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

[Richard Lamont]

On 10/01/18 18:53, Andy Walls wrote:

[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.

FWIW they are not my numbers, but appeared to come from a reliable source.

> 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.

It bends up because your plot has got one log axis and one linear one.

If plotted with a log frequency axis rather than a linear one then the
slope at the right should tend to a straight line of 6.02 dB/octave or
20 dB/decade. Indeed the idea of dB/octave or dB/decade only makes sense
if you use a log frequency axis.

Alternatively, you could use a linear voltage axis instead of dB, and a
linear frequency axis. That should also tend towards a straight line on
the right hand side.

-- 
Richard Lamont