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Re: [Discuss-gnuradio] Off-line signal processing (GR in slow motion)

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Off-line signal processing (GR in slow motion)
Date: Thu, 29 Aug 2013 12:03:05 -0400

On Thu, Aug 29, 2013 at 11:53 AM, Monahan-Mitchell, Tim
<address@hidden> wrote:
>>> Off-line example (3 separate flowgraphs, run one at a time):
>>> 1. USRP --> File
>>> 2. File --> AM Radio Demodulation, produces audio data --> File
>>> 3. File --> Throttle @ audio rate --> Audio Sink
> As Tim O pointed out, #3 would just be File --> Audio Sink, since the Audio 
> Sink consumes at a fixed rate.
>>> Note that #2 can take as long as needed to complete, no issues with loss of 
>>> data.
>>> If so, how would I set up flowgraph #2 above?
>> Hm, I'm not quite sure if I understand your problem. You can simply connect 
>> a file source to the am demod and then to a file sink.
> Yes, #2 was meant to depict whatever processing blocks are needed to go from 
> RF samples to audio samples.
>>> This flowgraph will then run unlimited ('cause no throttle). But that's OK 
>>> if the input file is limited in size. Your CPU will be running full power 
>>> until there's nothing left to process.
> Yes, I understand that part. What I was not sure about was whether the real 
> time rate of data passing through the system was required to be able to do 
> proper signal processing? Or whether some of the processing blocks are simply 
> told what the rate is?
> Here is another example: suppose I have a file of audio samples. To determine 
> the spectrum of the data , isn't the sampling rate of the data a factor? Or 
> must a scalar be applied somewhere (related to the sampling rate) to convert 
> relative values to energy per Hz?
> If the latter is true, then I'm still unclear on whether something like the 
> rx_ofdm example under gr-digital can work, just given a file of samples and 
> no throttle block (CPU limitations aside)?

GNU Radio doesn't care about sampling rate. It just runs data through
as fast as possible.

Those blocks that accept a sampling rate is for convenience only. In
the end, everything is operated off a normalized sampling rate. So
when you have a signal source with sampling rate 32k and a frequency
of 1k, the math that goes into generating that sine wave is really
working as 1000/32000. We allow you to state the sampling rate in
blocks like this because it allows us to think more naturally about
the frequency without doing the normalization process ourselves. But
in the end, it just how fast it's turning around the unit circle every

So no, running without throttle has no difference on the signal
processing that's happening.

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