Hi all,
In a recent post, Ramakrishnan, VU3RDD, referred to raised
cosine and root raised cosine filters. Yes those filters are
used on digital data systems, for minimum inter-symbol
interference a raised cosine filter is quite good and he
correctly says the system works best if the filtering is shared
between the Tx and Rx.
But is that any good for CW, are my ears raised cosine?
The ITU has a recommendation, which as an ex member I cannot now
access, on the occupied bandwidth of a transmission (AM = 2*Fmod
max etc) and covers data and CW. It is the shape of the rise
and fall of the envelope which determine the occupied bandwidth.
Pulses give a comb envelope with a frequency domain 'harmonic'
or spur at 1/t where t is the pulse width; on a (sinx/ x)
pattern spaced at 1/T where T is the pulse repetition rate.
However here the pulse width is better described as the rise
time rather than the dit rate. (yes I did mean dit (as opposed
to dah))
The compromise is adequate clarity between two pulses with a
dit interval and excessive key click. from memory the ITU rec
gave a figure between 100 and 200Hz for manual keying.
Suggestion, set up a perfect keyer, ie vertical sides, filter in
one of the digital audio filters, reducing the bandwidth
(removing key clicks) until copy is poor. Post the findings.
I'd do it myself but don't now have the bits.
Regards Alan G0HIQ.
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