shown in the

header of that frame.

If this method has been used, the movie will often be described

as "soft-telecined", since the process only directs the

DVD player to apply pulldown to the movie rather than altering

the movie itself.

This case is highly preferable since it can easily be reversed

(actually ignored) by the encoder, and since it preserves maximal

quality.

However, many DVD and broadcast production studios do not use

proper encoding techniques but instead produce movies with

"hard telecine", where fields are actually duplicated in the

encoded MPEG-2.

The procedures for dealing with these cases will be covered

later in this guide.

For now, we leave you with some guides to identifying which type

of material you are dealing with:

NTSC regions:

If MPlayer prints that the framerate

has changed to 24000/1001 when watching your movie, and never changes

back, it is almost certainly progressive content that has been

"soft telecined".

If MPlayer shows the framerate

switching back and forth between 24000/1001 and 30000/1001, and you see

"combing" at times, then there are several possibilities.

The 24000/1001 fps segments are almost certainly progressive

content, "soft telecined", but the 30000/1001 fps parts could be

either hard-telecined 24000/1001 fps content or 60000/1001 fields per second

NTSC video.

Use the same guidelines as the following two cases to determine which.

If MPlayer never shows the framerate

changing, and every single frame with motion appears combed, your

movie is NTSC video at 60000/1001 fields per second.

If MPlayer never shows the framerate

changing, and two frames out of every five appear combed, your

movie is "hard telecined" 24000/1001fps content.

PAL regions:

If you never see any combing, your movie is 2:2 pulldown.

If you see combing alternating in and out every half second,

then your movie is 2:2:2:2:2:2:2:2:2:2:2:3 pulldown.

If you always see combing during motion, then your movie is PAL

video at 50 fields per second.

Hint:

MPlayer can slow down movie playback

with the -speed option or play it frame-by-frame.

Try using -speed 0.2 to watch the movie very

slowly or press the "." key repeatedly to play one frame at

a time and identify the pattern, if you cannot see it at full speed.

7.1.2. Constant quantizer vs. multipass

It is possible to encode your movie at a wide range of qualities.

With modern video encoders and a bit of pre-codec compression

(downscaling and denoising), it is possible to achieve very good

quality at 700 MB, for a 90-110 minute widescreen movie.

Furthermore, all but the longest movies can be encoded with near-perfect

quality at 1400 MB.

There are three approaches to encoding the video: constant bitrate

(CBR), constant quantizer, and multipass (ABR, or average bitrate).

The complexity of the frames of a movie, and thus the number of bits

required to compress them, can vary greatly from one scene to another.

Modern video encoders can adjust to these needs as they go and vary

the bitrate.

In simple modes such as CBR, however, the encoders do not know the

bitrate needs of future scenes and so cannot exceed the requested

average bitrate for long stretches of time.

More advanced modes, such as multipass encode, can take into account

the statistics from previous passes; this fixes the problem mentioned

above.

Note:

Most codecs which support ABR encode only support two pass encode

while some others such as x264,

Xvid

and libavcodec support

multipass, which slightly improves quality at each pass,

yet this improvement is no longer measurable nor noticeable after the

4th or so pass.

Therefore, in this section, two pass and multipass will be used

interchangeably.

In each of these modes, the video codec (such as

libavcodec)

breaks the video frame into 16x16 pixel macroblocks and then applies a

quantizer to each macroblock. The lower the quantizer, the better the

quality and higher the bitrate.

The method the movie encoder uses to determine

which quantizer to use for a given macroblock varies and is highly

tunable. (This is an extreme over-simplification of the actual

process, but the basic concept is useful to understand.)

When you specify a constant bitrate, the video codec will encode the video,

discarding

detail as much as necessary and as little as possible in order to remain

lower than the given bitrate. If you truly do not care about file size,

you could as well use CBR and specify a bitrate of infinity. (In

practice, this means a value high enough so that it poses no limit, like

10000Kbit.) With no real restriction on bitrate, the result is that

the codec will use the lowest

possible quantizer for each macroblock (as specified by

vqmin for

libavcodec, which is 2 by default).

As soon as you specify a

low enough bitrate that the codec

is forced to use a higher quantizer, then you are almost certainly ruining

the quality of your video.

In order to avoid that, you should probably downscale your video, according

to the method described later on in this guide.

In general, you should avoid CBR altogether if you care about quality.

With constant quantizer, the codec uses the same quantizer, as

specified by the vqscale option (for

libavcodec), on every macroblock.

If you want the highest quality rip possible, again ignoring bitrate,

you can use vqscale=2.

This will yield the same bitrate and PSNR (peak signal-to-noise ratio)

as CBR with

vbitrate=infinity and the default vqmin

of 2.

