[RTTY] FW: [1000mp] PCT III Not Legal USA? - Sound Card Digital/Ritty Modes

[Kok Chen]

See forwarded message below.  Another salvo fired against
relying on DSP modems.  This time, over at the FT-1000MP
reflector.  

Until 24-bit A/D converters become the norm, keep using
narrow analog filters!  Remember, friends don't let
friends use DSP filters.

BTW, you can already buy 24-bit USB based A/D converters.
They are about the price of a new KAM.

That is the reason why I am very interested in buying a
real live ST-8000 to compare good DSP implementations
against.  So far, I haven't had much luck bidding for
a ST-8000 at what I think they are currently worth (the
price of a 24-bit A/D converter, HI HI. The rest, as
they say is "just software").

73
Chen, W7AY


------ Forwarded Message
From: "Tom Rauch" <w8ji@contesting.com>
Reply-To: 1000mp@mailman.qth.net
Date: Mon, 3 Jun 2002 14:27:52 -0400
To: <1000mp@mailman.qth.net>
Subject: RE: [1000mp] PCT III Not Legal USA? - Sound Card Digital/Ritty
Modes

> Current DSP-based TNC's provide mode-specific configurations in
> firmware, as do S/W TNC's operating with soundcards, and some use
> detectors with performance superior to the limiter/discriminator
> approach, such as linear matched filters.

Even the very best digital filters are limited to dynamic ranges in
the 50dB range or less. While they handle multiple signals that are
not extremely different in level OK, they fall apart when some
signals are significantly louder that others..especially when there
are multiple signals.

The reliance on digital filtering, even very good specialized digital
filtering, is why PSK and MFSK people complain so loudly and so often
about "high power stations". The problem is the  receiving systems
can't copy weak signals in the presence of multiple signals of
reasonable strengths.

Best to have the option of using some reasonable amount of IF
selectivity, should a high power 100-watt station with a good antenna
happen to come on nearby.73, Tom W8JI
W8JI@contesting.com

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------ End of Forwarded Message