Audio Sync

[SE14man]

When you reencode on Virtual Dub having WAV Audio for audio but Direct Stream copy for video.
So basically just changing the audio.
Can this degrade quality for the video or NOT because its a direct stream copy?

Also can you list me everything to make a digital video's sound go out of sync?
The causes of it?
Obviously some codecs issue.
But what about the video itself?
Would it being a VBR Audio make it more likely to go out of sync?

[laspis59]

"When you reencode on Virtual Dub having WAV Audio for audio but Direct Stream copy for video.
So basically just changing the audio.
Can this degrade quality for the video or NOT because its a direct stream copy?"


I haven't seen any decrease in quality using direct stream copy.In order to not lose quality,you just need to save with a lossless codec like huffyuv.If your saving with a crap codec then you will lose quality.

I never use it,I use WAV or mp3 constant bitrate,but I think that audio does go out of sync with vbr audio.I'm damn sure I've read it on this site in the past.

[ZippyP.]

Direct Stream means that the video is untouched, no quality loss, no change at all.

The causes of loss of sync can be many and varied. In my experience they are: bad frames in the source, VBR audio and changing frame rates. Cut out the offending frames, decode the audio to wav and use the same frame rate as the source.

[SE14man]

With VBR though, even if the file isn't corrupt, over time does the divx file just go out of sync (even if it has no corruption?)
Will i also get different issues with the audio like loudness and quietness?
Same with the video will it just randomly degrade (the quality) or is that impossible because its digital?

[SE14man]

A,lso you say changing frame rates how can i tell if the framerates are changing through out the whole video id be watching?
Am i right in thinking i'd have to do this.
Drag the vid in VDub.
Audio>Full Processing Mode>Compression>No Compression PCM.
File>Save WAV.
Then Reopen the Vid in VDub and Go Audio>Full Processing Mode>Compression>Mpeg Layer-3>56 kb/s.
Then Video>Direct Stream Copy.
File>Save AS AVI.
Then do i reload the vid back in the VDub?
Then concentrate on the frozen frames and cut them out.
How would i make it a constant framerate instead of having it keep change?

Cheers.

[laspis59]

I've never seen framerates change.If you add video of 25fps it comes out 25fps unless you change the settings.Like advised avoid using vbr as it could cause sync issues.

[SE14man]

will it cause sound sync issues as soon as its been ripped?

[laspis59]

If your using WAV or constant bitrate mp3 and your video is fine then all should come out in sync.I've never had any problems.
Try it,you'll soon find out.

[SE14man]

I know that mate cheers.
But over time Videos with VBR Audios go out ogf sync or would they be out of sync straight away as soon as ripped?

[SE14man]

well??

[SE14man]

c'mon man, someone must know!!

[SE14man]

??

[Cornucopia]

I'm going to go out on a limb here, as I don't do much DivX-type ripping/reconverting...

Digital is Digital is Digital.
Once it's saved as a file, assuming there is no corruption because of media/transmission bit errors, your file will be EXACTLY the same as when you originally created it.

Now, players vary in their ability to keep sync, and in their algorithms for recovering from bitrate throughput issues. Some drop/skip frames of video in order to keep audio smooth, some stutter, some come to a dead stop, etc. Once you can get the pipeline infrastructure big enough, those type of issues usually go away.

Note: Direct-Stream copy is just that--doesn't change codecs or anything.
Anything else may NOT be identical, even if saved to a lossless/uncompressed format. This mainly has to do with the math--rounding errors, YUV<-->RGB conversions, etc. You should assume that unless you are doing a straight copy, there will be some change, that's why it's best to start with the highest rez video you can, use the highest quality tools, save to the highest quality format, and re-edit/re-save as few times as possible. Whenever PROCESSING, digital audio and video start to act more like analog, and degrade.

As a longtime audio engineer, my advice to you is: Don't ever use VBR audio unless you specifically are absolutely required to use it. It is tricky and cause much more problems than it's worth.

Scott

[SE14man]

Note: Direct-Stream copy is just that--doesn't change codecs or anything.
Anything else may NOT be identical

Do you mean if i do a direct stream copy for the video the quality will be a little worse than the original file loaded?

use the highest quality tools

What is recommended?

PROCESSING digital audio and video start to act more like analog, and degrade.

