selecting the best chip

Early detection means producing a DTMF event before the length of the digit, and its true validity, can be tested. 10-15ms of tone seems to be the typical point at which these early detectors trigger. There are two reasons I can think of to want early detection. The first is to mute the channel, so the DTMF is not passed through. The second is to create things like RFC2833 events for VoIP.

Delayed detection is the normal form, where the true validity of the digit is tested before declaring it to exist. This can be expected to be considerably more resistant to false indications than the early form.

DTMF "chips" don't usually offer these features. DTMF software packages to run on general purpose DSPs increasingly do.

Regards, Steve

Steve Underwood wrote in news:dqksgc$slq$1 @home.itg.ti.com:

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The one point that Steve didn't make is that virtually any current DSP has plenty of MIPs to perform DTMF detection. If you are using a programmable DSP, you would pick the DSP for other reasons, (cost, support, additional software requirements, power consumption, etc)

There is no need for DSP. Any microcontroller will do the DTMF detection algorithm. The lazy folks can use the traditional 8870 clones.

VLV

Hello Vladimir,

For those interested in how to squeeze that into a bare bones uC look at TI app note SLAAE16. It's under MSP430.

Regards, Joerg