Chaning sample rates mid-recording?

Midnightlightning · Post by **Midnightlightning** » Tue Oct 16, 2007 7:35 pm

I've been using Audacity to record audio from a USB mixer for a while, and I've been getting a symptom more and more frequently that's quite annoying. I record the audio at 48000 Hz, and after the recording is done, randomly, parts of the recording sound like chipmunks, as they've been sped up. If I slice out that section and adjust the rate down to ~22000 Hz, then it sounds fine. If I re-record the session, sometimes the breaks happen, sometimes not; they seem to happen at random times in the recording. Is this a preference trying to 'auto-adjust' the rate of the recording?

alatham · Post by **alatham** » Tue Oct 16, 2007 7:45 pm

We've been seeing more and more of this kind of odd sample rate problem showing up lately. I wonder why?

Just a question, is your computer playing other sounds while you're recording (such as OS system sounds, or sounds from a different program)? I can see how that would throw it off, but it seems unlikely to me.

Do you have your computer speakers plugged in through the mixer, or plugged into a different sound card on your computer? If they're plugged into the mixer, try temporarily setting it up so the output is through the sound card and the input is through the mixer. If you do that, does the problem still occur?

Midnightlightning · Post by **Midnightlightning** » Tue Oct 16, 2007 9:13 pm

The computer's OS sounds are on during the process, but don't know whether that would be it. Currently I have a USB mixer (a MoblePre USB box) which has two line inupts, a USB out, and a speaker out. Currently the OS sounds are going out through the USB to the speakers hooked up to the mixer. The audio input is coming in on channel 1 and coming into the computer via the same USB cable, and is being streamed to the speakers as it goes.

Post by **kozikowski** » Tue Oct 16, 2007 10:53 pm

Wait a minute. We missed a step there. If the one mixer USB is going to the speaker system, how is Audacity getting the show? USB doesn't do "live" audio and video as well as FireWire does, and it doesn't share at all.

Koz

Midnightlightning · Post by **Midnightlightning** » Wed Oct 17, 2007 2:03 pm

kozikowski wrote:USB doesn't do "live" audio and video as well as FireWire does, and it doesn't share at all.

Mmmm, not sure; this is plugged into a USB 2.0 port and the OS audio output has been set to use the USB out as the sound output, so it's been a two-way street for a while. I don't know if the audio signal coming into the mixer is going down to the computer and then back up to the mixer before going out to the speakers or if it's going directly to the speakers through the mixer, but the OS sounds come through at the same time the sound is going down the USB channel.

I've switched it so that's not the case; USB is only sound-in now, and the sound card has the line-out for the speakers, and I'll see if that makes a difference.

alatham · Post by **alatham** » Wed Oct 17, 2007 6:56 pm

Midnightlightning wrote:I've switched it so that's not the case; USB is only sound-in now, and the sound card has the line-out for the speakers, and I'll see if that makes a difference.

Let us know if that helps or doesn't help. This problem if kind of baffling to me.

Midnightlightning · Post by **Midnightlightning** » Thu Oct 18, 2007 8:24 pm

Switching to using the USB to only be one-way (sound-in) and the sound card handling only sound-out didn't help the problem; when the project was set to 48000 Hz it still periodically "chipmunked" down to 22050 Hz. What I have found that cures it is having the project set to 22050 Hz. Since I did that there hasn't been any further random adjustments in speed.

alatham · Post by **alatham** » Fri Oct 19, 2007 3:27 pm

It's good that it works, but 22050 is a low sample rate, the quality won't be very good.

So, I think you've probably got a hardware problem, but there's no way to be sure without trying.

Midnightlightning · Post by **Midnightlightning** » Fri Oct 19, 2007 5:47 pm

The current project is importing/converting cassette tapes to MP3s, so the quality isn't that good to start with, but I do use this setup to do voice recordings from an XLR microphone too, which I did have the same issue with. How would you test for a hardware problem (hopefully other than getting a new USB mixer to try out)?

alatham · Post by **alatham** » Fri Oct 19, 2007 5:53 pm

You can try using Windows Sound Recorder to record some audio and see if it turns out alright. That should test the hardware, I think.

Also, even if you're using Cassettes, I would still record them at 44.1KHz. Recording at a lower rate would definitely be noticeable.

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Chaning sample rates mid-recording?

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