[Discuss-gnuradio] Re: About Latency

[Eric Blossom]

On Thu, Oct 04, 2007 at 09:46:36PM +0800, KC Huang wrote:
> Hi Eric:
> 
> I have already read that paper before. As that paper mentions, the USB 
> delay is according to the equation
> 
> f(512, fusb_block_size*fusb_nblock)
> -------------------------------------
> fs*sample_size                                                     , which 
> f(x,y) is at least x and at most y
> 
> So, does that mean if u have large product of fusb_block_size and 
> fusb_nblock, then the theometical maximum delay will be increased? 
> Therefore, I think that the range of USB latency is also increased. Please 
> correct me if I am wrong. Thanks!
> 
> KC

Yes that is true.

If you're trying to minimize latency you want the smallest values that
work reliably (no over/underruns) and with acceptable overhead.  If
you enable real time scheduling, you can reliably use smaller values.

Try fusb_block_size 2048 and fusb_nblock 4 or 8.  You may be able to
run with fusb_block_size 1024.  It depends on your data rate across
the USB.

Eric