> 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
On 6/9/2011 11:29 AM, Kok Chen wrote:
>
> On Jun 9, 2011, at 5:51 AM, Joe Subich, W4TV wrote:
>
>> Because some of the early "multi-mode" hardware boxes were
>> designed around TI modem chips that implemented the Bell 103
>> standard with a fixed 200 Hz shift.
>
> 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.
>
> 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).
>
> Even the TNCs that can change shift from 200 Hz to 170 Hz, such as
> the PK232 and the Kantronics KAM Plus could not adapt to a narrower
> filter because their filters were implemented in hardware.
>
> 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. Garry had written it
> up in a Communications Quarterly article back then, and those PK-232
> had gone to some DXpeditions. I believe Garry himself had modified
> two of them. One of them definitely went with Garry to the 3D2CU
> Conway expedition and he had also loaned a modified PK-232 to a few
> other DXpeditions that he didn't go himself to. A number of you
> probably could thank the modifications for making it through HI HI.
>
> I had modified a KAM Plus, which I still own, to also narrow it down
> for 45.45 baud instead of 300 baud. The KAM Plus was much easier to
> modify since it had used switched capacitor filters. The only things
> that needed changing are a dozen or so 1% resistors. You compute the
> resistor values, go buy them and that is all there was, no fuss and
> no muss since precision capacitors were not involved. I ran that KAM
> for a number of years, until the Timewave DSP-599zx came out and I
> used the 599zx as the regenerator for the KAM. The ST-8000 was later
> added as a regenerator ahead of another KAM to provide two-modem
> operation, but that second KAM need not be modified since it was just
> decoding from a clean regenerated AFSK signal from the ST-8000.
>
> The problem with hardware mods of course is that they are not easily
> reversible by a switch. So, once modified, they could no longer be
> used on HF Packet, which was fine by most of us who hate the wide HF
> Packet signals that use to QRM us when they ran up in the upper 14090
> kHz region.
>
>> When the signal is sufficiently strong one has plenty of reserve
>> margin and 3 dB doesn't matter.
>
> The degradation from receiving a 200 Hz shift signal with a proper
> modem is not as bad as 3 dB. However, if you don't tune a 200 Hz
> shift signal carefully under selective fading, one of the tones can
> bias the ATC of the decoder and throw all sorts of errors. The way
> to mitigate it somewhat is to tune the signal so that the vertical
> and horizontal ellipses of a crossed banana are off by about equal
> angles.
>
> The degradation is much worse when you use an unmodified TNC to
> receive a regular RTTY signal. When you try to receive an RTTY
> signal with a demodulator that is matched to 300 baud, the
> degradation is a whopping 8 dB or so if the receiver does not have a
> narrow filter. The transmitting end need to send about 7 times the
> power for you to copy them with an unmodified TNC.
>
> You still see a number of 200 Hz shift signals during RTTY contests
> today. Sad that people are still using TNCs, when software modems
> are literally free and can be used with the purchase of an
> inexpensive $30 sound card. Unlike the PCI bus, days there are no
> longer any really "bad" USB sound cards (unless you buy the truly
> cheap $6 ones that are meant for USB boom headsets). Remember how
> bad some of the early PCI Sound Blasters were? Now those same guys
> manufacture the outstanding E-MU 0404 and 0204.
>
> 73 Chen, W7AY
>
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