Chen,
> There is no need to introduce another "mode du jour" even.
>
> 7-bit ASCII (CCITT ITA-5) RTTY has been FCC approved (see part
> 97.309(c)) for a long time now. fldigi supports it, so does MultiPSK
> and cocoaModem, among others.
Does cocoaModem provide a way to set the data rate for ASCII down to
45.45 baud? Moving to 45.45 baud ASCII (ITA-5) would provide the
benefits discussed while maintaining compatibility with all the
Windows installations that are linked to FSK using EXTFSK in MMTTY
(MMTTY will also do 7bit ASCII although one needs to be careful since
the "HAM" function will reset MMTTY to 5bit code).
> Anyhow, the solution is already at your fingertips, and the FCC has
> blessed it for years now. It is called ASCII. And 110 baud with 170
> Hz shift is a breeze.
Why 110 baud? There is a significant bandwidth and s/n penalty in
the higher speed even if the shift were held constant at 170 Hz.
If one were going to use a higher speed, a slight increase to 50
baud or possibly 75 baud would be enough to provide increased
efficiency vs. 45.45 "Baudot" without the s/n and bandwidth hit.
73,
... Joe, W4TV
On 6/27/2012 4:36 PM, Kok Chen wrote:
>
> On Jun 27, 2012, at 12:47 PM, Alex Malyava wrote:
>
>> Why don't we just invent/introduce some new RTTY standard -
>> the one with 6 bits instead of 5 - covering whole alphabet and digits
>> without any FIGS/LTRS and speed it up a little bit to compensate an extra
>> bit?
>
> There is no need to introduce another "mode du jour" even.
>
> 7-bit ASCII (CCITT ITA-5) RTTY has been FCC approved (see part 97.309(c)) for
> a long time now. fldigi supports it, so does MultiPSK and cocoaModem, among
> others.
>
> In a discussion (a year or even longer ago) on this reflector, I had shown
> that for most RTTY contest exchanges, ASCII RTTY beats out Baudot RTTY in
> speed, even when both are running 45.45 baud.
>
> You get rid of the FIGS/LTRS confusion (thus problem with USOS
> incompatibility either; USOS is a Baudot problem), allows lower case, and it
> still beats out Baudot in contesting speed. It is when sending paragraphs of
> upper case text that Baudot wins over ASCII.
>
> Because of the Teletype Models 33/35, the popular speeds for running ASCII
> RTTY was 110 baud. At that speed, it will wipe the floor with Baudot RTTY.
>
>> Or drop one stop bit to save the length? Or use 3 frequency FSK -
>> shift left is "0", shift right is "1" and middle is sync/start/stop ?
>
> 3FSK may not be a good idea. The reason is that the equalizer to compensate
> for selective fading will be at best very complex to build.
>
> 2FSK has the very unique ability to fight selective fading with a very simple
> thresholding scheme. Once you add more tones, you can no longer build simple
> ATC circuits.
>
> For that reason, you will find that there is nothing in MFSK16 (16 tones),
> DominoEX (18 tones) or Olivia that explicitly fixes the selective fade
> problem -- they all use long interleaved codes to fight QSB in general -- and
> you may not want to use long interleavers with short contest exchanges; the
> latency will need to be over 1 second to be effective. You will need to add
> latency to the exchange time. Selective fading happens quite often. You can
> almost not avoid it with a Rayleigh path.
>
> Anyhow, the solution is already at your fingertips, and the FCC has blessed
> it for years now. It is called ASCII. And 110 baud with 170 Hz shift is a
> breeze.
>
> 73
> Chen, W7AY
>
>
>
>
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