[RTTY] Shift 170 -vs- 200

[Kok Chen]

On Jun 9, 2011, at 9:16 AM, WS7I wrote:

> Also we later used "real" TU's like Hal ST-6000/ST-8000 as devices
> behind the PK232 and strangely enough I am going to try that again
> with N1MM one of these days.

I have actually never used the Timewave 599zx or the ST-8000 in a standalone manner.  I had always used them as a receiving regenerator for the KAM Plus, and using the KAM Plus for transmissions.  

The reason for it is that I'd never bothered to write software to support these two TUs, while I had already written software to use the KAM on Mac OS 9 and Mac OS X.

When a TU is used as a receiving regenerator for the KAM, you essentially get the demodulator performance of the TU because the regenerated AFSK signal is clean enough that the KAM can decode with no additional error.

The ST-8000 and Timewave 599zx can similarly be used as receiving front ends to sound card modems.  

I.e., if you already have a sound card modem that is running properly, you can check how a ST-8000 or Timewave 599zx compares to it by just using the TUs as receiving regenerators for the sound card modem. 

Absolutely no extra software of any kind needed, and you can likewise transmit through your current sound card modem and not to have to go through the ST-8000.  (Unless for some reason you need the current loop from the ST-8000.)

Just think of the regenerating TU as a filter that is placed between the receiver output and the input sound card of the software modem.

One of the neat things about the Timewave is that with a single button press, you can turn it into a pass-thru mode (you can't do that with the ST-8000; there are myriads of dip switches inside for configuration :-).  

With the Timewave in pass-thru mode, the sound card modem does the hard lifting.  You can very rapidly switch between the two demodulators.  But a better comparison methodology is to run two sound card modems (or a software modem that can print from two audio streams concurrently) with the receiver audio output split to feed both the TU and one of the sound card modems.  The second sound card modem takes its audio from the regenerated AFSK of the TU.

The "objective" way to measure is of course to use AFSK from an HF Channel Simulator, instead of using off-the-air signals.  There are free HF CHannel Simulators for Windows (pathsim) and for Mac OS X (cocoaPath), so there is no excuse anymore not to use them.

One huge advantage software modems have over hardware modems is that it takes less time to test.  You can play back a recorded signal at very high sampling rates, or modify an HF channel simulator to run at very high sampling rates.  And if you modify the software modem to also sample at the same higher sampling rate, instead of having to collect half an hour of real-time data, you can do the equivalent measurement pass in a minute or less.

73
Chen