Front Panel controls of the HAL ST-8000A

              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 “2^nd” 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.

Operating Clover

Clover is one of my favorite high frequency (HF) modes.  It was developed in the early 90's and was brought out as a proprietary digital mode in the early 1990's by Hal Communications.  There are currently 3 variations of the mode in current use, Clover II, Clover 2000, and Clover 2500.  A good history and technical discussion can still be found at Hal's website http://www.halcomm.com  I won't try to rehash all of the ins and outs of the mode, but will instead give a fairly brief overview of the mode.

Clover II was the first commercially available iteration.  It consisted of 4-tones sent sequentially in a 500 Hz pass band.  Each tone shifted phase in order to pass binary characters. The number of phases increased with better band conditions until it reached 16 phase shifts.  In addition, in the highest modulation schemes, amplitude modulation was added to pass more data.  All of this was done at the low symbol rate of 31 Hz.  Clover II was targeted at both the amateur and commercial interests.  Unfortunately the cost of the mode was too much for the amateur community and only caught on with some, primarily US based, commercial interests.  Data rates for Clover II range from 125 bps to 750 bps and was available new until recently in several modems.  The last Hal unit to carry Clover II was the DSP4100

In 1999 Clover 2000 was introduced.  The basic modulation was the same, but the number of tones was increased to 8 and the bandwidth spread to 2 KHz.  Additionally the symbol rate was doubled for more throughput.  Hal decided that this mode was a bit of overkill for the amateur community and marketed it almost exclusively to commercial interests.  The modems remain expensive, but the increase in throughput was impressive at 500 to 3,000 bps.

In 2011, Clover 2500 was introduced in the late model DSP4100/2K and DSP4200 modems.  The symbol rate was increased even more to 71.125 baud which lead to an increase of bandwidth to 2.5 KHz.  This shows the practical limit of Clover in standard SSB transceivers and seems to be marketed in response to a rival's introduction of a new mode at approximately the same time.  The data rate for Clover 2500 is 625 to 3750 bps.

While there are published reports about the technical specification of Clover on the internet, there are few, if any on-air reports available except those found in relatively old equipment reviews of clover II.

I have fairly extensive experience with Clover II over the last 10 years or so, primarily with my DXP-38 modem.  This was the last modem marketed to amateurs with Clover in it.  It featured a tuning indicator which was omitted on commercial modems which primarily operate on fixed frequency channels and should not require manual tuning.

Clover II operation requires precise tuning, to within 20 Hz for link establishment.  The modem does not have the ability to compensate for frequency drift, but with statistics passed between the modems on a regular basis, the radio can be slowly (no more than 10 Hz at a time) zeroed in to each other.  Amateurs using the mode tend to have a set frequency to meet on (14.065.5 LSB dial is the most common) and fine tuning for rig errors can be done once the link is established. 

Once the link is established, Clover is pretty much on auto pilot.  All the operator has to do is type or send files.  The mode works on automatic overs after certain amounts of data is passed.  The length of these blocks between overs is determined by the amount of data to be sent and can last several seconds.  Overhead and a small amount of keyboard data are sent via short blocks with a very robust waveform while data blocks are longer and will be sent at the highest speed possible as determined by conditions on the other end of the link.

I have also experimented with Clover 2000 several times.  This mode is much more difficult to operate than Clover II, especially with lower power systems such as amateurs often use, and me especially.  The same amount of power is now spread over 2 KHz and 8 tones instead of 4 tones in 500 Hz.  This means that there is a lower average power on the other end of the link requiring more ERP and better conditions.  Clover 2000 is also set up to run with fewer error retries and is very prone to link failure if struck by fading.  Still, when the conditions are right, Clover 2000 will move data very quickly, even at it's more robust waveforms.

Clover is unique in current modes in that statistics for both sides of the link are readily displayed.  Waveform , amount of online error correction used, S/N ratio, frequency offset, throughput, and phase dispersion are all shown to the operator.  The most important to judge the link condition are S/N and PHS (phase dispersion).  The former shows the relative strength of the signal and higher numbers are desirable.  The second shows how much phase distortion is being caused as the waveform is propagated.  lower numbers are better in this case.  Since Clover relies on being able to determine phase shift, higher amounts of dispersion require fewer shifts per tone, and that will mean less throughput.  SN around 30 and PHS in the teens indicate a good link and will allow speedy transfer of data.  PHS in the 40's and S/N in the teens will expect low throughput.  Any worse than those numbers, you can expect link failure or at least numerous error signals. 

For Keyboard chats, using Clover 2000 is both frustrating and wasteful of spectrum.  The enhanced signal conditions that are required coupled with the relatively slow rate typing and reading make the mode difficult to use.  Save this mode for passing traffic.

Clover II is a very nice keyboard mode due to it's speed and semi-duplex nature.  Also the link status panel is of interest to radio enthusiasts.  It can also serve as a relatively fast file transfer mode provided the file isn't very large.

There is a yahoo group dedicated to Clover operating with a small but dedicated following.  So if you see a Hal modem online that has the mode for cheap, have a go at it.  If you think that clover alone isn't worth even the modest used prices of these modems, you can still use them as great RTTY or Pactor I modems.