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VE2HAR > MT63 11.03.05 20:50l 312 Lines 11369 Bytes #-7344 (0) @ WW
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Subj: [MT63] RE: MT-63 robustness or not
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I think you are confirming what I been saying...
> Thanks for all the comments, Walt.
>
I would like to see the performance specs on Olivia is anyone has them.
> The Olivia mode can change the baud rate, but the problem with Olivia that I
> find is that in order to have reasonable throughput, say 3 cps minimum,
> requires such an enormous BW, in the most robust form that I personally
> think it is not appropriate unless used for experimenting on a "dead band."
>
3 CPS is about 30 WPM. What is the BW and how robust is it. If 30 WPM can be
"copied" at a -15 dB SNR (reference poor CCIR channel), then its almost a robust
as MFSK16.
> Has anyone really found Olivia to be better than MFSK16 in bad conditions? I
> have not found that, but also have not seen any lab tests that do a fair
> comparison. Has anyone done this now or know of better weak signal
> performance by Olivia over all other modes?
Here's where we need that "standard" or reference file so comparisons can be made.
>
> The baud rate of Olivia can be either 31.25, 62.5, 125, --->, etc. But I
> have found that on 80 meter nighttime weak signals, it can be inoperative at
> the 125 rate (as you might expect). But MFSK16 works well at that time and
> at a fraction of the BW, and at a similar speed compared to Olivia using
> 31.25 baud rate > 3 cps for both. (I consider this really too slow for me
> but for many it will be a comfortable speed for keyboard modes.)
I don't know what detection method Olivia is using, but if its the same as MT63,
I can flat tell you that unless you have a low number of hops, 2 or 3 at most, a
stable ionosphere, and a very good signal, 125 baud anything isn't going to
provide 125 baud throughput.
I would expect 62.5 baud to work rather well on 160M through perhaps 20M but
conservatively only through 40M...and you might expect full throughput using a
NVIS antenna pattern, and stable ionosphere and where the signal was in the area
of 0 to +5 dB SNR. 31.5 SNR should give you full throughput down to around -5
dB SNR.
3 CPS is fine for my typing capability. Hi Hi...that's why I normally try to
send files. Hi Hi.
>
> If you compared throughput with the weakest signal per BW, wouldn't MFSK16
> at least equal or even exceed MT63 in all attributes but would be able to go
> way down into the noise well below MT-63?
Oh yes, at max. typing speeds (50-60 WPM) MFSK16 is going to be copied down
around -15 to -18 dB SNR...down you you can't even see it on a waterfall
display. You must be able to see MT63, even though it might be weak, on the
waterfall to copy it...you may not hear it but if tuned correctly, and you can
see it, they you should be able to copy it.
MFSK16 copies down around -18 dB before it falls out and MT63 falls out at
around -7 dB.
>
> Is it possible you have some misunderstanding on baud rates?
I don't think so.
>
> Here is my understanding based upon a useable symbol rate for HF < 100 and
> at least 3 CPS net throughput:
>
> MODE Symbol rate (baud) Net CPS BW
>
> OLIVIA 31.25 and 62.5 3 or 5 1000 or 2000
>
> MT-63** 10 8.3 1000
>
> MFSK 15.625 3.125 316
>
>
> ** Note: MT-63 can have baud rate doubled or halved with corresponding
> doubling or halving of the net CPS and BW
Good info. Its the first I've seen on Olivia.
MT63-1K (which you reference) is capable of 100 WPM in measurements on a channel
simulator. MT63-2K (long interleave) is capable of 200 WPM in measurements on a
channel simulator.
> By the way, are you sure that the lower bands can handle the faster baud
> rates and the higher bands need slower ones. I always thought it was the
> opposite. Perhaps it depends upon how many hops? One hop HF such as on 10
> meters can be a pretty stable signal (went it is propagating).
Hummm, I'll have to rethink that. But with the more hops and less stable
ionosphere, the greater the intersymbol interferrence, the less data you will
recover.
This is not to say that just because you are on 80 or 40 that you have a good
CCIR channel. But perhaps the 40-60 baud range that the government found to be
the range for HF is why they chose 45.5 baud, the low end of that range as the
fixed baud rate. I do know that their communications plan only considers
frequencies under 22 MHz and they never use (ideally) a frequency higher than
90% of the MUF. Also, they are always careful to use an antenna take-off angle
that will produce the least number of hops.
>
> What would happen if you increased the baud rate of MFSK16 to 40 - 60 baud?
> Is this technically not possible due to losing other attributes? Otherwise,
> wouldn't you increase the throughput of by a factor of 3 to 4 times? And the
> main tradeoff would be the increased BW?
First I think you would have to increase the bandwidth and then signal detection
and FEC become a problem so yes I think you lose some of the mode's attributes.
>
> On one other note, if we ever went to ultra wide modes on HF, > 3 KHz, what
> equipment did you have in mind to do this? Could you easily adapt existing
> rigs?
There are two rigs out now that already do this. One is the TenTec Jupiter (and
now out of production Pegausus) and the FlexRadio SDR-1000. Also the out of
production Kachina (sp). All have variable receive and transmit bandwidth
filters going out to
>From the Tentec Jupiter specs...
