[RTTY] Shift 170 -vs- 200

[k0bx at arrl.net]

Here is the link to the modification of the PK-232 for 170 shift and to narrow 
the filters.

http://www.qsl.net/k0bx/align.htm

Joe K0BX

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________________________________
From: "Joe Subich, W4TV" <lists at subich.com>
To: rtty at contesting.com
Sent: Thu, June 9, 2011 11:03:17 AM
Subject: Re: [RTTY] Shift 170 -vs- 200


> I believe part of the reason is that the TNC manufacturers were
> really selling the TNCs for HF Packet (200 Hz shift), with RTTY/Amtor
> (170 Hz shift) tagging along just to pad the specs sheet.

That's probably true ... all of the 200 Hz TNCs included HF (and VHF)
packet.

> What probably hurt more than 200 Hz shift was the fact that the TNCs
> had such broad filters (needing to pass 300 baud HF Packet data
> rate).

That's probably true as well ... but the 200 Hz transmission certainly
did not help with the ability to be heard due to the tuning issues.

> Garry NI6T had modified the PK-232 to narrow the filters down.  I
> forget now, but I recall vaguely that the filters use bi-quad
> topology with opamps, resistors and capacitors.

The modification is still floating around the net ... as I recall it
was something like four resistors.  When the PK-232 was modified for
the narrow filters and the PLL tuned for 170 Hz shift it wasn't a bad
HF modem but still doesn't hold up to even the "Standard RTTY" profile
in MMTTY much less some of the better software (e.g., cocoaModem<G>).

However, a good 400 Hz IF filter and 90 dB DR receiver can do a lot
with even moderately good decoders. Proper filtering and AGC can take
a lot of the strain off the decoder although flutter, multi-path and
selective fading will still take their toll.

73,

    ... Joe, W4TV


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