Bug in FFmpeg conversion? Work around found : )

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audio_pete
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Bug in FFmpeg conversion? Work around found : )

Post by audio_pete » Sun Jun 24, 2018 3:00 pm

More adventures in video editing.

Copy VOB in Audacity
Use Change Speed to get size 00:15:02 + 43,153 samples
Use Noise Reduction to get rid of DVD recorder motor noise
Use Change Speed to get size 00:15:02 + 43,153 samples
Save as AC3
open AC3 in Audacity
Use Change Speed to get size 00:15:02 + 43,776 samples

or
00:15:02 + 22 PAL frames
00:15:02 + 23 PAL frames
or
43,339,153 samples
43,339,776 samples

Don't see anything on front
Don't see anything on back

Listening test of seconds 2 to 3
look and sound the same.

Is this a bug?
makes it that much harder to fix my VHS to DVD video conversion.
Last edited by audio_pete on Mon Jun 25, 2018 3:38 pm, edited 1 time in total.

steve
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Re: Bug in FFmpeg conversion?

Post by steve » Mon Jun 25, 2018 12:44 pm

audio_pete wrote:Use Change Speed to get size 00:15:02 + 43,153 samples
Use Noise Reduction to get rid of DVD recorder motor noise
Use Change Speed to get size 00:15:02 + 43,153 samples
Why are you using Change Speed twice? Noise Reduction does not change the duration, so after changing the length to 00:15:02 + 43,153 samples in the first step, the third step should be unnecessary.
audio_pete wrote:Save as AC3
Be aware that AC3 is a lossy format. Lossy compression formats discard some information as part of their effort to reduce the file size, so there is an inevitable loss of sound quality. Also, as with MP3 (another lossy compression format), the start of AC3 files are padded with a small amount of silence up to the first frame boundary, so the exported length will be a little longer than the length of the audio that you are exporting from Audacity.

To guarantee the correct length and avoid loss of sound quality, it is better to export in a lossless format such as WAV.
audio_pete wrote: Listening test of seconds 2 to 3
look and sound the same.
What are you expecting?
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

audio_pete
Posts: 105
Joined: Fri Jul 26, 2013 1:03 am
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Re: Bug in FFmpeg conversion?

Post by audio_pete » Mon Jun 25, 2018 2:40 pm

Thanks for the help,
discovered your most helpful suggestion just before I read this.

I just made a AIFF copy of the noise reduced file
and it is 00:15:02 +22 PAL frames


I used Change speed ONLY to verify the exact length of the file,
maybe there is an easier way

I went with AC3 because that is what most videos do, right?

Was hoping the "extra" samples where at the beginning or end of file
where I could easily delete.
Did not find that to be the case.
The file is just slightly longer than the original (not lossy)
and for most purposes that would be ok
but 1 frame is a killer for audio video synch : (
It is a BUG

Oh and Filmora uses AIFF files.
Have not tried yet but am alway hopeful

steve
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Re: Bug in FFmpeg conversion?

Post by steve » Mon Jun 25, 2018 3:06 pm

audio_pete wrote:I used Change speed ONLY to verify the exact length of the file,
maybe there is an easier way
Double click on the track to select it, then look in the "Selection Toolbar" https://manual.audacityteam.org/man/sel ... olbar.html
By default, the final time box shows the time at the end of the selection.
audio_pete wrote:I went with AC3 because that is what most videos do, right?
Yes, but when the audio and video are encoded together they can be synchronised by the container format. When you are encoding the audio separately there is no such synchronisation. If you import the audio into your video editing application in a lossless format, then export the video+audio from that program, the video editor should handle the encoding of both the audio and the video and ensure that they remain synchronised.
audio_pete wrote:Oh and Filmora uses AIFF files.
AIFF is also a lossless format, so that should be as good as using WAV (AIFF is more common on Macs than on other computers).
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

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