Audio sync question; sampling rate to blame?

Urban Reflex · 14 Sep 2009 at 03:40

Okay, here's an odd one for you.

What I'm trying to do is create an Xbox Live gameplay video, with the voice chat overlaid on top.

I have recorded the gameplay, and because of the limitations of the Xbox 360, the gameplay video has my team's speech (what they say to me) on it, but not my speech (what I say to them).

I set up a dual mic system so that my speech was recorded as a wave file to my PC.

Using VirtualDub, I ripped the audio from the gameplay video also to wave format.

Now I'm trying to create one audio file in Audacity, which contains both my speech and my team's speech. The problem I'm getting is that the sync between what is said does not match up. For example, as you listen to the merged file I am answering questions before my friend has even asked them.

Despite being recordings of exactly the same session, started and ended at the same time, the length of my speech file seems 'compressed'. This gets worse as time goes on, with larger gaps appearing between answer and question (and it's always that way around) later in the recording.

Here are the details of the recordings (They're both 2 channel stereo PCM audio .wav files):

- Team's Speech file: 2h12m52s long, 1.42GB in size, 48000Hz sampling rate, bitrate of 1536kbps.
- My Speech file: 2h08m29s long, 1.26GB in size, 44100Hz sampling rate, bitrate of 1411kbps

I know these are fairly big files, but my machine is more than capable of handling the processing (see sig for spec). If you look, there's a gigantic difference in length of 4 minutes and 23 seconds between the two files, why could this be? Like I said, I started and ended both recordings at pretty much the exact same time (give or take a second or two).

Could it be the fault of the sampling rate? I don't know a lot about audio, but doesn't that affect the speed of the recording or something? I tried using Audacity to save the shorter file in 48000Khz but it didn't change anything at all.

Any help you guys can give me would be most appreciated. I'm really not too intelligent on audio stuff!

Jotun · 14 Sep 2009 at 04:03

Can't offer much advice for why it's happened, but your first track is 7972s, the 2nd is 7709s, so if you increase the 2nd track by 3.411597% they should have the same length so they should match up.

You can do it in audacity under Effect > Change speed.

Hope that's some help

lord filbuster · 14 Sep 2009 at 04:08

It's possibly something to do with the video's frame rate. The sync problems are obviously caused by the difference in lengths, but i couldn't say for sure what caused it. The sample rate definitely wouldn't though. A quick and dirty fix would be to use audacity to strech the shorter file. goto effect->change tempo, and in the box change the length to 7972s. (2h12m52s). This may effect the pitch a little, but since the added length is a small percentage of the total, you won't notice.
edit: beaten

Urban Reflex · 15 Sep 2009 at 00:31

Thanks for your help guys, but that didn't work :/ Basically getting the same problem, it just doesn't line up.

The only solution I can think of is to go through the whole two hours and align everything manually, which I wouldn't mind TOO much... but then if I add it to the video there's no guarantee it's going to match up with the action properly. God damn it.

lord filbuster · 15 Sep 2009 at 20:23

There is one more thing you could try, if you add both audio streams to the video in virtual dub, there is an option under video frame rate that says 'change frame rate to match audio length'. it's worth a shot.

Urban Reflex · 17 Sep 2009 at 03:27

Ah, didn't work either :/ Thanks though, was worth a shot.

Anyone know of a program that allows you to edit audio like Audacity (copy/paste segments of audio, overlay another track, etc) while actually viewing the video?

I'm going to do it manually, though it'll take forever. But I need to be able to view the video to make sure everything syncs up correctly.

ChazHurst · 17 Sep 2009 at 09:36

If you get a proper video editor (like Sony Vegas) you can see all 3 elements at once while previewing video. The version I use allows me to "time-warp" any media on the timeline in whatever way so that I can make anything (within reason) fit.

Urban Reflex · 30 Sep 2009 at 18:47

Vegas, Soundbooth, Audacity; nothing works to fix this problem. I can do the manual syncing in Soundbooth but, really, I'm not sure I have the patience. My thoughts have since turned to preventing this problem in future, for which I'll need one of two things:

1) Live playback of my PC microphone input via my speakers, so I can record it together with the audio output of my capture card (and thus team's voices) using Creative's 'What U Hear' feature.

2) A program that will record from two sources at once (ie my microphone and optical input) and allow me to save it as a single file. As I'm looking to eliminate sync problems, I can't simply record to two seperate files.

Can anyone help me? For reference, I'm using Vista, a Blackmagic Intensity Pro capture card, and a Creative X-Fi Titanitum Fatal1ty Pro sound card.

Urban Reflex · 5 Oct 2009 at 21:35

No ideas?