can anyone help me to do:
how change the PWM voice signal (Square Waves)  to DAC sin Waves??
I use the sunplus 6502 cpu to generation the DTMF voice signal,It can work
use the speaker close the telephone MIC  to send out the DTMF signal.
but I can not connect the DTMF SPK output pin to the phone line directness
I use the oscilloscope to watch the DTMF is a Squre Waves,
but another DTMF IC is the Sin Waves,So how can I exchange the PWM to Sin
Waves???calculus circuit is need??????? how can I do it????????