To: | RTTY Reflector <rtty@contesting.com> |
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Subject: | Re: [RTTY] IC 756 Pro II and rtty |
From: | Kok Chen <chen@mac.com> |
Date: | Mon, 17 Jan 2005 07:33:37 -0800 |
List-post: | <mailto:rtty@contesting.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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