[GreenKeys] Freqency shift standards and the HAL ST-6000 (And other rants)

[Jim Haynes]

On Tue, 4 Nov 2008, Jeffrey Angus wrote:

> If my somewhat faulty at times memory serves me correct, the standard for
> RTTY has been
> 850 Hz (wide) and 170 Hz (narrow) shift since the time it was first allowed
> by the FCC on
> amateur bands.
That's about right.  At first 850 was mandated, then the FCC changed the
rules to allow anything below 900.  There was a certain amoung of 
dithering around before 170 was settled upon.  170 is 1/5 of 850, and
is twice the 85 Hz shift commonly used in multi channel carrier
telegraph systems.
>
> I had always had problems with other stations running the AEA PK-232. (Back
> when it was
> introduced in the early '80s) And I seem to recall that their idea of
> "standards" were different
> from others. (I.e. everyone else).

It's more a matter of a compromise.  200 Hz shift is standard for AMTOR,
and then it is also standard for HF packet since that uses Bell 103 modem
standards.  It was hard to make their box switch between 170 and 200,
so they decided 200 is good enough for everything.

>
> And again, digging into my questionable memory bucket, on RTTY the tones were
> 2125 and 2975 Hz (850 Hz shift) and 2125 and 2295 Hz (170 Hz shift.)

Originally, yes.  But with the arrival of SSB radios that have an upper
cutoff frequency for audio of perhaps 2.5 KHz it was necessary to slide
the audio tones down to be within the passband.
>
> Obviously the reason AEA chose to play "close enough" with the frequency
> shift
> was they were using a Bell 103 Modem chip set. And the reasoning was most hams
> buying the PK-232 were interested in AX.25 packet radio. That it would do
> RTTY
> was extra, and not specific design criteria.

They aren't using a Bell modem chip; they built active filters out of 
parts.  They just didn't have a way to tune them.
>
> I dug up my original copies of the Kantronics KAM manual and verified what I
> thought
> they were doing. By using capacitive switched filters for the decoding, they
> could be
> set to any shift accurately. Although they default to 170, 425 and 850 Hz.
> Much like
> the later model HAL ST-8000 with the shiny knobs. ;-)
>
I've seen modification information published to put the PK-232 on 170 Hz,
assuming you don't care about AMTOR (and Pactor in the models so equipped)
and HF packet.  I wonder if the PK-2232 with digital filters uses 170
Hz for RTTY.  I'm surprised the number of PK-232s is increasing - even
if they are cheap the sound card RTTY modems are so much better.  I don't
know why anybody would use a hardware modem these days.  And if you are
doing it to set up an old-time RTTY station you wouldn't be using a
PK-232, since it is requires an ASCII terminal.

Jim W6JVE