Post by thread:Telephone Tone Detection

I'm working on a PIC-based telephone call costing device, that indicates the cost of calls made by recognising the number dialled, looking up the time per unit for that dial-code in a table, using the correct call rate for the time of day, and timing the call. (Why? The devices hired by our telecoms company that rely on metering pulses are expensive & there's a long waiting list for them). I'm having major difficulties reliably establishing when the call actually starts - i.e. when the chap at the other end answers. Sometimes, the line current reverses when the call is answered, and I can use this. But for the other cases, I'm looking at the ringing tone and seeing when it disappears, & assuming the call has been answered. I'm using the CML FX633 Call Progress Tone Detector chip at the moment to do this - it indicates if there's a valid tone (ring, engaged, etc) on the line - (you have to check the cadence in the uP software). But it's too expensive here to make the project feasible. I've tried to build my own filter (400Hz), but without much joy. It definitely lacked the noise immunity & gain control of the i.c. Can anyone offer any advice? Many thanks & happy new year. Rob. ************************************************************* * Save a fortune on international calls, and earn commission* * on people you sign up. Ask me how. * * Rob Bruce-Brand Independent Telephone Consultant * * spam_OUTbruce-brandraTakeThisOuTdurban.gov.za * *************************************************************

> I'm using the CML FX633 Call Progress Tone Detector chip at the > moment to do this - it indicates if there's a valid tone (ring, > engaged, etc) on the line - (you have to check the cadence in the uP > software). But it's too expensive here to make the project feasible. > I've tried to build my own filter (400Hz), but without much joy. It > definitely lacked the noise immunity & gain control of the i.c. If you are trying to detect a single tone, an edge detector with a PIC can do so fairly easily. If you're looking for two tones which are in a close harmonic relationship (e.g. 400:450 = 8:9) you may be able to use pattern matching on the output of an edge detector. If you're trying for the general case, a PIC is powerful enough to handle it with an edge detector for a reasonable number of tones (e.g. PIC16C620 picking up 40ms DTMF tones) but I don't know if my employer would be too happy if I gave away all my secrets. Sorry.

Hello Rob, > indicates the cost of calls made by recognising the number dialled, > looking up the time per unit for that dial-code in a table, using the > correct call rate for the time of day, and timing the call. > (Why? The devices hired by our telecoms company that rely on metering > pulses are expensive & there's a long waiting list for them). In Italy you can call the phone company and ask for their pulse even if you didn't rent their meter. The service is inexpensive (five dollars every two months) and is available two weeks after the request. Are you sure that you have to rent the meter to get the tone? I'd be surprised to hear that the italian Telecom offers something better than the rest of the world. I've built a meter using a PIC, it costs about 40$ (but I've bought some components from RS, their prices are quite high). Bye, Flavio