The problem with constant quantizing is that it uses the given quantizer

whether the macroblock needs it or not. That is, it might be possible

to use a higher quantizer on a macroblock without sacrificing visual

quality. Why waste the bits on an unnecessarily low quantizer? Your

CPU has as many cycles as there is time, but there is only so many bits

on your hard disk.

With a two pass encode, the first pass will rip the movie as though it

were CBR, but it will keep a log of properties for each frame. This

data is then used during the second pass in order to make intelligent

decisions about which quantizers to use. During fast action or high

detail scenes, higher quantizers will likely be used, and during

slow moving or low detail scenes, lower quantizers will be used.

Normally, the amount of motion is much more important than the

amount of detail.

If you use vqscale=2, then you are wasting bits. If you

use vqscale=3, then you are not getting the highest

quality rip. Suppose you rip a DVD at vqscale=3, and

the result is 1800Kbit. If you do a two pass encode with

vbitrate=1800, the resulting video will have

higher quality for the

same bitrate.

Since you are now convinced that two pass is the way to go, the real

question now is what bitrate to use? The answer is that there is no

single answer. Ideally you want to choose a bitrate that yields the

best balance between quality and file size. This is going to vary

depending on the source video.

If size does not matter, a good starting point for a very high quality

rip is about 2000Kbit plus or minus 200Kbit.

For fast action or high detail source video, or if you just have a very

critical eye, you might decide on 2400 or 2600.

For some DVDs, you might not notice a difference at 1400Kbit. It is a

good idea to experiment with scenes at different bitrates to get a feel.

If you aim at a certain size, you will have to somehow calculate the bitrate.

But before that, you need to know how much space you should reserve for the

audio track(s), so you should

rip those first.

You can compute the bitrate with the following equation:

bitrate = (target_size_in_Mbytes - sound_size_in_Mbytes) *

1024 * 1024 / length_in_secs * 8 / 1000

For instance, to squeeze a two-hour movie onto a 702MB CD, with 60MB

of audio track, the video bitrate will have to be:

(702 - 60) * 1024 * 1024 / (120*60) * 8 / 1000

= 740kbps

7.1.3. Constraints for efficient encoding

Due to the nature of MPEG-type compression, there are various

constraints you should follow for maximal quality.

MPEG splits the video up into 16x16 squares called macroblocks,

each composed of 4 8x8 blocks of luma (intensity) information and two

half-resolution 8x8 chroma (color) blocks (one for red-cyan axis and

the other for the blue-yellow axis).

Even if your movie width and height are not multiples of 16, the

encoder will use enough 16x16 macroblocks to cover the whole picture

area, and the extra space will go to waste.

So in the interests of maximizing quality at a fixed file size, it is

a bad idea to use dimensions that are not multiples of 16.

Most DVDs also have some degree of black borders at the edges. Leaving

these in place will hurt quality a lot

in several ways.

MPEG-type compression is highly dependent on frequency domain

transformations, in particular the Discrete Cosine Transform (DCT),

which is similar to the Fourier transform. This sort of encoding is

efficient for representing patterns and smooth transitions, but it

has a hard time with sharp edges. In order to encode them it must

use many more bits, or else an artifact known as ringing will

appear.

The frequency transform (DCT) takes place separately on each

macroblock (actually each block), so this problem only applies when

the sharp edge is inside a block. If your black borders begin

exactly at multiple-of-16 pixel boundaries, this is not a problem.

However, the black borders on DVDs rarely come nicely aligned, so

in practice you will always need to crop to avoid this penalty.

In addition to frequency domain transforms, MPEG-type compression uses

motion vectors to represent the change from one frame to the next.

Motion vectors naturally work much less efficiently for new content

coming in from the edges of the picture, because it is not present in

the previous frame. As long as the picture extends all the way to the

edge of the encoded region, motion vectors have no problem with

content moving out the edges of the picture. However, in the presence

of black borders, there can be trouble:

For each macroblock, MPEG-type compression stores a vector

identifying which part of the previous frame should be copied into

this macroblock as a base for predicting the next frame. Only the

remaining differences need to be encoded. If a macroblock spans the

edge of the picture and contains part of the black border, then

motion vectors from other parts of the picture will overwrite the

black border. This means that lots of bits must be spent either

re-blackening the border that was overwritten, or (more likely) a

motion vector will not be used at all and all the changes in this

macroblock will have to be coded explicitly. Either way, encoding

efficiency is greatly reduced.

Again, this problem only applies if black borders do not line up on

multiple-of-16 boundaries.

Finally, suppose we have a macroblock in the interior of the

picture, and an object is moving into this block from near the edge

of the image. MPEG-type coding cannot say "copy the part that is

inside the picture but not the black border." So the black border

will get copied inside too, and lots of bits will have to be spent

encoding the part of the picture that is supposed to be there.

