Re: Alternative(s) to 200 Hz CW Filter

Roelof Bakker

Hello all,

- Yes, I realise that, but old style CW ops we are used to wide band RX and doing ALL the
filtering in our ears/brain

This is an interesting discussion.

The human brain can simulate a bandfilter with a width of 50 Hz, which makes it possible to
select a single signal from many, received with a receiver with a wide filter. The brain needs
a reference, which can be provided by the band noise on a quiet band. This calls for a wide
filter setting. What also works is adding a little white noise when using a narrow filter.

One can combine the output of a 2.4 kHz filter in the audio chain at e.g. a 20 dB lower level
with the output of a 200 Hz filter. This way you can still hear what is happening around the
channel you listen to. In the QCX this can be done by by-passing the 200 Hz filter at a
reduced level.

Besides providing selectivity on a crowded band, narrow filters can help in reducing local
noise interference. The noise power in a 50 Hz wide filter is 10 times lower, that is 10 dB,
than the noise received with a 500 Hz filter. Unless you are lucky, these days most people are
suffering from local noise ingress on 160 and 80 meter. Decreasing the bandwidth might be more
effective than using a noise blanker.

Receiving Morse code is an excellent opportunity to experiment with narrow audio filters.
Using an on-line filter design tool from Texas Instruments, I have build filters 10 Hz wide
that don't ring. This is wide enough for keying speeds up to 12 WMP. A 20 Hz filter is good
enough for normal CW speeds.

The first time I listened with these narrow filters, it was rather unpleasant. However after a
couple of hours my brain had already adopted itself. I am using a centre frequency of about
500 Hz. Even using these narrow filters the perceived pitch can be changed by changing the
frequency setting of the reciever a couple of Hz.

Roelof Bakker, pa0rdt

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