Audacity Forum

Hi,
I am a newbie to this and I thank you in advance for your help.
I have played around with audacity today and managed to record a track being fed into my Macbook Pro via the optical/toslink digital input.
My question is how do I determine the native bit rate of the incoming LPCM bit stream? Ideally, to preserve bit accuracy in the digital domain, I would
like the sampling rate as well as the output bit width to be identical to the incoming; well at least thats what I am thinking. For example, if the source
of the LPCM bit stream is 24bit/88.2KHz, then I would like my ourput wave file to be also 24bit/88.2Khz without compression.

Go to Applications / Utilities and open the application Audio Midi Setup. in the Audio Devices tab select Properties for: Built-in Audio. Set Source to Digital In. You can then set the bitrate and depth beside "Format:".

I notice you have posted in the 1.2.x forum. You should consider using 1.3.12 instead. 1.2.6 is no longer being developed, and has issues with OS X 10.5 and 10.6.

In any case, you need to go to Audacity > Preferences and set Audacity to match your digital input settings. In 1.2.6 and 1.3.12 the appropriate setting will be found under the "Quality" tab.

-- Bill

Thanks for the reply, and for pointing out that I posted in the wrong forum. You are right, I am actually running version 1.3.12 beta (Unicode). But another quick question.

I know the digital data stream feeding thru' the optical spdif is LPCM 24bit/88.2kHz. But Midi setup does not have native support 88.2kHz sampling rate. If I choose 24bit/96KHz as the format for built-in input, does it mean Midi will re-sample/up-sample the LPCM data stream to 96kHz before audacity can see it?

Is there anything that I can do or any application that I can use to bypass Midi so that LPCM data stream coming in on the optical spdif gets routed to Audacity to preserve bit accuracy?

IS there a way that I can transfer this thread to the 1.3.x forum?

moved for you

Audio Midi Setup doesn't do anything other than tell you what interfaces are available and what settings are available. AFAIK it doesn't actually do any work - I think that is controlled by the hardware and low-level drivers.

How do you know the bit rate is 88.2 kHz? Is that the spec of the device you are connecting to the optical port? Do you know for sure that it is S/PDIF?

<<If I choose 24bit/96KHz as the format for built-in input, does it mean Midi will re-sample/up-sample the LPCM data stream to 96kHz before audacity can see it?>>

That I don't know Sounds like a question for Apple support.

But if you have successfully made a recording using the digital input you are obviously doing something right. What were your settings in Audacity > Preferences > Quality and in Audio Midi Setup?

-- Bill

Thanks for helping me move this thread to the right forum.

Bill,

Thanks for the response again. I have since experimented more.

Yes, I am quite sure it is optical spdif that I am feeding thru the mini toslink into the digital in on the Macbook pro.
I have experimented more by configuring the source of the LPCM data stream differently. The format on Midi Player for
Line In actually follows the format of the incoming LPCM data stream. At least that is true for LPCM 24bit/44.1kHz
as well as 24bit/48kHz. Midi player does not allow me to change format settings when there is a LPCM data stream.
For example, when it is LPCM 24bit/48kHhz, it does not allow me to pick 96kHz sample rate, Midi player very quickly reverts
back to 48kHz. My rookie interpretation from this observation seems to suggest Midi does no upsampling/downsampling business
but to just follow and report the format of the incoming LPCM data stream if it is 44.1kHz or 48kHz.

In both cases (LPCM 24bit/44,1kHz and 24bit/48kHz), Audacity sees the LPCM when I configure the preference to input to
built in. And I can change the recording sampling frequency as well as bit depth to anything I want. And when I export it into
uncompressed AIFF, I can specify the sampling frequency and bit depth again in the export parameters.

Yes, I have the open question now and I will try to post it in Apple forum. What happens when the LPCM data stream at the
digital in input is not 44.1/48/96kHz, like 88.2kHz? Can Midi or the macbook hardware handle that?

I am quite excited as this might be a method for me to perform some hi-res music rip, like from sacd or blue-ray music.

This is from Apple Support.
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Optical audio input is SPDIF format and uses a standard toslink cable with a toslink mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.
==================

WikiPedia.
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S/PDIF is used to transmit digital signals of a number of formats, the most common being the 48 kHz sample rate format (used in DAT) and the 44.1 kHz format, used in CD audio. In order to support both systems, as well as others that might be needed, the format has no defined data rate. Instead, the data is sent using biphase mark code, which has either one or two transitions for every bit, allowing the original word clock to be extracted from the signal itself.
===============

And less desirable...
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SCMS copy protection looks for bits written in the subcode data in a digital link. There are three states of these bits: copy allowed (00), copy once (11) and copy prohibited (10). If the source has the copy bits 00, and a copy is made of the source, the copy itself will have the bit set as 00 as well, allowing copies to be made from copies. If the source has the copy bits set as 11, every copy of this material will have the bits set to 10, and making a copy from the copy would be prohibited.
==============

Koz