If the picture runs all the way to the edge of the encoded area,

MPEG has special optimizations to repeatedly copy the pixels at the

edge of the picture when a motion vector comes from outside the

encoded area. This feature becomes useless when the movie has black

borders. Unlike problems 1 and 2, aligning the borders at multiples

of 16 does not help here.

Despite the borders being entirely black and never changing, there

is at least a minimal amount of overhead involved in having more

macroblocks.

For all of these reasons, it is recommended to fully crop black

borders. Further, if there is an area of noise/distortion at the edge

of the picture, cropping this will improve encoding efficiency as

well. Videophile purists who want to preserve the original as close as

possible may object to this cropping, but unless you plan to encode at

constant quantizer, the quality you gain from cropping will

considerably exceed the amount of information lost at the edges.

7.1.4. Cropping and Scaling

Recall from the previous section that the final picture size you

encode should be a multiple of 16 (in both width and height).

This can be achieved by cropping, scaling, or a combination of both.

When cropping, there are a few guidelines that must be followed to

avoid damaging your movie.

The normal YUV format, 4:2:0, stores chroma (color) information

subsampled, i.e. chroma is only sampled half as often in each

direction as luma (intensity) information.

Observe this diagram, where L indicates luma sampling points and C

chroma.

LLLLLLLLCCCCLLLLLLLLLLLLLLLLCCCCLLLLLLLL

As you can see, rows and columns of the image naturally come in pairs.

Thus your crop offsets and dimensions must be

even numbers.

If they are not, the chroma will no longer line up correctly with the

luma.

In theory, it is possible to crop with odd offsets, but it requires

resampling the chroma which is potentially a lossy operation and not

supported by the crop filter.

Further, interlaced video is sampled as follows:

Top fieldBottom fieldLLLLLLLL CCCC LLLLLLLLLLLLLLLL CCCC LLLLLLLLLLLLLLLL CCCC LLLLLLLLLLLLLLLL CCCC LLLLLLLL

As you can see, the pattern does not repeat until after 4 lines.

So for interlaced video, your y-offset and height for cropping must

be multiples of 4.

Native DVD resolution is 720x480 for NTSC, and 720x576 for PAL, but

there is an aspect flag that specifies whether it is full-screen (4:3) or

wide-screen (16:9). Many (if not most) widescreen DVDs are not strictly

16:9, and will be either 1.85:1 or 2.35:1 (cinescope). This means that

there will be black bands in the video that will need to be cropped out.

MPlayer provides a crop detection filter that

will determine the crop rectangle (-vf cropdetect).

Run MPlayer with

-vf cropdetect and it will print out the crop

settings to remove the borders.

You should let the movie run long enough that the whole picture

area is used, in order to get accurate crop values.

Then, test the values you get with MPlayer,

using the command line which was printed by

cropdetect, and adjust the rectangle as needed.

The rectangle filter can help by allowing you to

interactively position the crop rectangle over your movie.

Remember to follow the above divisibility guidelines so that you

do not misalign the chroma planes.

In certain cases, scaling may be undesirable.

Scaling in the vertical direction is difficult with interlaced

video, and if you wish to preserve the interlacing, you should

usually refrain from scaling.

If you will not be scaling but you still want to use multiple-of-16

dimensions, you will have to overcrop.

Do not undercrop, since black borders are very bad for encoding!

Because MPEG-4 uses 16x16 macroblocks, you will want to make sure that each

dimension of the video you are encoding is a multiple of 16 or else you

will be degrading quality, especially at lower bitrates. You can do this

by rounding the width and height of the crop rectangle down to the nearest

multiple of 16.

As stated earlier, when cropping, you will want to increase the Y offset by

half the difference of the old and the new height so that the resulting

video is taken from the center of the frame. And because of the way DVD

video is sampled, make sure the offset is an even number. (In fact, as a

rule, never use odd values for any parameter when you are cropping and

scaling video.) If you are not comfortable throwing a few extra pixels

away, you might prefer to scale the video instead. We will look

at this in our example below.

You can actually let the cropdetect filter do all of the

above for you, as it has an optional round parameter that

is equal to 16 by default.

Also, be careful about "half black" pixels at the edges. Make sure you

crop these out too, or else you will be wasting bits there that

are better spent elsewhere.

After all is said and done, you will probably end up with video whose pixels

are not quite 1.85:1 or 2.35:1, but rather something close to that. You

could calculate the new aspect ratio manually, but

MEncoder offers an option for libavcodec called autoaspect

that will do this for you. Absolutely do not scale this video up in order to

square the pixels unless you like to waste your hard disk space. Scaling

should be done on playback, and the player will use the aspect stored in

the AVI to determine the correct resolution.

