Hello Kok,
your posting is very interesting!
But I have two questions:
> MO and SO ...
> It should also be available on software modems since it is so trivial
to implement.
1) Do you know any software with MO/SO other than ritty?
2) How do you decode a weak signal with the 500 Hz Filter if the weak
signal is near a very strong one? The AGC in the receiver kills the weak
signal.
73 Bernd, DC3HB
Am 01.06.2010 06:42, schrieb Kok Chen:
> On May 31, 2010, at 8:04 PM, Don Hill AA5AU wrote:
>
>
>> Not much you can do about that although I have heard
>> rumors of a modem that could actually copy Baudot hearing only one of the
>> two tones, but this could have been a HAL myth.
>>
> No, definitely not a myth, Mark-Only (MO) and Space-Only (SO) works quite
> well. I have used it during times when Mark-Space copy is not possible
> because QRM is overlapping one, but not both of the tones.
>
> MO and SO is selectable right in the "Data Control" part of the front panel
> of the ST-8000 (I believe also on the ST-6000'e front panel). It should also
> be available on software modems since it is so trivial to implement.
>
> Basically, you filter everything (QRM and all) away except for a single tone
> and its keying sideband. At that point, the FSK (frequency shift keyed)
> signal looks like an OOK (on-off keyed) signal. What this means is that you
> lose about 3 dB in terms of SNR when there is no selective fading, but
> otherwise a perfectly decodable signal as long as your modem knows how to
> handle it.
>
> MO and SO is useless when there is selective fading, however.
>
>
>> I think I remember someone, maybe Chen, saying a 200 Hz filter would not
>> work on RTTY or something to that effect. Maybe I don't
>> have the story just right, but it appears to be working great. Too tight
>> for contesting, but works when required.
>>
> If you have a true brickwall 200 Hz filter, you will not be able to reliably
> copy 45 baud RTTY.
>
> In practice, an amateur "200 Hz filter" is never 200 Hz wide, of course.
>
> The RTTY shift is 170 Hz. With a 200 Hz filter, that leaves 15 Hz on each
> side of the two FSK carriers. RTTY baud rate is 45.45, that means that the
> keying fundamental is 22.7 Hz. If you really only have 15 Hz on each side of
> the RTTY shift, a Baudot character that has alternating 1s and 0s will be
> decoded wrongly with finite probability even under good SNR condition.
>
> Nowadays, it is easy enough to check the rig's real actual width by just
> measuring band noise (even better, injecting something like the Elecraft NGEN
> noise generator into the receive antenna connector of the rig) with a sound
> card spectrum analyzer, especially if the software lets you average the
> spectrum. You won't need expensive RF spectrum analyzers and tracking
> filters like in the bad old days anymore.
>
> The narrowest brickwall filter that would copy 45 baud RTTY reliable is
> something like 170+(45.45/2)*2 = 215 Hz. (The 170 is the shift, the
> "45.45/2" is the fundamental of the keying signal, and the "*2" represents
> the two outer edges of the FSK signal.)
>
> A 250 Hz filter will therefore copy RTTY signal quite reliably as long as the
> SNR is good. And you even have 35 Hz of wiggle room :-).
>
> However, when SNR becomes poorer, you want the passband to accept at least
> the 3rd harmonic of the keying sideband to capture most of the energy of the
> modulation (this is the "Eb" part of "Eb/No"). To pass the third harmonic of
> the keying sideband, the filter needs to be 170 + (45.45/2*3)*2 Hz wide
> (i.e., about 305 Hz). I suspect that many "250 Hz" I.F. filters are in
> reality wider than 300 Hz :-).
>
> Theoretically, when there is neither multipath nor Rayleigh fading, to get
> best weak signal copy, you actually want a filter passband that is very soft
> (drops off as 1/f from each of the two tones, where f is scaled to the
> fundamental of the keying signal). This is the so called "AWGN matched
> filter" -- many software modems, including RITTY implements a matched filter
> for weak signal work.
>
> When the band is stable (no selective fading or flutter), I prefer using a
> receiver filter that is at least 500 Hz wide and let the matched filter in
> the decoder to pull out the really weak ones.
>
> 73
> Chen, W7AY
>
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>
>
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