Re: [Discuss-gnuradio] Throttle block form wav files

[Kevin Reid]

On Sep 3, 2015, at 8:24, Murray Thomson <address@hidden> wrote:

> Even selecting source and sink from the audio card, if I have a wav file 
> playing in another input of the selector without a throttle, the CPU goes to 
> 100%. To avoid it I put the wav file, then the throttle and the the selector. 
> I've read that is bad practice but I couldn't reduce the CPU usage in any 
> other way.

(Please make sure to CC the mailing list in your replies.)

Ah. I just looked at the implementation of the selector block, and found that 
its documentation lies. It does not leave its unused inputs “disconnected” 
(which is actually good, because that would cause errors in some cases), but 
connects them to null sinks (which consume all the input they're given).

So, using a throttle is a workaround for this. (The cost is that since the 
throttle's timing is based on the CPU clock, not the audio card clock, you'll 
occasionally get either underrun or overrun.)

Here are some better solutions, from simplest to best:

1. Using some method to force the wav source and audio source to match sample 
rates. Specifically, you could use a “Multiply by Matrix” block to replace the 
function of the Selector entirely: give it a matrix value of either ((1, 0),) 
or ((0, 1),) to select just one input.

2. Modifying, or creating a replacement for, the selector block which does not 
connect to a null sink but rather to a block of some sort which never accepts 
any data and so stops that branch of the flowgraph. (I don't think such a block 
actually exists, but it could, and there might be something else that acts like 
it.)

3. Actually remove from the flowgraph whichever of the wav source and audio 
source are not currently in use. This is the most general, efficient, and 
straightforward solution, but unfortunately it cannot be built within GNU Radio 
Companion -- you must write some C++ or Python code. If you don't want to stop 
using GRC entirely, you could isolate that part by putting the connection 
changing logic inside a hierarchical block; this would be similar to the 
selector block itself, but it would be a source, which has the wav and audio 
sources "built into" it. Alternatively, you could write your main()/top block 
as a Python program and put the _rest_ of your logic (the demodulator and sink) 
into a GRC block which is used by the Python program. Either way works.

-- 
Kevin Reid                                  <http://switchb.org/kpreid/>