Unfortunately, not all players enforce this auto-scaling information,

therefore you may still want to rescale.

7.1.5. Choosing resolution and bitrate

If you will not be encoding in constant quantizer mode, you need to

select a bitrate.

The concept of bitrate is quite simple.

It is the (average) number of bits that will be consumed to store your

movie, per second.

Normally bitrate is measured in kilobits (1000 bits) per second.

The size of your movie on disk is the bitrate times the length of the

movie in time, plus a small amount of "overhead" (see the section on

the AVI container

for instance).

Other parameters such as scaling, cropping, etc. will

not alter the file size unless you

change the bitrate as well!

Bitrate does not scale proportionally

to resolution.

That is to say, a 320x240 file at 200 kbit/sec will not be the same

quality as the same movie at 640x480 and 800 kbit/sec!

There are two reasons for this:

Perceptual: You notice MPEG

artifacts more if they are scaled up bigger!

Artifacts appear on the scale of blocks (8x8).

Your eye will not see errors in 4800 small blocks as easily as it

sees errors in 1200 large blocks (assuming you will be scaling both

to fullscreen).

Theoretical: When you scale down

an image but still use the same size (8x8) blocks for the frequency

space transform, you move more data to the high frequency bands.

Roughly speaking, each pixel contains more of the detail than it

did before.

So even though your scaled-down picture contains 1/4 the information

in the spacial directions, it could still contain a large portion

of the information in the frequency domain (assuming that the high

frequencies were underutilized in the original 640x480 image).

Past guides have recommended choosing a bitrate and resolution based

on a "bits per pixel" approach, but this is usually not valid due to

the above reasons.

A better estimate seems to be that bitrates scale proportional to the

square root of resolution, so that 320x240 and 400 kbit/sec would be

comparable to 640x480 at 800 kbit/sec.

However this has not been verified with theoretical or empirical

rigor.

Further, given that movies vary greatly with regard to noise, detail,

degree of motion, etc., it is futile to make general recommendations

for bits per length-of-diagonal (the analog of bits per pixel,

using the square root).

So far we have discussed the difficulty of choosing a bitrate and

resolution.

7.1.5.1. Computing the resolution

The following steps will guide you in computing the resolution of your

encode without distorting the video too much, by taking into account several

types of information about the source video.

First, you should compute the encoded aspect ratio:

ARc = (Wc x (ARa / PRdvd )) / Hc

where:

Wc and Hc are the width and height of the cropped video,

ARa is the displayed aspect ratio, which usually is 4/3 or 16/9,

PRdvd is the pixel ratio of the DVD which is equal to 1.25=(720/576) for PAL

DVDs and 1.5=(720/480) for NTSC DVDs.

Then, you can compute the X and Y resolution, according to a certain

Compression Quality (CQ) factor:

ResY = INT(SQRT( 1000*Bitrate/25/ARc/CQ )/16) * 16

and

ResX = INT( ResY * ARc / 16) * 16

Okay, but what is the CQ?

The CQ represents the number of bits per pixel and per frame of the encode.

Roughly speaking, the greater the CQ, the less the likelihood to see

encoding artifacts.

However, if you have a target size for your movie (1 or 2 CDs for instance),

there is a limited total number of bits that you can spend; therefore it is

necessary to find a good tradeoff between compressibility and quality.

The CQ depends on the bitrate, the video codec efficiency and the

movie resolution.

In order to raise the CQ, typically you would downscale the movie given that the

bitrate is computed in function of the target size and the length of the

movie, which are constant.

With MPEG-4 ASP codecs such as Xvid

and libavcodec, a CQ below 0.18

usually results in a pretty blocky picture, because there

are not enough bits to code the information of each macroblock. (MPEG4, like

many other codecs, groups pixels by blocks of several pixels to compress the

image; if there are not enough bits, the edges of those blocks are

visible.)

It is therefore wise to take a CQ ranging from 0.20 to 0.22 for a 1 CD rip,

and 0.26-0.28 for 2 CDs rip with standard encoding options.

More advanced encoding options such as those listed here for

libavcodec

and

Xvid

should make it possible to get the same quality with CQ ranging from

0.18 to 0.20 for a 1 CD rip, and 0.24 to 0.26 for a 2 CD rip.

With MPEG-4 AVC codecs such as x264,

you can use a CQ ranging from 0.14 to 0.16 with standard encoding options,

and should be able to go as low as 0.10 to 0.12 with

x264's advanced encoding settings.

Please take note that the CQ is just an indicative figure, as depending on

the encoded content, a CQ of 0.18 may look just fine for a Bergman, contrary

to a movie such as The Matrix, which contains many high-motion scenes.

