USB audio device

I have a program that reads samples from usb audio device (ie /dev/dsp1).
Therefor the program sets the sample rate. This all works fine only if the
sample rate 11025 or greater! For lesser sample rates I read not enogh
samples per sec, about a half. But the sample rate for my work is 8000. 
The sound device is not the problem, on a linux pc this all works fine.
Has any body an idea?

Thanks
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