John,
The first thing I notice is that at the time of the image your MMTTY
demodulator was not set for 2125 Hz Mark. Notice the "2093" in the "Mark"
box in the control panel. This is the frequency to which MMTTY's mark demod
is set (along with 2263, as opposed to the usual 2295, for space, at your
170 Hz shift setting). Thus, the signal is not properly "tuned in" as far as
MMTTY is concerned, even though it might have been crack on 2125/2295.
With AFC on, the demod will follow the signal's mark tone if it moves or as
you tune, as you know. Simply turning AFC off does not set MMTTY's mark
frequency back to the default. You have to do that manually if you want to
(use the down arrow in the mark box).
Also, using 72 taps in the demod filter, as is the case in your image, is
rather broad, but if you want off frequency copy of "clear" signals it's
obviously OK. If your computer power will sustain it, try more taps (sharper
filters) and you'll see quite a difference amidst QRM.
Can't speak at all for the DXP-38 situation.
Interesting comparison, though.
73, John W2KI
================
WA9ALS - John wrote:
>
> If you're interested, please take a look at
> http://www.qsl.net/wa9als/dxp38.gif
>
> This is print from a RTTY signal on 20M awhile ago. It sounded fairly weak;
> Although I didn't get a look at the S-meter, it doesn't look that weak on
> the MMTTY display. The window on the left is MMTTY in default profile, and
> the one on the right is from the DXP38.
>
> They aren't getting the EXACT same audio, because I didn't parallel it.
> Rather the soundcard is getting it from the ACC connector on the rig (fixed
> level), and the DXP is getting audio from the speaker out jack of the rig
> (sensitive to AF gain control).
>
> Anyway, what I find interesting and confusing are the TUNING indicators.
> The DXP was set to delta-f mode and tuned to center. As I recall, each LED
> segment is equal to 50 Hz, so a properly tuned signal in the delta-f display
> should be within 50 Hz. However, look at the MMTTY display (AFC OFF). It
> obviously doesn't agree that the signal is properly tuned. I guess the
> off-peaks amount in the MMTTY display might only represent about 50 Hz, so
> maybe they agree better than I first thought. I didn't get a chance to tune
> to the MMTTY display and see if the DXP was still centered before they
> signed. (It's sure difficult to play with this stuff when noone's on the
> air! HI)
>
> Even less than perfectly tuned, the print in the MMTTY window was sure
> superior.
>
> I don't consider this a fair face-off of the demodulators, but I'd like any
> comments on the tuning displays.
>
> Thanks! 73 - John
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