selecting the best chip

[npp]

how do i decide which is the best chip for detection of DTMF tones?
and what does "early" and "delayed" detection mean??

 

[Steve Underwood]

how do i decide which is the best chip for detection of DTMF tones?
and what does "early" and "delayed" detection mean??

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

 

[Al Clark]

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

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)

 

[Vladimir Vassilevsky]

Al Clark wrote:

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

 

[Joerg]

Hello Vladimir,

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

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