[RTTY] Shift 170 -vs- 200

[WS7I]

Mike Lamb of AEA himself put out the fixes years ago and sent me one
which I installed in my old PK-232.  http://www.qsl.net/k0bx/align.htm

K0BX has it saved on his site.

Also we later used "real" TU's like Hal ST-6000/ST-8000 as devices
behind the PK232 and strangely enough I am going to try that again
with N1MM one of these days.

Jay WS7I

On Thu, Jun 9, 2011 at 9:03 AM, Joe Subich, W4TV <lists at subich.com> wrote:
>
>  > 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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