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Hi Chas,
> Shouldn't the touch tone pad tone be set at the maximum deviation or
> nearly so? No, and here's the explanation:
1. The pre-emphasis in your transmitter causes the tone with the higher
freq to deviate more than the other tone (for example, the 1477 Hz
tone will deviate more than twice as much as the 697 Hz tone).
If the DTMF level is too high, its waveform will be clipped in the
transmitter -- and the higher-freq tone will be clipped more
than the lower-freq tone because of the pre-emphasis.
At the receiving end, de-emphasis restores flat response --
but it causes the higher-freq tone to now be lower in level than
the other tone. If this "twist" (difference in level between the two tones)
is great enough, the decoder won't recognize the DTMF as valid.
2. You can end up with 'DTMF intermod'. The star (*) is 941
Hz plus 1209 Hz. If you multiply 1209 by two and subtract 941, you get
1477. The pound (#) is 941 Hz and 1477 Hz. That means if the system
has distortion, you can end up with the decoder seeing (*) and (#) at the same
time.
So, it's important to keep the DTMF level below the transmitter's
clipping point.
73,
Bob
Bob Schmid,
WA9FBO, Member S-COM, LLC PO Box 1546 LaPorte CO 80535-1546 970-416-6505 voice 970-419-3222 fax www.scomcontrollers.com YAHOO! GROUPS LINKS
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