Date 26.1.2013 17:35, Jonas Petersen wrote:
Hi there,
I'm trying to get spdif capture to work properly on the M-Audio Audiophile 192 (VT1724/Envy24HT). It is generally working, but I don't get a clean signal.
I have "Multi Track Internal Clock" set to "IEC958 In" (spdif in). When I capture via Jack, arecord or Audacity I experience the following two phenomenons (always):
1.) The captured signal ends up 6 dB(FS) to loud (200%)! Everything above -6 dB is distorting. A 1 kHz sine test signal at -12 dB (25%) will be captured as -6 dB (50%).
2.) The left and right channels are shifted by one sample! When I feed a 1 kHz test signal (L+R are _exactly_ the same signal), the right channel will be offset by exactly one sample. Zooming into the waveform clearly shows that. Analyzing the signal with a goniometer shows a (slight but obvious) vertical eliptical shape and not the expected single vertical line.
To make sure the hardware actually works fine, I did install a Windows 7 on the exact same machine, installed the drivers from M-Audio and did some recording with audacity. The result is as expected: the -12 dB signal ends up as -12 dB and the left and right channel exactly match each other. So the hardware is willing.
I am running a Lubuntu 12.10 and I'm am able to compile and run the current alsa-driver source with the 3.5.0-22 kernel. I played around a bit with the alsa driver source (e.g. pci/ice1712/ice1724.c, pci/ice1712/revo.c) and I am able to compile and load a modified driver. So far I only was able to make the problem worse though.
I'd now like to ask for some advice on how to approach the problem. I guess the fact that the left and right channel differ - even though they shouldn't - might be a thing to look for. This must be happening at some stage in the capturing. Is there a way to hook in at different places to narrow down what causes this?
Maybe this even already rings somebody's bells?
I'll be glad to deliver more information when needed.
I believe that there must be a S/PDIF receiver IC somewhere on the board and this IC may be wrongly configured. This IC is not handled in the current ALSA driver at all. Could you look to your board and check the used chips? From pictures on internet, a suspicious IC is in the middle of top on this PCI card. The AKM IC's are for analog audio outputs.
The ICE1724 chip has only serial SPDIF input and SPDIF input clock pins. The input samples should be copied to the DMA without any modifications (no volume control etc.).
Jaroslav