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 -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] Sent: Wednesday, October 11, 2006 2:17 AM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: Digital Voice: Some thoughts after one week. Some more info: I found a paper that describes some tests done with 2.4Kbps and 1.2kbps voice transmission over HF paths. It sounds like the 1.2 kbps gives useable voice quality. I talked to a friend who had done some research for the military back in the '80s on digital voice transmission over HF. He said that the state of the art back then was about 2.4Kbps but there was a technique developed for decoding the speech into text with a very limited vocabulary for tactical operations - around 300 words - and sending the words with minimum coding required for the limited vocabulary. The transmitted data was then reconstructed into speech at the receiving end. The data rates achieved with this method were as low as 300 bps. There were a few problems with this method, though. First it introduced substantial delays due to the processing required for the speech recognition. Second, the Aussies got very upset when their speech came out the other end of the link with a midwestern accent. :) Finally, when attempts were made to apply this method to an application for the Marines for fire control communications, the Marines were unable to come up with a 300 word vocabularly that didn't contain profanity or obscenities so the project was killed. Seriously, long distance HF propagation imposes some very challenging problems with long fades that can only be overcome with long interleaving of the data which creates long delays in the transmission. I will continue to study the issue and report any interesting ideas that show promise for weak signal digitzed voice communications. Ed --- In digitalradio@yahoogroups.com, "Ed Hekman" <[EMAIL PROTECTED]> wrote: > > --- In digitalradio@yahoogroups.com, Tony <DXDX@> wrote: > > > > Ed wrote: > > > > > Are there any communications engineers in this group that can > > > give us some idea whether a useable quality digitized voice can > be > > > sent over a 2.5 KHz wide HF channel with SNR comparable to or > less > than > > > what is required for analog voice? > > > > I was thinking about this today Ed. I'd sacrifice a bit of voice > qaulity for > > better SNR performance. > > > > WinDRM defaults to the MELP codec and I was wondering if the SPEEX > or LP-10 > > offer an improvement in SNR performance? > > > > Tony KT2Q > > Tony, > > The key to better weak signal performance will be primarily in how > the data is sent over the air. Observing the signal for digital > SSTV on 20M has been very interesting. Often a hole in the spectrum > can be observed as it moves across the spectrum. It takes out about > 20% of the spectrum and has a time span at one frequency of around a > second. The packaging of the data must be done with enough > redundancy spread throughout the spectrum so it can be recovered in > spite of these spectrum holes. On 80M atmospheric noise tends to > have short impulses that take out the entire signal for much shorter > periods of time - much less than a second. To mitigate this, the > data redundancy must be spread over time so the data can be > recovered in spite of a complete loss of signal for a few > milliseconds. > > Of course adding redundancy means reducing the data rate for the > encoded voice data. This would probably require some adjustments to > the voice coding algorithm. With cellular signals, although the > maximum data rate may be 8K bps, the effective rate is usually less > than 1/2 that due to the fact that speech is not a constant signal - > there are holes in it for short periods of time during which no data > needs to be sent. Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! 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