Better to Decompress First?

[FrozenInferno]

Hi, I'm doing some resizing with VirtualDubMod and I was wondering whether it would be better to decompress the video first, then re-encode the resulting uncompressed file. Or does it not make a difference whether you decompress it first?

[Baldrick]

No.

And you can't really "decompress" video, you can only reconvert it.

[jagabo]

The process of resizing the frame always consists of first decompressing the video, resizing, then compressing (or not) the video. So VirtualDubMod is automatically decompressing the video before applying the resize filter. Saving as uncompressed would be redundant and a waste of time and drive space.

[FrozenInferno]

No.

And you can't really "decompress" video, you can only reconvert it.

Does the same apply for audio as well?

[jagabo]

What Baldrick means is that you can't restore details that were lost by the compression. The same applies to audio.

[FrozenInferno]

Then this applies to both video and audio files? So when re-encoding a VBR audio stream from a video file into CBR, there's no point in first saving it as an "uncompressed WAV"? What exactly does it then mean by uncompressed, and If nothing is being restored, why are the file sizes so huge?

EDIT: Why does it instruct to save it as a WAV first?

[jagabo]

Take a list of 1000 zeros:

0000000....000

apply a plain english compression scheme:

repeat zero 1000 times

One thousand characters have been compressed down to 22 characters.

Apply decompression:

0000000....000

One thousand characters once again.

Now suppose there were a few ones in that string of zeros:

0001000....010

Apply a lossy plain english compression scheme:

repeat zero 1000 times (you'll barely notice a few ones are missing)

Once again, 1000 characters have been reduced to 22. Apply decompression:

0000000....000

One thousand characters once again. But the ones are gone.

[FrozenInferno]

Ok, so your point is that compression degrades quality, and you won't get it back by decompressing.

Do you then know why VirtualDubMod is prescribing to decompress the audio stream first, before re-encoding with a CBR in the screenshot I provided? Really curious about all this.

[filmboss80]

Like the VirtualDub message says: better compatibility. Depending on the playback hardware and/or software employed, VBR (variable bitrate) audio has the tendency to drift out of sync with the video. For keeping picture and sound in lockstep, it is usually best to have CBR (constant bitrate) audio. Simple as that.

[jagabo]

know why VirtualDubMod is prescribing to decompress the audio stream first

VirtualDubMod often has problems with VBR audio streams. By decompressing and saving as WAV it will be easier for the program to deal with.

[jagabo]

Ok, so your point is that compression degrades quality, and you won't get it back by decompressing.

And that decompressing restores the original size, even if not all the details are restored. Audio and Video must be decompressed before it can be modified (for the most part, there are some operations you can do while it's still compressed) or recompressed.

[Cornucopia]

Yup, what he said.

IMO, there's no point in having VBR audio around, anyway. There's so many things it's not compatible with, it often drifts out of sync, and DANG IT!, it doesn't really help compress that much more. Audio is already a great deal smaller than the video if you're doing CBR; why complicate things.

Now, of course, I'm guessing you probably didn't CREATE this clip you're having to deal with...

Scott

[manono]

Why does every rookie walking through the door always have to try and reinvent the wheel? Just answer 'No' to the question and go about your business.

[jagabo]

Just answer 'No' to the question and go about your business.

I find that works about 90 percent of the time.

[manono]

For me it's more like 100%. I can't even remember a single time it didn't work. And answering 'Yes' leads only to disaster, unless you happen to like out-of-synch audio. But then I don't download garbage from the Internet and create all my VBR MP3s myself.

[FrozenInferno]

know why VirtualDubMod is prescribing to decompress the audio stream first

VirtualDubMod often has problems with VBR audio streams. By decompressing and saving as WAV it will be easier for the program to deal with.

So what would be the difference between first saving it as an uncompressed WAV file, then recompressing it versus recompressing right off the bat? Didn't you say it decompresses the stream automatically anyway?

Now, of course, I'm guessing you probably didn't CREATE this clip you're having to deal with...

You'd be guessing right. I usually just go for CBR, seems a lot more straight-forward.

Why does every rookie walking through the door always have to try and reinvent the wheel?

I don't really get what you mean by this. Just genuinely curious.

[jagabo]

know why VirtualDubMod is prescribing to decompress the audio stream first

VirtualDubMod often has problems with VBR audio streams. By decompressing and saving as WAV it will be easier for the program to deal with.

So what would be the difference between first saving it as an uncompressed WAV file, then recompressing it versus recompressing right off the bat? Didn't you say it decompresses the stream automatically anyway?

When VirtualDub is converting audio and video at the same time it doesn't decompress all the video and all the audio all at once. It works frame by frame, decompressing as necessary. It has to locate the audio that corresponds to each video frame. The audio isn't necessarily multiplexed on a frame per frame basis so it can be difficult to determine exactly which audio goes with a particular frame. If you decompress the audio first it can just start at the beginning and read through the audio data, saving it as a CBR uncompressed WAV. With CBR data locating what you want is as easy as:

position = rate * time

The biggest problem with VirtualDub and VBR audio is that the author claimed the way some programs packaged VBR audio in an AVI file was incorrect. For years he refused to accommodate it even though it was pretty easy as VirtualDubMod showed. He eventually gave in and VirtualDub now handles VBR audio properly in most cases. I wish he would finally give in and accommodate interlaced YV12 video too.

[FrozenInferno]

know why VirtualDubMod is prescribing to decompress the audio stream first

VirtualDubMod often has problems with VBR audio streams. By decompressing and saving as WAV it will be easier for the program to deal with.

So what would be the difference between first saving it as an uncompressed WAV file, then recompressing it versus recompressing right off the bat? Didn't you say it decompresses the stream automatically anyway?

When VirtualDub is converting audio and video at the same time it doesn't decompress all the video and all the audio all at once. It works frame by frame, decompressing as necessary. It has to locate the audio that corresponds to each video frame. The audio isn't necessarily multiplexed on a frame per frame basis so it can be difficult to determine exactly which audio goes with a particular frame. If you decompress the audio first it can just start at the beginning and read through the audio data, saving it as a CBR uncompressed WAV.

I meant what the difference was between setting the audio stream to uncompressed and saving as a WAV, then re-importing the WAV and recompressing, then saving it, versus just recompressing the original stream. Video aside.

[FrozenInferno]

I assume I didn't get a reply because I didn't word my comment as a question. Video aside, is there any difference between setting the audio stream to uncompressed, saving it as a WAV, then re-importing the WAV and recompressing, versus just recompressing the original stream? If not, why does VirtualDub recommend doing this? And if there is a difference, why doesn't the same apply for video?

[jagabo]

VirtualDub is a video editor. It does things based on the timing of video frames. Whether you're recompressing the video or not, it has to locate the audio that corresponds to each video frame to mux them properly.

If you're talking about using only the audio functions, ie,

import video, export audio as uncompressed WAV, import uncompressed WAV, save as recompressed WAV

vs.

import video, export recompressed WAV

there may be little difference since it's not trying to coordinate video frames and audio chunks. You'll have to try it. Or write the author.

[FrozenInferno]

Cool, thanks a lot for your help.