[RTTY] PSK31 Clueless?/PSK31 Info & Clubs

[Kok Chen]

On Jan 19, 2005, at 11:25 AM, ws7i wrote:
> Raise your hand if you have operated a narrow-band mode, PSK31, which 
> is very very narrow, with 2.4 Khz filters and then complained about a 
> loud signal that was over 2.0 Khz away from your signal.

Actually, wide band PSK31 operation is not a significant problem if 
your audio chain can take the A/D converters with over 100 dB dynamic 
range, _and_ you make sure the audio levels are adjusted so the most 
significant bit of the A/D converter is never, ever exceeded.

A lot of the problems that people encounter with using wide IF filters 
to receive PSK31 are due their own A/D converter saturating (it also 
cause them to give out completely bogus IMD numbers to louder 
stations).

Using A/D converters and OS drivers that can handle a full 24 bits, 
combined with FIR filters that have floating point coefficients, you 
can build very nice wide band implementation of PSK31... until the A/D 
gets saturated.

Saturating the A/D at the receiving end is like overdriving at the 
transmitting end -- the IMD of your receiving system goes way up and a 
narrow band PSK31 signal now looks like it is wider than (in fact, on 
paper worse by a couple of dB) an FSK signal with no keying 
waveshaping.

73
Chen, W7AY