Is the only time digital video degrades is when you reencode it or corruption of course?
The same goes with audio?
If a file gets corrupted what would the caues be to the audio part, Distortion, and sync issues?
My biggest worry at the moment though is most of my divx video is VBR i have one hell of a lot of divx video.
As it's VBR (even if no corruption happens to it) will it still eventually loose it's sync them ore its watched and copied from drive to drive and media to media OR not at all unless it gets corrupted?
I find VBR a pain too.
Would you mind telling me the bad things about having a Variable Bitrate instead of Consant?
About the video degrading thing, for example a bad quality film would look very blocky not corruption blocky but low quality blocky.
DivX or any digital video cant end up like that can it if watched over and over?
Would it either have to be a bad encode to start with (in that case it would be bad quality anyway) or would it have had to have been encoded loads of times to end up blocky?)

Cheers sorry about this im a bit stupid.

Scott[/quote]

[Cornucopia]

Note: Direct-Stream copy is just that--doesn't change codecs or anything.
Anything else may NOT be identical

Do you mean if i do a direct stream copy for the video the quality will be a little worse than the original file loaded?

use the highest quality tools

What is recommended?

PROCESSING digital audio and video start to act more like analog, and degrade.

Is the only time digital video degrades is when you reencode it or corruption of course?
The same goes with audio?
If a file gets corrupted what would the caues be to the audio part, Distortion, and sync issues?
My biggest worry at the moment though is most of my divx video is VBR i have one hell of a lot of divx video.
As it's VBR (even if no corruption happens to it) will it still eventually loose it's sync them ore its watched and copied from drive to drive and media to media OR not at all unless it gets corrupted?
I find VBR a pain too.
Would you mind telling me the bad things about having a Variable Bitrate instead of Consant?
About the video degrading thing, for example a bad quality film would look very blocky not corruption blocky but low quality blocky.
DivX or any digital video cant end up like that can it if watched over and over?
Would it either have to be a bad encode to start with (in that case it would be bad quality anyway) or would it have had to have been encoded loads of times to end up blocky?)

Cheers sorry about this im a bit stupid.

Scott

[/quote]

#1--note the "ELSE". Everything else besides Direct stream copy (or obviously file copy in Explorer) may change the video. And with change usually comes degradation. Same with audio.

#2--Depends on what you need done... If you're talking about video processing, then VirtualDub and AVISynth are quite good. AfterEffects is better, just because it is supposed to do a better job with colorspace conversions and is also capable of 16bits-per-color (aka 48bit) math. But I'm not talking "easy-to-use" apps here. For batch conversion, I think Canopus ProCoder is one of the best. What categories were you thinking of?

#3--Try this sometime: Take a short (15sec) piece of uncompressed video (not DV/MPEG/MJPEG etc.) and export uncompressed bmp stills to a folder. Then, using that original piece of video, apply a gamma change ( PC->Mac ), then apply a reverse gamma change ( Mac->PC ). Save as 2nd uncompressed video, and also export uncompressed bmp stills to a 2nd folder. Now, if you did a bitwise compare of corresponding still pix ( Pix001of450a.bmp vs Pix001of450b.bmp ), there usually are differences. These differences come from the rounding of the math done during the processing. Compound this with lossy encoding, and that's why there is often degradation, not even taking into account bad math from bad algorithms.

//RANT//Not enough video apps apply dither during processing! The better audio apps do this a a rule during any dsp/bitdepth reduction. By dither, I mean a (psuedo-)random signal applied to the LSB. In audio, this keeps background noise and small signals (reverb!) sounding natural. In video, this helps avoid banding. Also, not enough apps do floating point dsp! Combine these 2 and all processes will benefit from a more beautiful, natural signal.//End RANT//

#4--Corrupted video can get out of sync, but usually there is some other telltale sign along with it--picture breakup, hash in the audio, etc.
Just having the file be played over and over doesn't corrupt it. If it did, that would be like having your applications be trashed after you used them a certain # of times. If that's happening to you, you need new HardDrives! If you're really worried about media corruption, use Scandisc, Norton DiscDoctor, DiskKeeper, etc on your HD's, and use ISOBuster to scan your optical media.

#5--VBR Audio. There's nothing inherently wrong with VBR audio in a theoretical sense. It's just not implemented very well, either on the encoding side, or more particularly on the decoding side. Lots of apps have trouble with it. Particularly as regards SYNC. If you've got a lot of VBR audio, I don't know what to say. Is it worth it to re-encode to a slightly higher bitrate at CBR? I don't know. Remember, your not saving a lot of bitrate by going VBR on the audio. The real savings is when using it on video.

