Mac Pro optical in problem - stuck 44.1 + low pitch

Audacity Help Forum macOS
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Hi, Sound is like transformed to low frequences and is very murky and distorted when I’m trying to record a voice with the optical digital in on my Mac Pro 2012 using Toslink.

I tried the sound/Midi settings app to configure my ports. However, the optical digital in seems to be stuck at 44.1 kHz sampling. After a change to 48 or 96 kHz, it resets itself to 44.1 kHz within a few seconds no matter what I do!

I use a small RCA to Toslink adapter to convert mic amplifier to digital opto. If I plug out the Toslink and plug it in again – the first 2 or 3 seconds will make a normal sound then it always distorts to low pitch and murky.

If I plug the adapter into a MacBook Pro (with a Toslink) 3.5 mm adapter the optical sound works great!

Could it be a fault in the Mac Pro optical digital input?
Why isn’t it possible to select 48 kHz sampling for the optical in?

Thankful for any ideas about this.

Johan

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I would have expected 48KHz to be the default. DAT and other digital machines all run at 48KHz Television sample rate.

What are the two Mac versions? Both running Yosemite?

Koz

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Does it help if you change the Audacity default sample rate to 48,000?

Audacity > Edit > Preferences > Quality: Default Sample Rate.

Koz

4

44.1 KHz is default. Providing the Mac Pro’s port isn’t able to sync, it would stay at that sample rate. This points to a dirty optical port in your Mac Pro. You can clean those with isopropyl alcohol. You need a tiny cotton swab, around 2 mm to get into the 3,5 mm connection. And you need to take care, as these damage easily.

There’s a tiny switch in the back of these connectors and these get stuck sometimes if not used for a long time. That switch switches from analog to optical digital.

5

Hi Koz,
Both run Yosemite 10.10.5.
Problem is the Mac Pro Audacity recording through Toslink sounds like a tape recorder at half speed; slow and low pitch! What can cause this? (Analog Line in signal sounds normal.)

The same Toslink signal sent to my MacBook Pro sounds OK. The MacBook Pro is possible to switch with the Sound/MIDI application from 44.1 to 48 or 96 kHz sampling. However, it automatically changes to 44.1 when the A/D converter is connected. It tells me the converter output is 44.1 kHz. Change of Audacity from 44.1 to 48 kHz makes no obvious difference.

(BTW, the A/D-converter is a “goobay” AVW 8, article no. 31965. The vendor says it can do 32; 44.1; 48 and 96 kHz sampling. However, how is this set? There is no switch and no software … And i assume the Toslink is not biderectional and handshaking about this but I might be wrong here.) I’m really confused.

6

The only page I found for the Goobay doesn’t seem to agree with multiple sample rates:

supports output sampling rate set at 48 kHz

http://www.serigo.eu/avw8.html

which is what I’d expect from Toslink. It’s uncompressed, 2 channels, 48 KHz or compressed 6 channels Dolby AC. AFAIK it can’t do anything else, certainly not 96 KHz. And that’s hardware, not software, eg no settings…

7

I did some more experments and got very confused now. Also tried a brand new 2 m Toslink cable without any difference. Also tried to clean the opto input with isopropanol.

  1. I can play an audio file on the Mac Pro, send it through Toslink to the MacBook Pro and record it in Audacity. Works well with 44.1; 48 or 96 kHz. Sound/MIDI is possible to change to any sample frequency.

  2. The other way round does not work. The Mac Pro only hears a burst of pulses repeating with about 5 Hz. The sound/MIDI refuses to stay at 48 or 96 kHz even if the MacBook Pro outputs those. Input resets itself to 44.1 kHz after a few seconds.

  3. If i connect the Toslink cable from out to in at the Mac Pro, I can output a Spotify stream to digital out, then record it in Audacity without problems at any sample rate! So the input and cable seem to work. But the input doesn’t work if I take the audio from the MacBook Pro nor from the A/D-converter!

Does anybody have an idea what is going on? Regards Johan :question:

8

After reading this, that makes two of us :unamused:

Also tried a brand new 2 m Toslink cable without any difference. Also tried to clean the opto input with isopropanol.

  1. I can play an audio file on the Mac Pro, send it through Toslink to the MacBook Pro and record it in Audacity. Works well with 44.1; 48 or 96 kHz. Sound/MIDI is possible to change to any sample frequency.

That’s Core audio in OSX resampling automagically.

  1. The other way round does not work. The Mac Pro only hears a burst of pulses repeating with about 5 Hz. The sound/MIDI refuses to stay at 48 or 96 kHz even if the MacBook Pro outputs those. Input resets itself to 44.1 kHz after a few seconds.

Sounds like the Mac Pro isn’t able to extract the clock?

  1. If i connect the Toslink cable from out to in at the Mac Pro, I can output a Spotify stream to digital out, then record it in Audacity without problems at any sample rate! So the input and cable seem to work. But the input doesn’t work if I take the audio from the MacBook Pro nor from the A/D-converter![/quote]

What version of OSX is on this Mac Pro? And which Mac Pro are we talking about?

Does anybody have an idea what is going on? Regards Johan > :question:

I’ve never seen this behaviour before. I’m afraid most Mac users don’t use the optical ports. I’m even pretty sure a lot don’t even know it’s there…

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Hi cyrano! Thank you for spending your time and knowledge.

The MacPro is a mid 2012, 3.2 GHz Quad-Core Intel Xeon running 10.10.5.

Yeah, I also found goobay specifies 48 kHz on the net. However, vendor says 32, 44.1, 48 and 96 kHz sample frequencys. I have contacted the vendor with no answer so far. Maybe it is a copy and paste error because they also sell the D/A adaptor and it might work with different sampling frequences in … Then they might have copied that info for the A/D converter description … But strange the working MacBook Pro autoselects 44.1 as soon as I plug in the goobay Toslink cable.

Could the problem be something with the clock sync? MacBook Pro Audio/Midi app says (translated from Swedish): Built-in digital input – Clock source: Preselected.
But the Mac Pro says: Built-in digital input – Clock source: Locked! Why a difference?

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We’re all just learning here…

The MacPro is a mid 2012, 3.2 GHz Quad-Core Intel Xeon running 10.10.5.

Thanks. This machine is not on my “problem machine list”.

Yeah, I also found goobay specifies 48 kHz on the net. However, vendor says 32, 44.1, 48 and 96 kHz sample frequencys. I have contacted the vendor with no answer so far. Maybe it is a copy and paste error because they also sell the D/A adaptor and it might work with different sampling frequences in … Then they might have copied that info for the A/D converter description …

Knowing the Chinese and their habit of citing every number available (or nothing at all), I suspect copy-and-paste at work.

But strange the working MacBook Pro autoselects 44.1 as soon as I plug in the goobay Toslink cable.

That’s what I don’t understand. Toslink = 48 KHz. But it works on the MB Pro and not on the Mac Pro.

Could the problem be something with the clock sync? MacBook Pro Audio/Midi app says (translated from Swedish): Built-in digital input – Clock source: Preselected.
But the Mac Pro says: Built-in digital input – Clock source: Locked! Why a difference?

That’s what it’s supposed to tell you if it’s working. Only, it’s not :unamused:

According to Apple’s docs, when the optical port locks, it works. If it doesn’t lock, it’ll default back to analog, 44,1 KHz.

Have you tried a PRAM reset on the Mac Pro?

Have you tried removing com.apple.audio.AudioMIDISetup.plist?
It’s in your user folder, under Library/Preferences.

Other than that, I can’t think of any suggestions.