The ST-8000A was designed for the military and as such, the controls are simple to use and robust in construction. There is a single toggle switch that powers the unit on, and the rest of the controls are membrane buttons. There is a “2nd” key that allows for most buttons to be used for two purposes. There are also LED’s used for tuning, status indicators, and data output for settings such as tone and baud rate that have been selected. Once quirk of the controls is that once the selection has been typed in, a green LED will blink until the enter key is selected to finalize the input.
The 8000A is a complex modem that can be configured for a wide variety of inputs. However, I use it only as an audio filter between the radio and the decoding modem, currently a HAL DSP-4100. In this case, the important settings for operation are the setting up the filters so that the signal gets processed, and ensuring that the audio path is correct.
There are two methods to tune the filters of the 8000A, both get to the same place, varying only in the parameters that are being utilized. The traditional way to set the filters is to have the unit display the mark and space frequencies along with the baud rate. First select the M/S display and enter each frequency in turn. The difference between the mark and space will be the shift and the average of the two numbers will be the center frequency. Since I am feeding a DSP-4100, I set the M/S at the standard Amateur RTTY tones of 2125 and 2295 at 45 baud. This gives the standard shift of 170 Hz. Since I am feeding the audio to a DSP-4100 that has it’s RTTY filters set at these tones, it is easy to match the settings. The Radio RTTY mode outputs are also set at these tones, so the M/S display and setting method makes tuning amateur RTTY easy.
The second method is to have the 8000A display the center frequency, shift, and baud. This method is more used when monitoring commercial or military RTTY that use alternative shift and baud rates. Since commercial and military stations are not as worried about bandwidth, the shift is often around 850 Hz, even with a relatively slow baud rate of 50. In this case, the radio will be set to USB and to more easily utilize the radio passbands, the 8000A and DSP-4100 tone selections need to be changed. To make things simple, I change the Cf to 1500 Hz, and select the shift to the appropriate amount and change the baud rate, all by direct entry. I then switch the display to M/S where the tone frequency are automatically displayed. I then go to the terminal software and use the configuration commands to align the DSP-4100 to the same M/S settings.
For an example of copying commercial RTTY over the air, German weather station DDK broadcasts weather over RTTY using a 425 Hz shift and 50 baud. I dial in the frequency on the radio set to USB to 10.099.315. This centers the tones at 1500Hz in the passband. I can then use the various radio filters to reduce noise and interference. I set the 8000A to 1500 Cf, 425 Shift, and 50 baud. Switching to M/S display, I see the tones at 1287.5 Hz for Mark and 1712.5 Hz for Space. I then go into the Hal Software and adjust the baud on the front panel, and go to the configuration page to change the M/S frequencies. Using these settings, audio passes through the radio filters into the 8000A where it is filtered and then passed to the 4100 where the audio passes through the DSP board and finally to the decoder where it is converted to serial data for display on the screen.
Traditional RTTY is sent on LSB but new amateur convention, which is nearly all soundcard modems now, uses USB for digital work. To correct for this problem when using legacy equipment, everything has to be set to Reverse. Since I am using FSK settings on the radio, the 8000A is set to REV as well. I keep this setting for commercial RTTY and reverse it again in the software rather than change the modems around.
There are some articles on the internet about using the HAL ST-8000 and ST-8000A as regeneration units to supply audio to a second decoder for conversion to serial data that is fed to a computer for display. This is known as regeneration. Basically, audio from the radio is filtered and decoded by the primary modem. Instead of outputting serial date, the modem generates high quality Mark and Space audio tones that are fed to a second decoder for conversion to serial data. The second decoder can be pedestrian in performance because the quality of the output from the regenerating modem is extremely high, even if that output is full of errors. In effect the signal is decoded twice, but any errors in the first decoding are carried through to the input of the second decoder. This is a stop-gap method to use the older TU with newer equipment. My setup does not work like this. In my system, the regeneration is on, but the pin that has the output signal is not connected to anything. In my set up, the audio is filtered in the 8000A but never decoded. The filtered audio is extracted before the demodulator and fed to the DSP-4100, where it is further processed and then decoded using higher end computers rather than just the analog decoders in the 8000A. I have a ST-8000 in line that directly decodes the same audio that is fed to the 8000A. The decoding of the 8000A/DSP-4100 is superior by a significant margin than the old “gold standard” that was the ST-8000.
The final article in this series will see how radio teletype is utilized at N1ZZZ, and it certainly is, if not unique, at least highly unusual.
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