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VE2HAR > MT63 11.03.05 17:04l 172 Lines 6343 Bytes #-7345 (0) @ WW
BID : 34307SENTTO
Read: GUEST
Subj: [MT63] Re: MT-63 robustness or not
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0EEO<DB0RES<ON0AR<VA2HAR<
VE2HAR
Sent: 050311/1459z @:VE2HAR.#MTL.QC.CAN.NOAM Laval #:41343 $:34307sentto
> 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
>
> 73,
>
> Rick, KV9U
>
>
>
>
> -----Original Message-----
> From: linlink-bounces@wetnet.net [mailto:linlink-bounces@wetnet.net]On
> Behalf Of dubose@texas.net
> Sent: Thursday, March 10, 2005 10:07 AM
> To: linlink@wetnet.net
> Cc: MT63@yahoogroups.com
> Subject: RE: [MT63] 20 kHz wide Digital Proposal (Was: ARRL MUST GET
> BETTER)
>
>
> Hi Rick,
>
> Quoting Rick Williams <mrfarm@mwt.net>:
> > Something to consider:
> >
> > In the last week or so, another station and I did some NVIS testing in the
> > evening on 80 meters. We were using Olivia and then went to MFSK16 and no
> > problem. We switched to MT-63 and the signal was light but you could hear
> > it, certainly enough for a CW contact. But no MT-63 signals got through.
> > Went back to MFSK16 and no problem.
>
> Ok...well I'm not sure why you didn't copy the MT63 signal...but Olivia
> should
> be more robust that MT63 and MFSK-16 more robust than Olivia. The
> difference
> between MFSK16 and Olivia and MT63 would be the throughput capability.
> Olivia
> and MT63 should be at least 4 times faster than MFSK16 but if you are direct
> keyboard input, its the same.
>
> MFSK16 is by far the best keyboard mode but Olivia and MT63 the best file
> transfer modes.
>
> It is possible that there was more symbol interferrence for the MT63 mode to
> decode but MFSK16 was able to...in this case it wasn't a signal level
> problem
> as much as a propagation problem. But that only a guess.
>
>
>
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