Re: [Discuss-gnuradio] Capturing maximum complex value

[John Ackermann N8UR]

On 3/17/19 3:01 PM, Kevin Reid wrote:

> One of the AGC blocks that has separate attack and decay rates (agc2_cc 
> or agc3_cc) could be used to do this: set attack to 1, decay to 0, 
> ignore the output, and read (1.0 / agc.gain()) to find the peak-hold 
> magnitude.
>
> In my own application, I am using this chain for peak detection 
> /without/ hold:
>      blocks.complex_to_mag_squared(),
>      blocks.stream_to_vector(itemsize=gr.sizeof_float, 
> nitems_per_block=window),
>      blocks.max_ff(window),
> blocks.probe_signal_f()
> but you would need to separately poll (at the sample rate divided by the 
> window length) and max-hold this to catch long-term peaks. On the 
> upside, as I just found out, it uses less CPU time than running the AGC 
> (on my processor).

This is where I get myself confused -- to get a magnitude do I need to 
do an FFT, or can I just look at the raw complex stream, or the output 
of complex_to_mag_squared without an FFT?  I've used something like the 
blocks you describe to capture the max within each FFT frame, but what 
I'm looking for now is any instant where I get clipping, no matter how 
brief.

> If the only thing that affects the peak/gain is the recording, perhaps 
> you could measure the peak in the file offline (that is, separately from 
> running your real-time flowgraph) and use that to set the gain, instead 
> of having to interactively monitor anything.

That's an idea.  Thanks!