audio sync - stretching audio

[abbymat]

I've captured a 3 hr VHS tape to Huffvy AVI using Virtualdub.
Audio captured at PCM 48000hz 16-bit stereo.

The audio is progressively going out of sync, so I used Vegas to stretch the audio
till its in sync with the video and saved the audio at the same PCM 48000hz 16-bit stereo.

Then used Virtualdub to split the AVI into video & audio files and then recombined
the original video with the stretched audio using direct stream copy and the sync is perfect.

Question is will there be any loss of video or audio quality after doing the above process ?

thanks

[johns0]

Depends on how how vegas resampled the audio while stretching it.If it did fast sample rate then it will be good,if it replicates or trandsforms for keeping pitch them it will start to sound a bit off,if you saved the recombined audio and video and used direct stream copy for video then there will be no loss for the video.

[abbymat]

where do I set the fast sample rate option in Vegas ? I've selected the "preserve pitch" option.

Should I also use the Auto-Ripple effect while stretching....the sync is progressively getting worse towards the end.

[johns0]

If the sound is good to you i wouldnt worry,not sure about vegas sample rate settings if it uses that option,just that some sound editing programs warble the sound a bit when trying to keep the pitch.

[Almost Human]

Before making any changes, like cutting out unwanted sections:

Open your AVI in VirtualDub, under “File”, “File Properties”, check to see if the video and audio are the same length. If they are not the same length, then…

Under "Video" select "Frame Rate".

In "Source rate adjustment" select "Change so video and audio durations match, OK.

Under "Video" select "Direct stream copy".

Under "File" select "Save as AVI".

Save

Check your new AVI's file info and you will see that the audio and video are the same length.

This will probably produce an AVI with a frame rate of 29.968 or thereabouts which is still within the normal parameters of most encoding programs (TMPGEnc has no problems with it).

[abbymat]

OK, the audio was about 1.27 seconds shorter than the video - original frame rate was 25 fps (PAL) .

I matched the audio/video durations using Virtualdub. The new frame rate is 25.004 fps.

There is good audio-video sync using this method also, but on some players like VLC the file stutters and hangs if I try to fast forward while playing. On Windows media player it plays fine. Wonder why this happens ? Also, will there be any loss in video or audio quality in the above method ?



Hence, all in all considered, which would be a better method - using Vegas to stretch the audio to match the video length or using Virtualdub to match the audio/video length by changing the frame rate ?

[Almost Human]

abbymat - I don't use Vegas, so your on your own there. I use VLC exclusively for all my video playbacks and have not witnessed the anomaly you wrote of - all mine are NTSC.

[abbymat]

I'm not sure why VLC alone is creating playback problems, all other players are fine.

OK, one other question, since I'm using "direct stream copy" for both Audio & Video while changing the frame rate to match the length in Virtualdub, anytime I want, can I can reverse the process and get the original AVI file back EXACTLY (25.000 fps length) without losing any quality in either Audio or Video ? (AVI file is Huffyuv and PCM 48000hz 16-bit stereo).

If so, the Virtualdub frame-rate change process will be defintely better than stretching the Audio using Vegas as the Vegas changes the Audio waveform while stretching which certainly can't be reversed.

[Almost Human]

Try it out. Let me know if it works.

[abbymat]

I tried it out, reconverted the AVI with frame rate of 25.004 back to 25.000 fps and observed the audio wave-forms at the beginning and end (not possible to do it for the whole file) of the frames. It appears identical.

I think its theoritically correct to presume that it will give back the exact original if direct-stream copy is used, as only the number of video frames/second timing is changed when changing the frame rate. The actual video & audio data is not touched in anyway. I hope I am correct ?

[borodisha]

7.29 method by Almost human worked for me, thanks!
Then I reencoded again with Xvid and 29.97 fps (also needed to correct audio pitch up 1.6 - my orig TV rip was out of synch and with low voice garbled narration).