34 IF-DSP receive filters. 25 choices from 1050-8000 Hz for voice modes and 9
more from 300 to 900 Hz for digital and CW modes. Of course, all receive
filters are selectable independent of mode.
18 different DSP-generated transmit bandwidths from 900-3900 Hz are available
for SSB to give your transmit audio a well-rounded sound tailored to your voice
characteristics
>From the SDR-1000 specs...
Max. Receive Bandwidth 40KHz (Limited by sound card)
The transmit bandwidth is also limited by the sound card so the assumption is
that it can create a 40 KHz BW mode.
That's all folks.
73,
Walt/K5YFW
>
> 73,
>
> Rick, KV9U
>
> -----Original Message-----
> From: dubose@texas.net [mailto:dubose@texas.net]
> Sent: Friday, March 11, 2005 8:40 AM
> To: mt63@yahoogroups.com; linlink@wetnet.net
> Cc: mrfarm@mwt.net
> Subject: Re: MT-63 robustness or not
>
>
> > I expected the wide BW and low baud rate Olivia to work well. The baud
> rate
> > of MT-63 is very low and ironically subject to doppler effects although
> very
> > immune to multipath.
>
> I don't know enough about Olivia to comment...but MT63 2K has a 31 Hz
> baudrate
> as I recall.
>
> Back in the early 50's (here goes my story again) the U.S. Government let a
> research contract to Standford Research Institute (SRI), with Collins Radio
> closly looking on, to study how fast the military could send RTTY. And at
> time,
> ASCII was becomming popular. SRI said that the fastest baud rate you could
> use
> on HF (up to 14 MHz) was 110 baud. Thus you see the "standard" speed for
> ASCII
> was 110 baud.
>
> In the early 80's the military took another look at what baud rate was best.
> This after deploying a number of "chirp sounders". Contractors in the
> project
> were Rockwell-Collins, Magnavox, Harris RF Comm Gp, ARINC and a couple of
> more.
> Their conclusion was that the best baud rates for HF were 40-60 baud. 60
> baud
> on the lower HF frequencies and 40 baud on the higher (above 10 MHz) HF
> frequencies.
>
> As a result of this research, MIL-STD-188-110 was formed with a standard of
> 45.5
> baud as the standard baud rate for their 16 and 29 parallel tone modems.
>
> MT63 is somewhat like the 29 parallel tone modem.
>
> Later MIL-STD-188-110a,b became FS-1052 (Fed Std) which you can find the
> specs
> to on the web. Take a look at this spec and them MT63.
>
> FS-1052 provides for 2400 BPS user throughput at a signal level of about a 0
> dB
> SNR on a poor CCIR channel. MT63 does better but with lower throughput.
>
>
> >
> > What would happen if the baud rate were increased from the native 10 baud
> > rate to perhaps 60 or more?
>
> Reference the above...60 baud is through 40M and maybe 30M. On 20M and
> above,
> you would want to drop back to 40 baud. I would be safe and split the
> difference and go with 50 baud; however, due to "timing", I think you go
> with
> 45.5 baud and the lower end rather than a higher figure around 55 baud.
>
> >
> > One of the problems with modes that transmit many tones at once is that
> the
> > power in an individual tone is very low and not as effective as when only
> > transmitting one or a few tones at a time.
>
> True but then again the multiple tones give you the speed and robustness you
> need.
>
> The current industry standard for creating high speed throughput and
> robustness
> uses a modem called a single tone modem. Harris has a proprietary single
> tone
> modem that provides 4800 BPS with strong FEC. This fits into a 3 KHz
> bandpass.
> In talking with some contractor folks, they feel that 4800 BPS is about the
> best you can do with the level of robustenss required of them by their
> customers
> using a 3 KHz channel. Their next effort will be a modem using 8 KHz
> bandwidth
> in a 10 KHz channel. Thoughput will be in the order of 14 KBPs.
>
> >
> > Perhaps that also explains why MFSK16 does so well in getting through with
> a
> > weak signal?
>
> It has a low throughput rate (50-60 WPM) so much of the modulation is
> "dumped"
> into the FEC. I you need to increase the throughput, you have to increase
> the
> baud rate, increase the number of tones and/or increase the FEC.
>
> >
> > I suppose the main drawback of MFSK16 is that it is can not handle 8 bit
> > ASCII? Otherwise, it would greatly outperform MT-63 in terms of speed and
> > particularly weak signal when you consider the BW.
>
> I don't think MFSK16 would have more throughput than MT63 because you would
> have
> to increase the number of tones to get more throughput and then have a wider
> bandwidth to deal with and require more FEC.
>
> >
> > If I understand it correctly, MFSK16 has a throughput of 42 wpm with FEC
> (no
> > ARQ) and a bit over 300 HZ BW. It seems that a wide BW MFSK mode should
> > still be the best weak signal mode with a similar speed and BW to MT-63.
>
> Multiple the speed by 5 and you have MT63-2K. Multiply the bandwidth by 5
> and
> you have 1.5 KHz bandwidth. But as the bandwidth becomes wider, more
> propagation problems to contend with so you and some different modualtion
> techinques and finally come out with a bandwidth of more than 1.5 KHz.
>
> There ain't no free lunch.
>
> 73,
>
> Walt/K5YFW
>
>
>
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