By Andrew SM6MOJ
There are two transmission modes: stop/start and quasi-synchronous. The letters are transmitted at the rate of 5 characters/s, which is exactly twice the speed of a Feldhell machine. In stop/start mode, there is a start pulse before each letter, as with Hell-GL. Unfortunately, there is not the rigid timing facility of the Feld-hell machine, so the quasi-synchronous mode produces a tape with letters jumbled up and down all over the place. This could easily be fixed by software in a modern implementation.
Another important feature is the stop signal. When you press
the stop button on the transmitting machine, this stops your machine.
It also causes a 1260 Hz tone to be transmitted, which stops the
receiving machine at the other end. And I can assure you that
the operator at the other end will have his work cut out to stop
his machine, if the sending machine does not transmit a long-enough
pulse. 1-2 seconds is normally about right.
|Transmission mode||Associated Frequency|
|Frequency for white value||1625 Hz|
|Frequency for black value||1925 Hz|
|Signal frequency||1260 Hz|
|Bandwidth required||from about 1100 to 2000 Hz|
The character set consists of 9 horizontal rows of pixels, and 7 vertical columns, which makes a total of 63 pixels. Two rows and 2 columns are reserved for the frame round each letter, leaving 7x5 = 35 pixels (black or white) for each letter.
There are two starts on the printer helix, which gives two lines of print, as with the Feldhell machine.
Because of the higher print speed, most people will probably prefer to use stop/start operation, or pre-punched paper tape.
I have not had the opportunity of experimenting with different character sets. There is no reason why a Hell-80 should not print any character set in quasi-synchronous mode, but experiments will have to be done to see what happens in stop-start mode.
It may be interesting to note that a Feld-hell machine can receive Hell-80 transmissions, if the receiver is tuned to the mark frequency. As with Fax at the wrong drum speed, alternate pixels are printed high and low on the tape. This means that you get four rows of tiny letters on the Feld-hell tape. Not desperately practical, but amusing to know...