Problems with DTMF Generatiom

[Angus Anderson]

Hi Group-

I have a project going with 2 16F876's talking to each other via PSTN using DTMF tones. I use an MT8870 to decode, and the pbp DTMFout command to generate the DTMF tones.

I am having trouble with DTMF generation - I get flaky decoding with the DTMF tones produced by the 'PIC , but not if I use DTMF tones from a telephone, that works fine. This leads me to think that it's the DTMFtones from PBP that are the cause of the problem. Does anyone have any experience with the quality of the DTMF tones generated by PBP? Could it be because I'm not lowpass filtering the tones properly? I use the filter in the PBP manual (2 caps and 2 resistors) and an opamp to buffer. Any advice would be welcome.

Secondly, can anyone point me toward a supplier that will provide sample quantities of a DTMF generator chip such as the Holtek 9200 or Samsung EFG7189 - one with serial or parallel interface? Nobody in South Africa carries anything like this on the shelf. If I can't get the PIC right, I'll have to go the hardware way. Again, any advice would be welcome

Regards,

Angus Anderson

[b1arrk5]

I got a 16F877 to generate dtmf ok, using the filter shown in the manual, however I had to amplify it to get a decoder to accept tones. I used a 358 op amp set up, non inverting, with a gain of around three, and it worked great. I used the Teridian 75T204 decoder. I can probably dig up the schematic if you need. Hope this helps!

Jerry.

[mister_e]

DTMFOUT work fine with a 20MHZ crystal.

If you already use a 20MHZ crystal, you may need to redesign or improve the low-Pass filter and, probably, increase the amplitude of the signal.

[Angus Anderson]

Thanks Guys-
I'm going to build a 2nd order LPF using an opamp, fiddle with the gain and then see what improvements I get. Thanks for the tips

Angus

[mischl]

i had also some problems a year before with dtmf generation...

at last, i used a 18f242 instead a 16f876 because it is possible to use a 10 mhz oscillator and on pll inside it is 40 mhz.

about filtering, bruce's hint helps me most :
http://www.picbasic.co.uk/forum/showpost.php?p=5399&postcount=3