[BC] dtmf tone filter solutions

[WBRadiolists at aol.com]

In a message dated 09/17/2008 1:32:59 PM Eastern Daylight Time, 
RichardBJohnson at comcast.net writes:

> Any attempt to filter [DTMF tones] out
>  will result in a severe loss of significant information
>  right in the middle of the audible frequency range.

A fast-acting, modern DSP device should be able to handle this pretty 
effectively. Sure, there will be some artifacts, but you won't get blasted by the 
tones, nor would you have a "hole" punched in the audio. With a hundred 
millisecond or so delay, you wouldn't get the "burst" of tones while the DSP was 
locking-in, either.

In Ham Radio, many modern rigs have a built-in audio DSP to cut carrier 
heterodynes and other noise. It's extremely effective, "constructing" a very tight 
notch filter right on the undesired tone's frequency. The only noticable 
effect is a slight change in the incoming audio when the offending signal is very 
strong. Similar filters should be quite capable of handing DTMF tones.

Willie...