Vortexbox / Linux USB Audio Samplerate Problem

[jmag99]

I am having a problem using the xmos reference board with a Vortexbox (Fedora). The Vortexbox is using the native USB Audio 2.0 driver. The problem is that every time I start playing a track, the I2S output on the xmos temporarily changes sample rates and causes the downstream DAC to spit out some bad samples. The attached scope plot shows the LRCLK on the top and the SCLK on the bottom. The plot shows the start of a new track.

When playing a 44.1 track, it jumps to 88.2.
When playing a 48 track, it jumps to 192.
When playing a 88.2 track, it does not jump.
When playing a 96 track, it jumps to 192.
When playing a 192 track, it does not jump.

Also strange is that the LRCLK stops one bit short of 32 bits, even though the incorrect clock goes 32 times. This tells me that one full sample is being output (the last bit in I2S is the first bit in the next channel).

I am trying to determine if this is a linux driver problem, the linux playback application problem, or a firmware problem in the xmos. Any thoughts for how I can debug this?

BTW, the same board works fine and does not do this using windows or mac.

[Ross]

The software (unmodified) would never change frequency without first changing the codec setup. Do you have a USB trace?