Re: [Discuss-gnuradio] Latency on TX for low data rate systems

[Eric Blossom]

On Mon, Dec 03, 2007 at 11:31:03AM -0800, Eugene Grayver wrote:
> Hello,
> 
> I have a very low data rate FSK system (<100 bps).  The problem is the 
> enormous latency introduced by the inter-block buffers.  (I am running in 
> ms windows).  The buffer granularity is apparently 64kB.  I 
> upsample/interpolate the data to around 500 kHz before sending it to 
> hardware.  The buffers get filled up on startup from the data source, and 
> any new data takes forever to come out. (64k/100 = 600sec!)  Is there a 
> way to make the inter-block buffers MUCH smaller?  Or, at least make them 
> different size depending on the actual sample rate?  What other options 
> can anyone propose?

Under Linux (and most other systems) you can reduce the buffering
down to 4KB (the page size).  The windows API we're using only allows
us to map an area of memory on a 64KB boundary, thus the high amount
of buffering.

However, you may want to reduce the rate at which you're generating
data.  From your description, it sounds like you have an unthrottled
source feeding a small pipe.  Are you transmitting continously or are
there bursts/packets/frames of bits?

Eric