BTW, how many generations of re-encoding are you doing with your DivX?
Me, it's only 1st or 2nd generation at worst. If I want to do something more with it, I go back to the DV- or MJPEG- or Un-compressed master.

Scott

[SE14man]

So can copying a file make it worse quality ru saying then along the top of your comment?
You certinaly seem pretty high up in the video encoding stuff :O

What could be the cause of VBR audios being out of sync?
It wouldnt just randomly happen would it?
We've pretty much covered all my questions here and am very grateful for that.
Theres not even anyway that watching digital video over and over it will make the picture fade/go blocky?
I know u said it wont corrupt but i was just wondering if it would fade or go less clear?
Im assuming not because i think of applications doing it and its impossible really isnt it lol.
Or isit different with vid?
So copying cant do harm to the videos quality?
It seems to me that the only thing to affect audio and video being distortion, out of sync etc would be a corrupt drive?
(with sync though it would be a bad rip at the start wouldnt it to be out of sync)
Thanks 4 all this Scott ,is appreciated.

[Cornucopia]

I think I answered all that already.

Think about this:
Master = 100% (100)
Copy of Master = 100% (100)
Copy of 1stCopy = 100% (100)
Copy of 2ndCopy = 100% (100)
Copy of 3rdCopy = 100% (100)
.
.
.
Copy of 9thCopy = 100% (100)

but, if processing or reencoding @ 95%

Master = 100% (100)
Copy of Master = 95% (95)
Copy of 1stCopy = 95% (90 1/4)
Copy of 2ndCopy = 95% (85 3/4)
Copy of 3rdCopy = 95% (81 1/2)
.
.
.
Copy of 9thCopy = 95% (60)

That's a big difference. Good thing is DV, DVCam, DVCPro re-encoding is better than that (prob. 98%), simple processing w/ Lossless re-encoding is probably 99.99%.



Watching and rewatching, Listening and relistening of DIGITAL audio/video WILL NOT degrade the material (unless rare case of media corruption--if you've got media corruption, you'll know it)

I would bet that 99% of playback (incl. sync) problems with DivX with VBR audio is that the software that isn't all that forgiving/smart and the PC infrastructure (CPU speed, RAM size, cache, Bus speed, HD seek and access speeds, etc) isn't hefty enough to continually serve it up smoothly.

How 'bout some other questions...

Scott

[SE14man]

Master = 100% (100)
Copy of Master = 95% (95)
Copy of 1stCopy = 95% (90 1/4)
Copy of 2ndCopy = 95% (85 3/4)
Copy of 3rdCopy = 95% (81 1/2)
Copy of 9thCopy = 95% (60)

So if i reencode a file in full processing mode i will loose a bit of quality. Then i save that file and load it in and do a direct stream copy for the video and audio i wouldnt loose quality then would i?
I wondered that from where u typed 'Master=100% then copy of master=95% did you mean direct stream copy or reencode copy with a different codec? If you meant direct stream copy i thought you get no loss with a direct stream copy do you?

Watching and rewatching, Listening and relistening of DIGITAL audio/video WILL NOT degrade the material (unless rare case of media corruption--if you've got media corruption, you'll know it)

When you ssaid media corruption, did you mean media corruption as in bad sectors on the disk drive? Or having them stored on DVD and the DVD being scratched or whatever?

How 'bout some other questions...

lol, i know i'm a real pest its just im really stupid and some of the things you say i have trouble understanding lol.
Think you've done a good job of explaining mate very grateful for this.

[Cornucopia]

#1-remember the "ELSE" answer earlier on? DirectStream copy is just the gathering of the video chunks and audio chunks (as chosen), remuxing them, and putting them into a new avi container--there is no change in the quality. Like, for example, you have AVIMovieFileA with VideoA and SoundtrackA. Then you have WAVEfileB with SoundtrackB. Opening AVIMovieFileA in VirtualDub, along with SoundtrackB from WAVEfileB and doing a DirectStream copy will "un-zipper" the V + A from AVIMovieFileA and "re-zipper the V with the A from WAVEFileB. But the contents of the "teeth of the zipper" are not modified at all.

#2-Corruption can be bad HD sectors, or scratches or both, or a few other things.

#3-Always helps to keep learnin'. 8)

Scott

[SE14man]

Yeah cheers Scott thats helped me a lot.
I think you've answered all the questions am very grateful.

Thanks again for that.