Bear with me here. I am a radio producer; I record my shows to DAT at the radio station then bring the tape home for post-production on my Mac in Peak Pro.

For many years I used a G4 running OS X 10.4.x with M-Audio Audiophile USB, but last year started having all kinds of problems with the recordings. It was usually a jittery, "motorboating" kind of sound, but occasionally there were artifacts from other parts of the recording popping up in inappropriate places or just generalized distortion. (This occurs in varying degrees whether recording into Peak or Quicktime.) When all my troubleshooting failed, I decided the M-Audio was the culprit and bought a Behringer UCA202 USB A/D converter. The problem persisted. I did a clean system install, replaced the logic board. When that didn't work, I gave up and purchased a new iMac 2.4Ghz Dual Core.

The first few recordings I made were OK, but then the problem came back with a vengeance. Here are just some of the other troubleshooting attempts I've made:
Upgraded all software
Changed the cables running from DAT to A/D
Tried using sources other than DAT (ie, a cassette deck)
Switched USB ports
For the hell of it I thought I'd try the M-Audio again, but the latest driver seems to be in be incompatible with Leopard and/or the Intel processor. My recording attempts came out blank.

Last night I contacted Apple. The tech guy I spoke with had never encountered this problem and suggested that USB might be the bottleneck and that I should get a Firewire A/D converter. Frankly, I don't understand why popular USB devices that work for others aren't cooperating with my setup. The Firewire converters I've seen online are very expensive and wa-a-ay overkill, since all I need is something that will accept a stereo analog or SP/DIF input and convert it to 16-bit, 44.1 Mhz. No mixing, no instruments, no midi.

Since this directly affects the way I make my living I HAVE to find a solution before I go totally nuts. I figured that if anyone had ever heard of this or could come up with a solution it would be you folks.

Cary
iMac 2.4 Ghz Intel Core 2 Duo
OS X 10.5.2
4 GB RAM

Just a guess but are your audio interfaces and DAW software both set up with the same sampling rate? This would definitely cause this.
Seems odd given you just about replaced everything and still get the issue.

Have you tried plugging directly in to the Mac with a stereo RCA to 3.5 mm cable? This would at least rule out USB as part of the problem.

If your DAT has toslink optical you could use that as well. The audio-in on the back of the iMac is both 3.5 mm stereo and optical.

the built in microphone and the audio-in both connect to the same audio board inside the iMac, so if you can record normally in Quicktime Pro with the microphone you should be able to record normally with the audio-in.

Thanks. My DAT (Tascam DA-20 MkII) only has RCA and SP/DIF outputs, so no to Toslink. But I did try your suggestion of going directly into the Mac via RCAmini-plug. Once I got the input levels set correctly the recording sounded fine -- at least for the six or seven minutes I recorded. OTOH, I have had USB recordings be fine for close to an hour before crapping out, so this will require more experimentation.

If this check out via repeated testing, that would SEEM to indict USB as the culprit, yes? (Don't want to go out on a limb because I've been fooled before in the hours and hours of troubleshooting I've spent on this.)

But, if this is correct then I'd like to know if the A/D converter built in to the iMac is of sufficient quality that I don't even need to mess around with an outboard converter. If so, it would sure save me a lot of $$$ and hassle.

Cary




Yes, Audio Midi is set to 44.1 Mhz. Peak Pro has been set to either "Auto" or 44.1 Mhz with no noticeable difference.

By Audio Midi I am assuming this is your iMac setting and Peak pro is your software.

My point was whether the device (M-Audio or DAT or whatever you are feeding the iMac your digital signal from) is set to send at 44.1 as well. The whole chain needs to be in synch to work at 44.1 or else you will get problems.

Again I dont know enough of your setup to be sure. Check every digital output and input device and software in the chain from start to finish all has the same sample rate set.

Your mini RCA going direct success definitely points the problem to digital issues and hence very likely the sampling rate assuming different devices you have replaced still result in the same problem.

Cheers