Post by thread:DTMF generation on digital output

Perusing, I've found one DTMF generation program for the PIC, by Stephen Hardy of Canberra. While this uses an interesting and heretofore unknown to me method of sine wave generation, it expects a DAC (typically 8 bits) to generate the summed sinewaves. I notice that the Basic Stamp 2 has a BASIC command (DTMFOUT) to generate DTMF via a single digital output (aka 1 bit DAC), which it says can be turned into proper waveforms with filtering. (It also has a command to output one or two arbitrary tones). My question is: How is this done? I can see that a single tone can be done via a square wave (and attenuating the 3, 5'th etc harmonics would approximate a sine wave). But directly summing two square waves of equal amplitude and different frequency would take a 3 level output. I don't see how to reduce it to 2 levels and keep the desired dual tones even approximately. My goal would be to transmit DTMF audio (via low powered radio) from a PIC (not Stamp) monitoring a pumphouse, to be decoded with a standard DTMF receiver chip (and a radio receiver) a few hundred feet away. It would be nice if I could avoid using a DTMF encoder chip (or 8 bit DAC) at the pumphouse end. Zhahai @ Zhahai Stewart spam_OUTzhahaiTakeThisOuThisys.com @ A Meme Gardener http://rainbow.rmii.com/~hisys/zhahai.html @ Standard Disclaimer YMMV - Your Maya May Vary

> I notice that the Basic Stamp 2 has a BASIC command (DTMFOUT) to > generate DTMF via a single digital output (aka 1 bit DAC), which > it says can be turned into proper waveforms with filtering. (It > also has a command to output one or two arbitrary tones). > > My question is: How is this done? > > I can see that a single tone can be done via a square wave (and > attenuating the 3, 5'th etc harmonics would approximate a sine wave). > But directly summing two square waves of equal amplitude and > different frequency would take a 3 level output. I don't see how to > reduce it to 2 levels and keep the desired dual tones even > approximately. There are a number of methods for this. Probably the simplest to understand is something like the following: Loop: bcf PORTB,0 btfsc Phase1,7 bsf PORTB,0 movf Freq1,w addwf Phase1 nop movf Freq2,w addwf Phase2 bsf PORTB,0 btfss Phase2,7 bcf PORTB,0 decfsz OverallTime goto Loop When both waves are high, the output is [almost] constantly high; when both are low, the output is [almost] constantly low. When the two waves are opposite each other, the output is a 50%-duty-cycle very rapid square wave.

> From: Zhahai Stewart <.....zhahaiKILLspam@spam@HISYS.COM> > > Perusing, I've found one DTMF generation program for the PIC, > by Stephen Hardy of Canberra. While this uses an interesting and > heretofore unknown to me method of sine wave generation, it > expects a DAC (typically 8 bits) to generate the summed sinewaves. > > I notice that the Basic Stamp 2 has a BASIC command (DTMFOUT) to > generate DTMF via a single digital output (aka 1 bit DAC), which > it says can be turned into proper waveforms with filtering. (It > also has a command to output one or two arbitrary tones). No need to use PWM: you can construct a quick-and-dirty DAC by outputting the sine values to, say, 4 bits of port B. Use a binary weighted resistor network (say 4.7K, 10K, 22K and 39K from the MSB to LSB) and feed the output into the summing node of an op-amp or even a common emitter transistor. When amplified, the tone sounds awful, but the telecom equipment doesn't seem to mind. I successfully dialled from my phone by running the resistor network directly into a pair of headphones held up to the phone. Entirely PIC-powered! Regards, SJH Canberra, Australia