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

[Andy Walls]

From:   Richard Lamont
Date:   Wed, 10 Jan 2018 20:41:06 +0000

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

[snip]

> > 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.

Ah yes, of course.

And on that note, the bilinear transform design method warps the
GNURadio FM de-emphasis filter line downward on the log-log graph,
which is expected.

Regards,
Andy

> 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.

Filter_responses_log_log.png
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