The one common theme I see with much of the digital modes that require
the higher level of speed is that the required minimum S/N ratio hovers
around 10 db S/N. It seems to be true with DV voice, with SCAMP, and
also with high baud rate modes (such as trying to exceed a few hundred
baud on HF).
To me there is a dividing line between real time voice
and high speed
digital texting vs. slower modes that get through under
difficult
conditions. Actually, difficult conditions tend to be more of
the norm
on HF.
Does anyone have any "inside" information that
would suggest that we
will be able to lower the required S/N ratios for a
given speed or are
we already too close to the Shannon Limit as it
is?
If we can not do this, I don't see how any of the higher speed
digital
modes will ever be able to replace analog communications for weak
signal
and general amateur radio communications with modest power levels
and
modest antennas. In other words, what most of us do most of the
time.
Of course this would not be true for VHF and up, only for the
restricted
BW's of HF.
73,
Rick, KV9U
DuBose Walt Civ
AETC CONS/LGCA wrote:
>Ed,
>
>Back in the fall of 1989,
we started using a unit called the TacTerm for DV. The best modem was a 39
parallel tone modem (Mil-Std-188-110?) and 2400 BPS. On HF it sounded
funny...robotic like...and while it did work on about the same SNR as a ~S3
SSB signal ...the SNR had to be about 10 dB, it worked much better on VHF and
UHF FM as well as UHF AM.
>
>In Jan-Feb 1990, after Operation Just
Cause, I got to play with a Harris Mil-Std-188-110? modem and DV at 2400
BPS AND 4800 BPS. The 2400 bps DV still sounded a bit robotic but worked quite
well at S2-S3 signals and even when the SNR was at or I think slightly below
10 dB. The modem had ALE and provided a SNR figure and my old ham radio ear
figured an S2-3 signal.
>
>When we switched to 4800 bps, the DV
sounded just like the individual speaking over a telephone...better than
SSB voice but it required a slightly better SNR.
>
>Data with the
TacTerm 39 tone modem was really good...text came through at about a page a
minute with a signal that you could hear. When the signal got down close to
the noise...where you had to strain to hear it, the throughput went down to
maybe 20-30 lines per minute....but still near 100% copy (could have been
typing errors). The actual computer software was KA9Q NOS running on a BIG
military laptop computer.
>
>When we went to the Harris modem and
2400 BPS/4800 BPS, the copy at 2400 BPS was about the same with a good signal
but better than the TacTerm with lower qualitity signals.
>
>4800
bps screemed unless the signal got below 10 dB SNR or
so.
>
>During Operations Desert Shield and Desert Storm, the U.S.
Marine Corp used a Hughes RT-1209 and Harris AM-6545A 400 watt amplifier to
make up the GRC-193A (http://www.nj7p.org/millist/pic/grc-193.gif)
used in some of the last productions Jeeps and Humvees. Most were using a
15-16 ft fold over whip. The whip went up just about 18" higher then the
Humvee top and then tilted over and ran horizontal the remainder of the
length. This provided a good NVIS antenna radiation
pattern.
>
>The Marine Corp did connect the Harris
MIL-STD-188-110x modem to the GRC-193 and used a computer to send data
from the field back to their command center.
>
>For digital voice
I believe that some units used the Navy's version of the TacTerm and some may
have been using the later ITT MinTerm DV unit. The TacTerm (KY-57) can accept
signal fades of up to 12 seconds without losing synchronization with the
transmitting station.
>
>I never noticed any delays or words that
were not understandable using the TacTerm or MinTerm or Harris Modem using
LCP-10 and UG-??? encryption unit. There were of course a second or two
between transmissions, but certainly not enough to prevent artillary spotting
or fire control.
>
>The PRC-109/GRC-193 systems were used at
least in 1985 and perhaps before with the TacTerm. The TacTerm was used in the
Viet Nam war on HF SSB, VHF and UHF AM and as far as I know without problems.
The C-130s tracking and Navy vessel captured by North Korea used Tacterm's on
HF and the crew of the C-130 never mentioned to me that there were
communications using the TecTerm.
>
>So when the Marine Corp have
problems with DV must have been shortly after the capture of the USS Pubelo
which I believe was in 1968.
>
>A good reference on the ANDVT
modem may be _HF modem evaluations for the Advanced Narrowband Digital Voice
Terminal (ANDVT)_ by Chase, D.; Bello, P. A.; Boardman, C.; Pickering, L.;
Pinto, R. published in Nov 1978
>
>Abstract:
>"During this
program, the specifications for the ANDVT HF modem have been refined and
detailed evaluation and simulation of the new technical features within this
modem have been conducted. These include a multiple-tone signal detection
format with an adaptive threshold, a multiple-tone/multiple-stage
Doppler estimation algorithm, a matched filter frame estimation algorithm
utilizing PN correlation properties, a low-rate error-correction coding
approach for protection of the KG sync sequences, an error-correction coding
approach specifically designed to protect the critical speech parameters, use
of soft-decision (channel measurement) information obtained from the
demodulator, and decision-directed Doppler tracking utilizing information from
all data tones. The analytical and simulation results provide the desirable
result that the preamble can be successfully received at a lower SNR than is
required for the reception of high-quality 2400-b/s digitized
voice."
>
>Walt/K5YFW
>
>
>
