Voice Frequency Telegraphy (VFT) Explained
From The RadioReference Wiki
Multiple channel voice frequency telegraphy (MCVFT or VFT for short) sounds complicated (and I suppose it is) but it is simply the function of using a modem to change the two states (mark/space) each of a number of telegraph channels into two tones (voice frequency) per channel and applying these to what is basically an SSB RT transmitter. USB is the usual choice.
There are quite a number of architectures in use from 2 to 16 channels. Some are to agreed International (CCITT) Recommendations since these were used between PTTs of different countries. I say were since the majority of PTT circuits have transferred to satellite.
Other VFTs are multiple channel carrying fully on-line encrypted military traffic either (one must suppose) with independent data per channel or each carrying the same traffic in a form of frequency and/or time multiplexed error correcting system. Some of these are variants on the International recommendations but often there is no requirement to meet such standards since they are internal to a single administration.
While far short of the results that can be obtained with the audio spectrum modules of CODE30, a good screen grabber and the graphics facilities of WP5.1 the following is a simple way of outlining the construction of a VFT within the limitations of a text file. The scale represents the 3 khz USB of the suppressed carrier f0. Channel placements are indicated at the approximate offsets (Hz relative to f0) with M and S representing Mark and Space tones respectively. For want of an alternate system I refer to channels incrementally away from the suppressed carrier ie Channel 1 is nearest to the suppressed carrier.
Note: Most of the stations listed here have either moved to using other modes and/or changed frequencies in the last several years
Example a. German Navy (DHJ59 and warships) M S M S M S M S M S M S ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| | | | | | | | | f0 1000 2000 3000
Regularly logged as voice transmissions for RTTY coordination the above shows the digital vft within the USB passband. Three channels appear offset +1190/1870/2550 hz from the suppressed carrier f0. Each channel is 170hz wide. Data is online encrypted at 150 or 200bd. One can only assume that the three channels carry parallel data possibly time delayed on each other (similar to the 6028 system example c below) to provide a frequency division multiplexed system to alleviate transmission errors.
Freq(f0): 2625 4154 6731 6779 8335.5 10271.8 10192.5 10772
Example b. British military Piccolo MS MS MS MS MS MS MS MS ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| | | | | | | | | f0 1000 2000 3000
These are often seen where 6-tone Mk.6 Piccolo is employed. These may be two, three, four channel. Channels are located at offsets 510, 910, 1310, 1910 Hz. Usually USB but LSB is not unknown. All traffic channels are fully online encrypted. Channel 1 is the engineering channel and will sit idle, often for hours. Idle is alternating two central tones, 20 hz apart.
Occasional engineer exchanges in the clear can be noted. Mk.6 12-tone Piccolo is normally found stand-alone ie not part of a vft.
Freq (f0): 7595 9226 9265 10261 10967.5 11018.5 12065 14510 14593 15855 16205 18057 18879 19053.5
Example c. Type 6028, or "Barrie" ! M S M S M S M S M S M S M S ! M S M S M S M S M S M S M S ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| | | | | | | | | f0 1000 2000 3000
This is a time-and-frequency multiplex system in which a single channel of traffic is transmitted, not quite simultaneously, on up to 7 FDM channels spaced 340hz. The individual channels are 170hz and are offset at +850/1190/1530/1870/2210/2550/2890 Hz with an unmodulated pilot tone at 561 hz. This is a common vft but the individual channels run at 75bd and are fully online encrypted. (However see Belgian Diplo above. They use ITA2 at 100bd either in the clear or offline encryption.) Otherwise the baud rate can be up to 110bd.
This system sends the data bit by bit over each channel. The receiving equipment then assesses each channel for a majority character ie over the seven channels the data is received as say P P A P S P P where the majority character is the letter P and this is the one passed to line. By mutual agreement the circuit may not support all channels; MFA Brussels currently does not use channel 2.
Freq(f0): 4083 6785 8172 10616 11010 11105 14821 14902 14474
Example d. RN Gibraltar/GYU S M MS M S M S S M MS M S M S ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| | | | | | | | | f0 1000 2000 3000
The RN employ various vft structures for broadcasts to ships. Here all channels are 75bd but each has a different shift.
- Ch.1 + 600 200 hz Note reversed ie low tone is the space. Used as the channel availability broadcast.
- Ch.2 +1105 85 hz 7bit with blocked encrypted traffic.
- Ch.3 +1785 340 hz Fully online encrypted.
- Ch.4 +2550 850 hz Fully online encrypted.
Freq(f0): 2824.3 4220.3 6370.3 8625.3 12823.3 16986.3 22668.3
Example e. RNZN Waiouru/ZLO M S M S M M S M S M ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| | | | | | | | | f0 1000 2000 3000
Two 75bd fully online encrypted channels (1- 200hz at +600 and 2- 850hz at +2000) with a CW marker at +2800hz.
Freq(f0): 6337 8598.5 12717 17224.8
16x85 type (CCITT R39-1) nicknamed in the States I believe as "bandsaw". This consists of up to 16 x 85hz wide channels spaced at 170hz intervals. Just a note to indicate this was (may still be by RAN) used by RAN and the USN. The RAN broadcasts were fully encrypted. The USN however had two clear channels - Ch.14 weather at 74.2bd from KAWN, and Ch.16 AFRTS(?) news at 50bd from APN/UPI. Unfortunately these seem to have disappeared in the last two/three years from HF to that big bird in the sky.
From the Digital Review column, WUN0205 written by Ary Boender published May 1996 (c) Worldwide Utility News