On Fri, 2025-04-11 at 22:30:30 +0300, Hans Summers via groups.io wrote:
So I'm not currently working on a simplified CW filter. But I will...
And I wouldn't say it's a difficult thing to do. Just that the
cascaded filter thing was almost trivially easy at the time Reg
suggested it to me, I got intrigued and thought "why not".
Hi all,
To explain my thoughts, I attach a small C-program that I am running on
my Linux PC. It takes the USB I and Q streams from the QMX and feeds
them through a CW filteri, mixes then with the sidetone, and then sends
them to the PC speaker. I have implemented four kind of lowpass filters:
a Gaussian, a Butterworth, a Chebyshev and an Elliptic filter, all of
order four (but any order will do).
The filters can be adjusted for bandwidth and center frequency. The
filters are defined in continuous time and then converted to discrete
time. This conversion, which is only done when changing the parameters,
involves the bandwidth setting. The adjustment of the center frequency
is implemented by a mixer. Since the filter works in the IQ domain, the
center frequency can be positive or negative.
The filters are implemented on a balanced state-space form, which avoids
many numerical problems, especially if the filter order is high. The
implementation runs at 48 kHz but this can be modified to 12 kHz for the
QMX.
Best regards
Anders
