Thank you very much for your suggestions, Brian. I totally agree with your
points. While I understand the IEEE754-compliant float representation and the
fact that 24-bit integer samples can be embedded in a lossless fashion in
32-bit float, the application I write really needs to be able to capture entire
32-bits intact. It is supposed to be a proprietary app bundled with high-end
ADC coming from one of the leading manufacturers. It does not need to work with
any audio device but the one it was designed for. Among requirements, some
point down the road we need to be able to record DSD stream (embedded in the
32-bit integers, all bits meaningful - sort of a non-audio data you mentioned,
non-PCM in this case which makes your point valid), so it looks like I really
need to pursue that 32-bit integer path.
Hubert Pietrzykowski
On Wednesday, May 31, 2017 7:34 PM, Brian Willoughby
<[email protected]> wrote:
I understand the need for lossless audio, but you don't need to change the
device format to preserve the original data. The maximum uncompressed audio bit
depth is 24-bit, and you can convert the 32-bit float to that without any loss
of resolution. You just have to be very careful of matching the sample rate
when using an AUConverter. If you handle the float to integer conversion in
another way, then you might not have to worry about sample rate.
If you're dealing with non-audio data, or compressed data, then that's a
totally different question.
As for why your code is not working, it could be that the device driver doesn't
really support what you're doing, or that you're accessing the wrong scope or
device. It also depends upon which method you're using to open the input device
as to whether there is a format converter in the middle.
At the very least, if you continue to pursue 32-bit integer input, your program
will not work with all audio input devices. It would be far more successful to
deal with the canonical 32-bit float input and use lossless conversion
techniques within your app.
I've written plenty of CoreAudio code that is totally lossless with regard to
passing pure 24-bit integer audio samples from input to output, all while using
32-bit float. It does take significant attention to detail, and it really helps
to have some sort of bit meter on the output or other analysis tools to verify
that there hasn't been an accidental loss.
Brian Willoughby
Sound Consulting
On May 31, 2017, at 7:17 AM, Hubert Pietrzykowski wrote:
> New to this list, so let me introduce myself. I've done a lot of freelancer
> DSP algorithm design and programming over the last 15 years (including
> numerous commercial products). Haven't used CoreAudio too much, one or two
> simple iOS projects using Remote IO.
> Now I face the problem of implementing something more challenging - I need to
> get the 32-bit integer data from the ADC to the OSX app (CoreAudio-based)
> without any conversion or any other kind of processing. Seemed easy but I was
> too optimistic, apparently. No matter how hard I try I cannot make it work. I
> can successfully enable the hog mode on the device. Once this is done I
> inspect the input stream format of the device, which is 32-bit floating
> point. This alone is a little strange, because Audio MIDI Setup says the
> device input is 32-bit integer (which is exactly what I need). So, the next
> step I try is setting the stream format (by setting corresponding property).
> The supported stream formats, obtained from the device by means of
> kAudioDevicePropertyStreamFormats includes integer streams. To my surprise
> the stream format does not change when I attempt to set it to the desired
> spec by calling AudioObjectSetPropertyData with
> kAudioDevicePropertyStreamFormat selector. The function does not return any
> error. It's just the stream that remains like it was before. To be more
> precise - the fields like sample rate do actually change. It's just the flags
> that always remain 32-bit floating point (value of 9) not matter what I do.
> Anyone has ideas? Is it possible? I heard the integer mode has been
> re-enabled in OSX starting from 10.9 if I recollect correctly, so there
> should be a way. Any help will be greatly appreciated.
>
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