[RTTY] HAL ST-8000 and 6000 vs. Sound Cards

[Kok Chen]

On Jan 28, 2005, at 5:40 PM, w4gkm at juno.com wrote:
> I sure would like to get some input as to what everyone thinks about 
> the HAL ST-8000 vs. sound cards.  If you have used the HAL and the 
> sound card what if any difference to you see?  I have used them side 
> by and side and see little if any difference.  At times one will print 
> when the other won't.

I have a very old ST-8000 (S/N in the low 100's), so the following 
comments may or may not apply to anyone else.  Also, I have only 
compared it against the only software modem I can run (cocoaModem), and 
the following are just my own opinions.  I had bought this particular 
ST-8000 a little after the original owner had it checked out by HAL in 
January 2002 just so I have a gold standard to compare software 
algorithms against, but I didn't start writing a software modem until 
about the middle of 2004.

The first thing to be mindful of is that ST-8000 comes with a very good 
audio AGC circuit which contributes to its 85 dB dynamic range.  If you 
have a 16-bit sound card, you may have to "ride the gain" of the A/D 
converter between very strong and very weak stations to match that.  If 
your A/D converter is capable of greater than 100 dB of dynamic range 
(noise floor to clipping), then you should have better headroom than 
the ST-8000.

Be mindful, though that you will only get this maximum dynamic range in 
the digital world if you had carefully adjusted the audio gain from the 
rig so that the loudest possible signal _barely_ does not clip the A/D 
converter of the "sound card."  Otherwise, just use the "armstrong" 
method and install a gain pot in the audio line.

Many of the gripes you hear about software modems not working well is 
likely to be because the audio level is not properly adjusted or there 
is something else wrong with the audio chain (hum and harmonics of 
50/60 Hz, for example).  After connecting the audio chain up, use a 
software spectrum analyser to look at the rig's output to make sure 
there are no spurs or harmonics of the power line that is stronger than 
-80 dB below the strongest signal.  Most "sound cards" do not come with 
balanced audio input (which both the HAL ST-8000, and a HAL DSP-4100 
which I also own, come with), so you may have to play around a bit to 
fashion a low noise audio chain.

A software modem should be able to out-shoot the ST-8000's rather 
primitive multipath circuitry, assuming they'd made the effort to do 
so.  The circuit in my old ST-8000 does not do much more than make sure 
that the mark and space demodulators outputs do not overlap.  There is 
no true equalizer in the line.

cocoaModem uses 9 separate demodulators, each with a slightly different 
delay line between the mark and space signals and also slightly 
different automatic threshold control (ATC, or adaptive slicer) time 
constants.  The former is to counter the case where mark and space 
signals arrive at different times and are overlapping, and the latter 
is to lessen the harm done to auto thresholding by very rapid (of the 
order of 25 ms) QSB/flutter signals.

The outputs of these demodulators then enter a soft decoder which does 
a majority vote on the bit patterns and the result goes to a character 
sync algorithm (just looking for the start bit is quite error prone) 
and then to the Baudot-to-ASCII translator to be printed.  The behavior 
of the multiple modems is similar to watching the output of two modems 
and sometimes one would print correctly and sometime the other would 
print correctly -- even a KAM Plus occasionally prints a character that 
is misprinted by the ST-8000.  In the case of cocoaModem, I have 9 
modems with slightly different characteristics watching the same signal 
and then doing a majority vote so you only see a single character 
stream coming out of it.  (The "squelch" in cocoaModem is not based on 
signal strength, but is a threshold on how many of the different modems 
agree with one another -- maximum squelch threshold means that all 9 
modems have to agree perfectly to print, minimum squelch threshold 
prints everything.)

> The reason for this is that I am thinking of going with the sound 
> cards with a SO2R to cut down on some of the equipment in the radio 
> room and thought that would be a good place to start but didn't want 
> to comprise the printing.

I can't vouch for the software that you are using, but I myself have 
stopped using my ST-8000 and Timewave 599zx and have been exclusively 
using only software modems for half a year now.

In addition to all of the above, there can be other differences in our 
set ups -- I use a couple of 24-bit A/D converters (M-Audio Transit and 
M-Audio Quattro) together with the MacOS's floating point sound 
library; all my filters are also implemented in floating point.  I only 
use recursive filters in places that don't impact demodulation; the 
rest are linear phase FIR filters.

Bottom line: your mileage may vary :-).  But from my one person's 
experience, the potential of the software modem is definitely there.

73
Chen, W7AY