WAV -> MP3 causes lag?

Jnzk · Post by **Jnzk** » Thu Apr 22, 2004 2:44 am

The difference is 51 ms which is about 1.2 frames, so it's not easy to say, but it feels to me like there is a slight difference in sync when I compare the videos with wav and mp3 respectively. I converted the mp3 back to wav with dBpowerAMP, but it didn't change the visualization: the lag stays. SoundForge 7.0 shows exactly the same 51 ms difference too. I don't know about the 32-bit float thing, Audacity seems to display MP3s like that although my default sample format is 16-bit.

Jnzk · Post by **Jnzk** » Thu Apr 22, 2004 2:48 am

Oh yeah, I forgot to mention that I'm using a 10385 ms audio delay in VirtualDubMod. As a last resort I guess I could try to tweak this by trial and error, but I'd rather have the wav and mp3 exactly the same.

AbsoluteDestiny · Post by **AbsoluteDestiny** » Thu Apr 22, 2004 2:51 am

10385ms audio delay????

whatever for?

I can see you putting in a 51ms delay after these results but not THAT much.

Jnzk · Post by **Jnzk** » Thu Apr 22, 2004 2:59 am

I have a bumper in the beginning of the avi, and I don't want the audio to start playing right away.

AbsoluteDestiny · Post by **AbsoluteDestiny** » Thu Apr 22, 2004 3:01 am

You should really make a wav file that has silence at the start and then convert that wav to mp3.

If you are wanting the kind of accuracy you are looking for then that will be the most reliable method due to the way mp3 is interleaved.

Jnzk · Post by **Jnzk** » Thu Apr 22, 2004 6:35 am

Yay! I managed to get the original wav and the mp3 line up. I cut off 2257 samples from the beginning of the uncompressed file, added the same amount to the end and used the new edited version for compression with LAME.

The mp3 is still a little longer than the original unedited wav, but that's not a problem as far as audio sync is concerned.

I've been using the delay option in VirtualDubMod in almost all of my videos, I always thought it was extremely handy. Now I have to figure out how to add the right amount of silence to the beginning with another method.

I put a an empty audio file on the Premiere timeline and exported it after adjusting it to proper length. I ran it through BeSweet to do the 24 -> 23.976 fps conversion. Then I measured the lengths of both the 24 fps and the 23.976 file in samples. The latter was about 0.9947x as long as the former, not 0.999x as you'd suppose. Since the length of the silence is 10 seconds, doesn't this mean that I get a 43 ms error if I add this to the beginning of my edited wav file?

Heheh, I'm driving myself nuts with this.

Corran · Post by **Corran** » Thu Apr 22, 2004 8:19 am

Janzki wrote:I have a bumper in the beginning of the avi, and I don't want the audio to start playing right away.

>_< In certain players I notice a bumper, and when it shows up the audio starts right away. When I don't see the bumper the audio starts right away but everything is synced. I noticed this while trying to put the AMV Lethal Embrace on to a DVD. I thought you were just trying to deter people from stealing your videos...

AbsoluteDestiny · Post by **AbsoluteDestiny** » Thu Apr 22, 2004 10:57 am

Having silent bumpers and using the 24fps method is problematic, I know, but there are ways of doing this.

I'd personally use an avisynth script such as:

vida=AVISource("yourpremiereexport.avi").AssumeFPS(23.976)
auda=WAVSource("youroriginalwavfile")
a=AudioDub(vida,auda)

vidb=AVISource("yourbumper").AssumeFPS(23.976)
audb=BlankClip(vidb, audio_rate=44100)
b=AudioDub(vidb,audb)

a+b

Then I'd put that in virtualdubmod and compress as usual. If you want the wav from that you can try exporting it from the stream if you want to process it externally.

This is presuming the audio in the original is going to sync correctly of course

Corran · Post by **Corran** » Thu Apr 22, 2004 11:38 am

If you're setting it up like that shouldn't it be:

b+a

AbsoluteDestiny · Post by **AbsoluteDestiny** » Thu Apr 22, 2004 11:41 am

Corran Productions wrote:If you're setting it up like that shouldn't it be:

b+a

heh, yeah. oops