Re: [Discuss-gnuradio] FM pre-emphasis and de-emphasis

[Andy Walls]

> Date: Tue, 9 Jan 2018 00:32:41 +0000
> From: Richard Lamont

[snip]

> Fair enough. I've just started playing with grc and did the usual FM
> broadcast RX thing as my Hello World - and, as a former broadcast
> engineer with pretty good ears - I immediately noticed that there was
> far too much 'top' in the the audio. I have been using the FM
> de-emphasis block that is shipped in the Ubuntu gnuradio 3.7.9
> package -
> I have added any others yet.

[snip]

> When working correctly, the amplitude/frequency response should be
> within, ideally, 0.1 dB of the values shown. I've shown values for
> both
> 75 and 50 microsecond time constants.
> 
> Freq (Hz)     Gain dB (75us)     Gain dB (50us)
> 
> 30             0.00                0.00
> 300           -0.09               -0.04
> 500           -0.23               -0.11
> 1000          -0.87               -0.41
> 3000          -4.77               -2.76
> 5000          -8.16               -5.40
> 8000         -11.82               -8.64
> 10000        -13.65              -10.36
> 12500        -15.52              -12.15
> 15000        -17.07              -13.65
> 

FWIW, the attached plots compare the 50 us de-emphasis filter response
of GNURadio 3.7.10 vs. the numbers you provided.

The filters stay within 0.1 dB of each other up through 5000 Hz, but by
then your tabular response has noticeably started to bend upward.  The
difference between the two filters is 3.44 dB at 15000 Hz.

So hopefully you won't hear to much 'top' in your audio using 3.7.10's
filter.

Regards,
Andy

Difference_of_filter_responses.png
Description: PNG image

Filter_response_curves.png
Description: PNG image