Optical digital input (S/PDIF) problem (sampling rates)

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Level 1

8 points

Optical digital input (S/PDIF) problem (sampling rates)

I have a huge problem with my 2009 iMac's digital input that I didn't run into on my older MacBook running the same OS (10.6.x), and it's driving me nuts!

Basically, I'm trying to record a digital feed from an ADC hooked up between my hand-built tube amps/turntable and my iMac, with the goal being the transfer of some of my LP records to 48/24 LPCM for archiving and editing. The ADC's output rate is fixed at 48000Hz and cannot be changed. My iMac, however, refuses to lock on to the device rate, instead forcing 44.1kHz in the audio/midi setup. In fact, I can't get it to do anything but 44.1kHz on anything I plug in (even my DAC's passthrough) despite the fact that I need to use 48kHz. The audio is correct for a few seconds, then forces back to the wrong rate again. No matter how many times I try, the computer insists on using the wrong sampling rate, which has rendered impossible any hope of using my iMac as an audio recorder.

I've tried setting up an aggregate device, but to no avail. Nothing I do seems to work, and since the problem is with Core Audio, any program used to catch the audio records gibberish. Is there a way to force Core Audio to use 48kHz in the terminal? It's not like the computer can't do it, as I get about 2 or 3 seconds of correct audio when changing the sampling rate manually. I just can't get it to stick!

iMac 2.66GHz, Mac OS X (10.6.3)

Posted on Apr 7, 2010 8:24 PM

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Answer 1

_dk

Level 1

0 points

Apr 16, 2010 7:57 PM in response to italiano79

Seeing the exact same thing here. External digital source is 48kHz, and this Mac Pro Desktop will only stay on that setting very temporarily (for about .10 or .15 seconds) then it reverts back to 44.1kHz. Even if I don't have a signal or even a Toslink cable plugged in to the input.

(I'm surprised you are getting 2 or 3 seconds of correct audio, when I'm getting about 1/10th that before it reverts back to the wrong sample rate. Maybe related to a buffer size option somewhere?)

I have tried every known option to reset or re-configure this, and even to the point of creating a new user, resetting PRAM, resetting SMC, etc. I was going to try upgrading to 10.6, thinking maybe it was a 10.5.x problem, but now I'm not sure that will help.

Had I the choice to set the setting to be 48kHz and lock it in, it should stay on that setting - (even if the chance exists that what I'm getting is not correct.)

More often than not, I know what I'm doing more than Mac OS does, and I want to force it to do what it needs to.

There really needs to be a "do it because I told you so" option for most of the Mac audio configuration system. This automatic "the hardware decided to do something for you" stuff is absolutely horrid for a system that is marketed as "Pro". A real "Pro" system would let me edit the configuration once, with the tool (text editor hopefully) of my choice, rather than attempting (and failing) to automate things.

+1 for another instance of this being "a huge problem" and "it's driving me nuts!"

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Answer 2

_dk

Level 1

0 points

Apr 16, 2010 8:26 PM in response to _dk

Noticed the same (and unanswered) question in this thread: http://discussions.apple.com/thread.jspa?threadID=2193210

I'm worried that there was a run of bad hardware from late 2008 to early 2009 that affects Macbook, iMac, Mac Pro Desktops... and there wouldn't just be a software patch for this.

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Answer 3

italiano79 Author

Level 1

8 points

Apr 20, 2010 8:32 PM in response to _dk

I really wish Apple would address this issue, be it a patch or otherwise. Macs are a natural for all sorts of audio work, so it makes absolutely no sense that this problem would go unnoticed or unfixed.

Luckily, this has literally been the only problem I've ever had with my '09 iMac.

If anyone has a Windows (gasp!!) disc laying around, perhaps booting into XP or Windows 7 and attempting to record via the optical input might shed some more light on whether this is a software or hardware issue. I would imagine that one could use DirectX drivers instead of Apple's Bootcamp ones, which would rule out software problems. I'd try this, but I don't have a Windows disc laying around and am not going to install BootCamp (and subsequently, waste HDD space) just to play around with it.....

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Answer 4

May 10, 2010 7:56 PM in response to italiano79

Just out of curiosity, what software are you using to record?

I have an early 2009 MacPro, and am able to record at 48k via the built in optical with QT7Pro.

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Answer 5

italiano79 Author

Level 1

8 points

May 17, 2010 10:17 PM in response to Jeffrey Lee

I've tried QT, Audacity, and some cheap freeware audio recorder. All show the same result - slow, out-of sync audio with constant glitches and synchronization errors. QT won't record for more than a second, and it erases whatever it records ("the audio was interrupted...."). Still no luck in getting the Audio/Midi setup to do anything but 44.1kHz.

FWIW, I did finally install Windows Vista on the other half of the drive, and the RealTek audio console in Windows does the same thing - it locks to wrong 44.1kHz rate and lets you choose nothing else. I can't even get garbled audio in Audacity in Windows - just gibberish. Looks like this may be a hardware issue - aka, I'm screwed. So much for doing any recording on my iMac....

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Answer 6

Jul 7, 2010 4:35 AM in response to italiano79

I'm having a different but related problem with the digital output on my 27'' iMac. The optical out is running to the optical in on my DAW, but anytime there is no audio playing, the DAW says the signal being sent is 0Khz. Didn't have this problem on my old mini as you guys mentioned. Seems like if there is no signal being produced by the audio card, it extinguishes any kind of monitor signal for the DAW to keep track of. It works fine when audio is being played, but I have to re-sync my DAW every time I play audio from a different source even though they are all at 44.1Khz? That get's old!
I agree, seems like the older models don't have this problem, but my guess is it can't be solved with a software update as stated on a thread related to this one.

Common' Apple! We audio people want to believe you're the best. Prove it and fix this problem!

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