On the other hand, it is worthless to raise CQ higher than 0.30 as you would

be wasting bits without any noticeable quality gain.

Also note that as mentioned earlier in this guide, low resolution videos

need a bigger CQ (compared to, for instance, DVD resolution) to look good.

7.1.6. Filtering

Learning how to use MEncoder's video filters

is essential to producing good encodes.

All video processing is performed through the filters -- cropping,

scaling, color adjustment, noise removal, sharpening, deinterlacing,

telecine, inverse telecine, and deblocking, just to name a few.

Along with the vast number of supported input formats, the variety of

filters available in MEncoder is one of its

main advantages over other similar programs.

Filters are loaded in a chain using the -vf option:

-vf filter1=options,filter2=options,...

Most filters take several numeric options separated by colons, but

the syntax for options varies from filter to filter, so read the man

page for details on the filters you wish to use.

Filters operate on the video in the order they are loaded.

For example, the following chain:

-vf crop=688:464:12:4,scale=640:464

will first crop the 688x464 region of the picture with upper-left

corner at (12,4), and then scale the result down to 640x464.

Certain filters need to be loaded at or near the beginning of the

filter chain, in order to take advantage of information from the

video decoder that will be lost or invalidated by other filters.

The principal examples are pp (postprocessing, only

when it is performing deblock or dering operations),

spp (another postprocessor to remove MPEG artifacts),

pullup (inverse telecine), and

softpulldown (for converting soft telecine to hard telecine).

In general, you want to do as little filtering as possible to the movie

in order to remain close to the original DVD source. Cropping is often

necessary (as described above), but avoid to scale the video. Although

scaling down is sometimes preferred to using higher quantizers, we want

to avoid both these things: remember that we decided from the start to

trade bits for quality.

Also, do not adjust gamma, contrast, brightness, etc. What looks good

on your display may not look good on others. These adjustments should

be done on playback only.

One thing you might want to do, however, is pass the video through a

very light denoise filter, such as -vf hqdn3d=2:1:2.

Again, it is a matter of putting those bits to better use: why waste them

encoding noise when you can just add that noise back in during playback?

Increasing the parameters for hqdn3d will further

improve compressibility, but if you increase the values too much, you

risk degrading the image visibly. The suggested values above

(2:1:2) are quite conservative; you should feel free to

experiment with higher values and observe the results for yourself.

7.1.7. Interlacing and Telecine

Almost all movies are shot at 24 fps. Because NTSC is 30000/1001 fps, some

processing must be done to this 24 fps video to make it run at the correct

NTSC framerate. The process is called 3:2 pulldown, commonly referred to

as telecine (because pulldown is often applied during the telecine

process), and, naively described, it works by slowing the film down to

24000/1001 fps, and repeating every fourth frame.

No special processing, however, is done to the video for PAL DVDs, which

run at 25 fps. (Technically, PAL can be telecined, called 2:2 pulldown,

but this does not become an issue in practice.) The 24 fps film is simply

played back at 25 fps. The result is that the movie runs slightly faster,

but unless you are an alien, you probably will not notice the difference.

Most PAL DVDs have pitch-corrected audio, so when they are played back at

25 fps things will sound right, even though the audio track (and hence the

whole movie) has a running time that is 4% less than NTSC DVDs.

Because the video in a PAL DVD has not been altered, you need not worry

much about framerate. The source is 25 fps, and your rip will be 25

fps. However, if you are ripping an NTSC DVD movie, you may need to

apply inverse telecine.

For movies shot at 24 fps, the video on the NTSC DVD is either telecined

30000/1001, or else it is progressive 24000/1001 fps and intended to be

telecined on-the-fly by a DVD player. On the other hand, TV series are usually

only interlaced, not telecined. This is not a hard rule: some TV series

are interlaced (such as Buffy the Vampire Slayer) whereas some are a

mixture of progressive and interlaced (such as Angel, or 24).

It is highly recommended that you read the section on

How to deal with telecine and interlacing in NTSC DVDs

to learn how to handle the different possibilities.

However, if you are mostly just ripping movies, likely you are either

dealing with 24 fps progressive or telecined video, in which case you can

use the pullup filter -vf

pullup,softskip.

7.1.8. Encoding interlaced video

If the movie you want to encode is interlaced (NTSC video or

PAL video), you will need to choose whether you want to

deinterlace or not.

While deinterlacing will make your movie usable on progressive

scan displays such a computer monitors and projectors, it comes

at a cost: The fieldrate of 50 or 60000/1001 fields per second

is halved to 25 or 30000/1001 frames per second, and roughly half of

the information in your movie will be lost during scenes with

significant motion.

