Wav File Source and Audio Sink

Hi all,

I am little bit puzzled by a simple flowgraph.
I have prepared a wav file using following graph: Signal Source (float,
sine, 1khz freq, sample rate 8000) -> Throttle -> Wav File Sink (sample
rate 8000, 8 bit per sym).

Then i created graph for listening: Wav file source -> Audio Sink
(sample rate 8000) + FFT Sink.
And then i ran this flowgraph, i faced with following problem: I don’t
hear anything from audio sink.

FFT sink shows me that data is coming, 1 khz peak is present on FFT, but
i can’t hear it.

Could somebody explain this strange behaviour.

Best regards,
Igor

Dear Igor,

can’t point this out of enough:

Signal Source (float, sine, 1khz freq, sample rate 8000) -> Throttle
-> Wav File Sink (sample rate 8000, 8 bit per sym).
Never, ever use a throttle block in a flowgraph where hardware dictates
the physical sampling rate.

Your GNU Radio companion should even have warned you about that, if your
GNU Radio version is rather recent:

Generating: “/home/igor/top_block.py”

Warning: This flow graph contains a throttle block and another
rate limiting block, e.g. a hardware source or sink. This is usually
undesired. Consider removing the throttle block.
If you’re not seeing aUaU… on the console, you’re lucky. Usually, you
would have built a system with two competing clocks. One is bound to
be a little slower than the other, and if it’s the throttle, your audio
sink would have no samples to play when it needs them.

I do, however, suspect that your sound system doesn’t support a 8kHz
sampling rate. Try 44.1kHz, as it’s supported by most systems.
If that doesn’t help: try explicitely setting the ALSA device to use;
“aplay -L” will usually list all available output devices.

Also, try going down with the amplitude a bit. Typically, 0.4 as
amplitude is enough to drive sound cards “loud”, and higher values often
result in clipping.

Best regards, and happy hacking,

Marcus

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