[NCARC] DTMF

[SCOMIND@aol.com]

Hi Guys,

Those attending today's NCARC meeting heard my presentation on repeater 
audio, and you'll recall we talked briefly about the bad effect that clipping 
and IM (intermodulation distortion) have on DTMF.

Here are the numbers I was trying to remember from an article in 
Communications Magazine:

Suppose a user sends a DTMF "*" (941 and 1209 Hz). Apply the 2A-B intermod 
equation: Twice 1209 is 2418, less 941 is 1477 Hz. So the user is putting in 
1209 Hz and 941 Hz, and he's getting out 941, 1209, and 1477 Hz. Well, 1209 
and 941 are the DTMF "*", and 941 and 1477 are the DTMF "#". So we have the 
components for two DTMF characters at the same time. The DTMF decoder will 
have a hard time with that!

The solution is to keep the audio level of the DTMF keypad below the point of 
limiting.

73,
Bob, WA9FBO


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