Therefore, if you are encoding for high quality archival purposes,

it is recommended not to deinterlace.

You can always deinterlace the movie at playback time when

displaying it on progressive scan devices.

The power of currently available computers forces players to use a

deinterlacing filter, which results in a slight degradation in

image quality.

But future players will be able to mimic the interlaced display of

a TV, deinterlacing to full fieldrate and interpolating 50 or

60000/1001 entire frames per second from the interlaced video.

Special care must be taken when working with interlaced video:

Crop height and y-offset must be multiples of 4.

Any vertical scaling must be performed in interlaced mode.

Postprocessing and denoising filters may not work as expected

unless you take special care to operate them a field at a time,

and they may damage the video if used incorrectly.

With these things in mind, here is our first example:

mencoder capture.avi -mc 0 -oac lavc -ovc lavc -lavcopts \

vcodec=mpeg2video:vbitrate=6000:ilme:ildct:acodec=mp2:abitrate=224

Note the ilme and ildct options.

7.1.9. Notes on Audio/Video synchronization

MEncoder's audio/video synchronization

algorithms were designed with the intention of recovering files with

broken sync.

However, in some cases they can cause unnecessary skipping and duplication of

frames, and possibly slight A/V desync, when used with proper input

(of course, A/V sync issues apply only if you process or copy the

audio track while transcoding the video, which is strongly encouraged).

Therefore, you may have to switch to basic A/V sync with

the -mc 0 option, or put this in your

~/.mplayer/mencoder config file, as long as

you are only working with good sources (DVD, TV capture, high quality

MPEG-4 rips, etc) and not broken ASF/RM/MOV files.

If you want to further guard against strange frame skips and

duplication, you can use both -mc 0 and

-noskip.

This will prevent all A/V sync, and copy frames

one-to-one, so you cannot use it if you will be using any filters that

unpredictably add or drop frames, or if your input file has variable

framerate!

Therefore, using -noskip is not in general recommended.

The so-called "three-pass" audio encoding which

MEncoder supports has been reported to cause A/V

desync.

This will definitely happen if it is used in conjunction with certain

filters, therefore, it is now recommended not to

use three-pass audio mode.

This feature is only left for compatibility purposes and for expert

users who understand when it is safe to use and when it is not.

If you have never heard of three-pass mode before, forget that we

even mentioned it!

There have also been reports of A/V desync when encoding from stdin

with MEncoder.

Do not do this! Always use a file or CD/DVD/etc device as input.

7.1.10. Choosing the video codec

Which video codec is best to choose depends on several factors,

like size, quality, streamability, usability and popularity, some of

which widely depend on personal taste and technical constraints.

Compression efficiency:

It is quite easy to understand that most newer-generation codecs are

made to increase quality and compression.

Therefore, the authors of this guide and many other people suggest that

you cannot go wrong

[1]

when choosing MPEG-4 AVC codecs like

x264 instead of MPEG-4 ASP codecs

such as libavcodec MPEG-4 or

Xvid.

(Advanced codec developers may be interested in reading Michael

Niedermayer's opinion on

"why MPEG4-ASP sucks".)

Likewise, you should get better quality using MPEG-4 ASP than you

would with MPEG-2 codecs.

However, newer codecs which are in heavy development can suffer from

bugs which have not yet been noticed and which can ruin an encode.

This is simply the tradeoff for using bleeding-edge technology.

What is more, beginning to use a new codec requires that you spend some

time becoming familiar with its options, so that you know what

to adjust to achieve a desired picture quality.

Hardware compatibility:

It usually takes a long time for standalone video players to begin to

include support for the latest video codecs.

As a result, most only support MPEG-1 (like VCD, XVCD and KVCD), MPEG-2

(like DVD, SVCD and KVCD) and MPEG-4 ASP (like DivX,

libavcodec's LMP4 and

Xvid)

(Beware: Usually, not all MPEG-4 ASP features are supported).

Please refer to the technical specs of your player (if they are available),

or google around for more information.

Best quality per encoding time:

Codecs that have been around for some time (such as

libavcodec MPEG-4 and

Xvid) are usually heavily

optimized with all kinds of smart algorithms and SIMD assembly code.

That is why they tend to yield the best quality per encoding time ratio.

However, they may have some very advanced options that, if enabled,

will make the encode really slow for marginal gains.

If you are after blazing speed you should stick around the default

settings of the video codec (although you should still try the other

options which are mentioned in other sections of this guide).

You may also consider choosing a codec which can do multi-threaded

processing, though this is only useful for users of machines with

several CPUs.

libavcodec MPEG-4 does

allow that, but speed gains are limited, and there is a slight

negative effect on picture quality.

