roskos@BBN.COM> (02/10/90)
lrj@fibercom.COM (Larry Johnson) writes: >In response to the earlier request about the frequency, I have been >monitoring 11620 kHz over the past few days, and have heard this >station. I believe that it is a multichannel radio-teletype, or >something similar. I remember reading about them a few years ago in Pop >Comm, but I'm fuzzy on some of the details. In any case, the signal is >made up of a number (perhaps about 20?) of narrow band frequency-shift >signals, spaced about 170 Hz apart. Yes, that is commonly called FDM (Frequency-Division Multiplexing), more accurately called MCVFT (Multi-Channel Voice-Frequency Telegraphy). There are a number of different schemes for doing this. Typically (R7B) there are 15 RTTY channels sent with 85 Hz shifts each, spaced from 425 to 2805 Hz off the nominal carrier frequency, on a single sideband. Another scheme (B9W) sends Independent Sideband with these 15 channels on the USB, and speech on the LSB. Another (J9W) sends SSB with speech on the USB in the area occupied by channels 1-14, and with a single RTTY channel in the small 85 Hz area around 2805 occupied by channel 15. A small number of VFT stations only send 1 or 2 channels with 400 Hz shifts; I believe some of the British military stations may do this. I believe the reason for using VFT is that you can send a number of separate RTTY channels with the same transmitter (the same sort of advantage you get sending two voice channels using ISB). Since transmitters are very expensive compared to the devices used to modulate them, this reduces the cost. You can receive some VFT transmissions with a conventional shortwave receiver with a CW filter installed, using a PK232. But they are subject to noise and fading, so usually a lot of garbage is interspersed with the text unless you are very near the transmitter, or have a very good antenna. Some of these VFT stations carry weather and news, although most of it appears to be encrypted traffic. One of the features of the (very expensive) M7000 from Universal is that it can receive VFT, although as I said above you can do it for about 1/3 the price if you have a radio with a narrow CW filter. Without the CW filter you can probably still do it with a PK232 by building your own very narrow bandpass filter; all the M7000 uses is a fairly simple bandpass filter, so it shouldn't be hard to do something similar. Some of the weather bulletins the amateur meteorologists on the Bitnet use come from US Coast Guard VFT transmissions received with a PK232 and a narrow CW filter, I've read. I've tried it a few times myself, but my antenna isn't good enough -- there's too much noise and fading most of the time. Monitoring Times had a picture (and some discussion) of one of the actual devices used to demodulate VFT transmissions about 6 months ago. It was a rack-mounted device with a set of 15 identical narrow sub-panels in it, apparently for each of the VFT channels. It also told where one could buy them, although they were priced in the tens of thousands of dollars. -- Eric Roskos (roskos@IDA.ORG or Roskos@DOCKMASTER.NCSC.MIL) "Some countries maintain `fast time' throughout the year, in which case it becomes `standard time'." -- DMA World Map 1150 (USGS)