Re: [RTTY] IC 756 Pro II and rtty

[Kok Chen <chen@mac.com>]

On Jan 17, 2005, at 4:44 AM, Jerry Pixton wrote:
> After investigating what was going on, I discover there are TWO 
> settings that involve the filters that are available in RTTY mode. In 
> previous tests, I was seeing the bad triangular shaped filters for 
> rtty with RTTY FIL OFF and the Twin Peak Filter with its excessive 
> processing noise with RTTY FIL ON which seemed to have sharp slopes 
> but unbearable noise in the earphones.

A quick word on rigs and DSP filters (e.g. Timewave 599zx) that have 
double peaked spectral responses:  it is may be better not to engage 
them if your demodulator is using a true matched filter.  The extra 
spectral shaping only makes the combined impulse response of the 
matched filter become unmatched.

Demodulators that don't use matched filters will benefit from the dual 
peak filters, but ones like RITTY (I know lots of you are still using 
RITTY) and cocoaModem use matched filters and it won't help and can 
hurt.  Probably just a small effect, but every dB can be a difference 
between printing a DX or not printing the DX.

With only a 45 baud rate and a relatively wide shift of 170 Hz, RTTY 
matched filters themselves already behave like filters with two peaks.  
But it is a little more precise than that -- just Google for the term 
"Matched Filter."

73
Chen, W7AY








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