Overload on S/PDIF input

[ianrt]

I am using 1.3.5-beta on Ubuntu 8.10 with an M-Audio Audiophile 192.
When I record a 24bit/88.2k stream from the S/PDIF input I am getting a large percentage of the waveform clipping - as if the record level is too high on an analogue recording! The quality of the recording in the low regions is fine and there are no other artifacts, just the clipping
But this is a digital signal - how can I get an overload?
Is there an issue with 24 bits, perhaps?

Ian

[steve]

But this is a digital signal - how can I get an overload?

What are you using as the input into the sound card?

[ianrt]

The input is a PCM stream from another device into the S/PDIF port of the sound card.
I can monitor that stream through a DAC so I know the data is OK (and as I said there is nothing wrong the audio at lower levels)
When recording I noticed that the level meters are both red permanently
Also I have checked that Audacity is correctly configured for 24bit/88200 data.
Looking at the results it is like the top one or two bits have been lost and the remaining data shifted up by 6 or 12 dB

[steve]

Are you certain that both pieces of hardware support 24 bit data? (SPDIF is usually 20 bit data, sometimes less).

[ianrt]

Pretty sure.
I am using the audiophile192 to successfully output 24bit/96k flac files (from Linn records) and it is supposed to be able to handle 24/192k in and out.
At the other end the system has transferred up to 24/192k from a DVD-Audio disc, so that tells me it (and the cables) can do 24/88.2k.
I have just been looking at a file I recorded on this system which happened to be mastered at really low levels (I have to turn the gain right up on replay) and it is OK, but now has peaks approaching full scale. So it still looks like something (the audiophile192, the ALSA driver or Audacity) has fiddled with the data to make it look like a gain change. Anything at higher levels runs into clipping!!
The confusing part is I can't imagine why any of this software would do this so I assume it is something in my setup but I can't find anything.

[ianrt]

Further info:

I rebooted the PC into Windows (XP pro) downloaded Audacity 1.3.7-beta and tried recording again.
No problem with overload!!

So, it appears to be the ALSA driver that is causing the problem (unless there is a huge difference between 1.3.5 and 1.3.7 betas)

[steve]

(unless there is a huge difference between 1.3.5 and 1.3.7 betas)

There have been some significant improvements for handling newer hardware, and there is still work going on in 1.3.8 alpha toward getting portaudio to correctly identify bit depths over 16 bit (which from what I hear seems to be a weakness in portaudio).