On Jul 27, 2010, at 7/27 6:45 AM, WS7I wrote:
> FWIW the Hal units DXP-38 DSP4100 play very well.
Back in the days when HAL only made PCI based cards for amateurs and
needed MS-DOS to run, I bought the stand-alone DSP-4100 to get a taste
of what HAL modems are like. (I bought a used ST-8000 much later.)
But I did later get some "face time" with the DXP-38 when HAL
graciously sent me one to play with back in early 2002.
It is less than 9 years ago but already feels like aeons -- ham modems
have evolved as rapidly as the computer industry in the last 10
years :-).
I sent HAL a report of my impressions when I returned the loaner after
a few months.
This is a portion of the comments that I sent back to HAL:
>> The DXP-38 actually performed very well with weak and clean
>> signals. It is about on par with the Timewave/KAM combination,
>> with the Timewave in remodulation mode. It beats the Timewave/KAM
>> with the Timewave in RTTY Filter mode, and definitely beats the
>> standalone KAM-Plus.
>>
>> However, the Timewave Remodulator comes out ahead with FSK
>> which exhibits transpolar or equatorial flutter. In a certain
>> case with a fluttery FO5, the Timewave provided much better print
>> than the DXP-38. The same with signals through the pole (most
>> of Europe) when there is significant flutter.
>>
>> How much better print? Well, I probably would not have lost
>> a single contact if I only had the DXP-38, and no Timewave on hand.
>>
>> That is to say that when the DXP-38 starts misprinting badly, the
>> Timewave would also be misprinting. Just that I can get a slightly
>> cleaner print from the Timewave.
>>
>> I can pull more callsigns cleanly with the Timewave. If I can print
>> a DX coming back to me on the Timewave, I am pretty sure I can
>> eventually print him on the DXP-38, if the DX repeats my call a
>> few more times.
The "weak and clean" signal mentioned above is probably something that
is close to the so called AWGN conditions (i.e., very little Rayleigh
propagation effects).
The "RTTY Filter mode" uses the Timewave's analog AGC and DSP filter,
but uses the demodulator in the KAM. The "remodulation mode" uses the
entire Timewave chain, including the DSP demoulator.
The subjective comments above was done way before I'd built my own HF
Channel Simulator to test modems with. Nowadays, I can make
repeatable comparisons much more objectively (as long as one trusts
propagation models which HF channel simulators use :-).
I also mentioned this to HAL at the time:
>> ... I do not notice any observed difference in performance
>> between the DXP-38 and my HAL DSP-4100, which has been the secondary
>> modem I have been using for a while now.
Which made the DXP-38 a pretty good bang for the buck at that moment
in time, if you remember what the DSP-4100 cost then.
Another thing that I had mentioned to HAL about the DXP-38 is that it
can really benefit from a better tuning indicator. One of its tuning
modes is fast and imprecise, the other mode is more precise but the
indicator is slow.
73
Chen, W7AY
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