[op25-dev] Re: Sample rate?

md123 at nycap.rr.com md123 at nycap.rr.com
Thu Mar 14 03:01:52 UTC 2013

---- ikj1234i <ikj1234i at yahoo.com> wrote: 
> As to the specific question about throttling - having a throttle block in a flow graph should /only/ be done if there is no hardware source or sink (usrp or soundcard).  If the throttle block is in the flow graph at the same time as a hardware block you have the two-clocks problem.
> As for optimal sample rates using the USRP the slowest rate supported by the hardware (i.e., the highest decimation rate) is still more than enough for recording/capture of P25.  2M is overkill unless you're trying to capture an entire band 2 MHz wide.
> In playing back a file and displaying the result in a GUI is the case where you need a software throttle...
> Max
Hi.  I'm sorry I didn't specify that I am using the e4k USB tuner and not yet the UHD.  Yes I am a little confused here because when I play back the capture I need to throttle back even though there is an audio sink in the flow graph.  I was under the impression that the the audio sink (at 44.1) would limit how fast the sample plays.  But no, it does not, without the throttle the gui immediately greys out. Is it possible that this has something to do with the fact that I get no audio with OP25?. I have to throttle it back to around 56k or 64k to be able to to play the sample without it greying out.  However, when I make the capture I'm using a sample rate of 1.2288M (gui does not go grey) because using RTL-FM I can hear that it takes a sample rate in this neighborhood until the audio coming from the speakers sounds like clear digital transmission.  Should I capture the signal at a lower sample rate?  I would think that the lower the rate the better right? if so then i would like to minimize the sample rate subject to the constraint that OP25 be able decode the captured signal.  
How will a high capture sample rate and a low replay sample rate effect the play back?  I am still having trouble getting the four lines in dbits but sometimes I can make them out.  The fsk4 line seems to get tight and jump down when the carrier wave pops up. Like I said, I have yet to hear audio from OP25 so I have to watch the carrier wave to know when the transmission is taking place.  So the sample rate is set at 1.2288M for the capture and 56k for the replay, decimation at 32.  Not sure if these variables are set correctly. Or is there some other variable that I can change?  I don't know what I am doing wrong with respect to the audio(or lack thereof).
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