[RTTY] Shift 170 -vs- 200

[Kok Chen]

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