Xvid's multi-threaded encoding,

activated by the threads option, can be used to

boost encoding speed — by about 40-60% in typical cases —

with little if any picture degradation.

x264 also allows multi-threaded

encoding, which currently speeds up encoding by 94% per CPU core while

lowering PSNR between 0.005dB and 0.01dB on a typical setup.

Personal taste:

This is where it gets almost irrational: For the same reason that some

hung on to DivX 3 for years when newer codecs were already doing wonders,

some folks will prefer Xvid

or libavcodec MPEG-4 over

x264.

You should make your own judgement; do not take advice from people who

swear by one codec.

Take a few sample clips from raw sources and compare different

encoding options and codecs to find one that suits you best.

The best codec is the one you master, and the one that looks

best to your eyes on your display

[2]!

Please refer to the section

selecting codecs and container formats

to get a list of supported codecs.

7.1.11. Audio

Audio is a much simpler problem to solve: if you care about quality, just

leave it as is.

Even AC-3 5.1 streams are at most 448Kbit/s, and they are worth every bit.

You might be tempted to transcode the audio to high quality Vorbis, but

just because you do not have an A/V receiver for AC-3 pass-through today

does not mean you will not have one tomorrow. Future-proof your DVD rips by

preserving the AC-3 stream.

You can keep the AC-3 stream either by copying it directly into the video

stream during the encoding.

You can also extract the AC-3 stream in order to mux it into containers such

as NUT or Matroska.

mplayer source_file.vob -aid 129 -dumpaudio -dumpfile sound.ac3

will dump into the file sound.ac3 the

audio track number 129 from the file

source_file.vob (NB: DVD VOB files

usually use a different audio numbering,

which means that the VOB audio track 129 is the 2nd audio track of the file).

But sometimes you truly have no choice but to further compress the

sound so that more bits can be spent on the video.

Most people choose to compress audio with either MP3 or Vorbis audio codecs.

While the latter is a very space-efficient codec, MP3 is better supported

by hardware players, although this trend is changing.

Do not use -nosound when encoding

a file with audio, even if you will be encoding and muxing audio

separately later.

Though it may work in ideal cases, using -nosound is

likely to hide some problems in your encoding command line setting.

In other words, having a soundtrack during your encode assures you that,

provided you do not see messages such as

“Too many audio packets in the buffer”, you will be able

to get proper sync.

You need to have MEncoder process the sound.

You can for example copy the original soundtrack during the encode with

-oac copy or convert it to a "light" 4 kHz mono WAV

PCM with -oac pcm -channels 1 -srate 4000.

Otherwise, in some cases, it will generate a video file that will not sync

with the audio.

Such cases are when the number of video frames in the source file does

not match up to the total length of audio frames or whenever there

are discontinuities/splices where there are missing or extra audio frames.

The correct way to handle this kind of problem is to insert silence or

cut audio at these points.

However MPlayer cannot do that, so if you

demux the AC-3 audio and encode it with a separate app (or dump it to PCM with

MPlayer), the splices will be left incorrect

and the only way to correct them is to drop/duplicate video frames at the

splice.

As long as MEncoder sees the audio when it is

encoding the video, it can do this dropping/duping (which is usually OK

since it takes place at full black/scene change), but if

MEncoder cannot see the audio, it will just

process all frames as-is and they will not fit the final audio stream when

you for example merge your audio and video track into a Matroska file.

First of all, you will have to convert the DVD sound into a WAV file that the

audio codec can use as input.

For example:

mplayer source_file.vob -ao pcm:file=destination_sound.wav \

-vc dummy -aid 1 -vo null

will dump the second audio track from the file

source_file.vob into the file

destination_sound.wav.

You may want to normalize the sound before encoding, as DVD audio tracks

are commonly recorded at low volumes.

You can use the tool normalize for instance,

which is available in most distributions.

If you are using Windows, a tool such as BeSweet

can do the same job.

You will compress in either Vorbis or MP3.

For example:

oggenc -q1 destination_sound.wav

will encode destination_sound.wav with

the encoding quality 1, which is roughly equivalent to 80Kb/s, and

is the minimum quality at which you should encode if you care about

quality.

Please note that MEncoder currently cannot

mux Vorbis audio tracks

into the output file because it only supports AVI and MPEG

containers as an output, each of which may lead to audio/video

playback synchronization problems with some players when the AVI file

contain VBR audio streams such as Vorbis.

Do not worry, this document will show you how you can do that with third

party programs.

7.1.12. Muxing

Now that you have encoded your video, you will most likely want

to mux it with one or more audio tracks into a movie container, such

as AVI, MPEG, Matroska or NUT.

