[RTTY] Shift 170 -vs- 200

[Joe Subich, W4TV]

> I've noticed over the years that some manufacturers (for whatever
> reason) have moved to use a 200Hz shift instead of the standard 170Hz
> shift. The reason for the move is unclear, perhaps it is merely for
> simplicity.

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.  The marketing departments of the manufacturers
pushed the lie that 200 Hz would work with 170 Hz systems without
degradation rather than spend the money and take the time to design
and build a proper selectable shift modem.

> I think Chen will bear me out that under strong signal conditions
> this offset doesn't matter.

When the signal is sufficiently strong one has plenty of reserve
margin and 3 dB doesn't matter.  However, a substantial amount of
RTTY activity occurs at or near threshold and the degradation due
to the unmatched shifts can be significant.

73,

    ... Joe, W4TV


On 6/9/2011 7:58 AM, Phil Sussman wrote:
> I've noticed over the years that some manufacturers (for whatever reason)
> have moved to use a 200Hz shift instead of the standard 170Hz shift. The
> reason for the move is unclear, perhaps it is merely for simplicity.
>
> Whatever the case, it is claimed that tuning between two stations, one
> using 170Hz shift and the other using 200Hz, is not all that great. When
> the tuning between the two is centered, there is an offset of only 15Hz.
> (200-170 = 30; 30/2 = 15) In the days when filters were a lot broader,
> this offset was insignificant. I think Chen will bear me out that under
> strong signal conditions this offset doesn't matter.
>
> The only time when such an offset is important is when very sharp filters
> with tight skirts are used AND the signal strength is weak or fluctuating.
> That would lead to more errors.
>
> I've been using an SCS modem for years with 200Hz shift on AFSK and I don't
> seem to have a problem decoding weak RTTY signals.
>
> Any thoughts or explanation?
>
> Thanks,
>
> 73 de Phil - N8PS
>
>
>
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