MEncoder is currently only able to natively output

audio and video into MPEG and AVI container formats.

for example:

mencoder -oac copy -ovc copy -o output_movie.avi \

-audiofile input_audio.mp2 input_video.avi

This would merge the video file input_video.avi

and the audio file input_audio.mp2

into the AVI file output_movie.avi.

This command works with MPEG-1 layer I, II and III (more commonly known

as MP3) audio, WAV and a few other audio formats too.

MEncoder features experimental support for

libavformat, which is a

library from the FFmpeg project that supports muxing and demuxing

a variety of containers.

For example:

mencoder -oac copy -ovc copy -o output_movie.asf -audiofile input_audio.mp2 \

input_video.avi -of lavf -lavfopts format=asf

This will do the same thing as the previous example, except that

the output container will be ASF.

Please note that this support is highly experimental (but getting

better every day), and will only work if you compiled

MPlayer with the support for

libavformat enabled (which

means that a pre-packaged binary version will not work in most cases).

7.1.12.1. Improving muxing and A/V sync reliability

You may experience some serious A/V sync problems while trying to mux

your video and some audio tracks, where no matter how you adjust the

audio delay, you will never get proper sync.

That may happen when you use some video filters that will drop or

duplicate some frames, like the inverse telecine filters.

It is strongly encouraged to append the harddup video

filter at the end of the filter chain to avoid this kind of problem.

Without harddup, if MEncoder

wants to duplicate a frame, it relies on the muxer to put a mark on the

container so that the last frame will be displayed again to maintain

sync while writing no actual frame.

With harddup, MEncoder

will instead just push the last frame displayed again into the filter

chain.

This means that the encoder receives the exact

same frame twice, and compresses it.

This will result in a slightly bigger file, but will not cause problems

when demuxing or remuxing into other container formats.

You may also have no choice but to use harddup with

container formats that are not too tightly linked with

MEncoder such as the ones supported through

libavformat, which may not

support frame duplication at the container level.

7.1.12.2. Limitations of the AVI container

Although it is the most widely-supported container format after MPEG-1,

AVI also has some major drawbacks.

Perhaps the most obvious is the overhead.

For each chunk of the AVI file, 24 bytes are wasted on headers and index.

This translates into a little over 5 MB per hour, or 1-2.5%

overhead for a 700 MB movie. This may not seem like much, but it could

mean the difference between being able to use 700 kbit/sec video or

714 kbit/sec, and every bit of quality counts.

In addition this gross inefficiency, AVI also has the following major

limitations:

Only fixed-fps content can be stored. This is particularly limiting

if the original material you want to encode is mixed content, for

example a mix of NTSC video and film material.

Actually there are hacks that can be used to store mixed-framerate

content in AVI, but they increase the (already huge) overhead

fivefold or more and so are not practical.

Audio in AVI files must be either constant-bitrate (CBR) or

constant-framesize (i.e. all frames decode to the same number of

samples).

Unfortunately, the most efficient codec, Vorbis, does not meet

either of these requirements.

Therefore, if you plan to store your movie in AVI, you will have to

use a less efficient codec such as MP3 or AC-3.

Having said all that, MEncoder does not

currently support variable-fps output or Vorbis encoding.

Therefore, you may not see these as limitations if

MEncoder is the

only tool you will be using to produce your encodes.

However, it is possible to use MEncoder

only for video encoding, and then use external tools to encode

audio and mux it into another container format.

7.1.12.3. Muxing into the Matroska container

Matroska is a free, open standard container format, aiming

to offer a lot of advanced features, which older containers

like AVI cannot handle.

For example, Matroska supports variable bitrate audio content

(VBR), variable framerates (VFR), chapters, file attachments,

error detection code (EDC) and modern A/V Codecs like "Advanced Audio

Coding" (AAC), "Vorbis" or "MPEG-4 AVC" (H.264), next to nothing

handled by AVI.

The tools required to create Matroska files are collectively called

mkvtoolnix, and are available for most

Unix platforms as well as Windows.

Because Matroska is an open standard you may find other

tools that suit you better, but since mkvtoolnix is the most

common, and is supported by the Matroska team itself, we will

only cover its usage.

Probably the easiest way to get started with Matroska is to use

MMG, the graphical frontend shipped with

mkvtoolnix, and follow the

guide to mkvmerge GUI (mmg)

You may also mux audio and video files using the command line:

mkvmerge -o output.mkv input_video.avi input_audio1.mp3 input_audio2.ac3

This would merge the video file input_video.avi

and the two audio files input_audio1.mp3

and input_audio2.ac3 into the Matroska

file output.mkv.

Matroska, as mentioned earlier, is able to do much more than that, like

multiple audio tracks (including fine-tuning of audio/video

synchronization), chapters, subtitles, splitting, etc...

Please refer to the documentation of those applications for

more details.