The present invention provides a modulator/generator circuit which includes the following features: a 1200 baud 4-phase differential phase shift keying (DPSK) modulator, a 300 baud frequency shift keying (FSK) modulator, a dual tone multi-frequency (DTMF) generator and 2100/2225 Hz answer tone generators. The DPSK modulator utilizes time-domain filtering techniques. It includes a spectrum controller that shapes the in-band frequency spectrum and attenuates the adjacent channel frequency components to eliminate the conventional requirement of band-pass filters after the modulator. The FSK modulator also utilizes a spectrum controller which, during each data transition, sends out six intermediate frequencies to smooth the frequency changeover. DTMF generation is accomplished by multiplexing two sine-wave counters into a DPSK sinewave look-up ROM. By varying the duty cycle of the multiplexing control signal, the DTMF generator provides a digital pre-emphasis function that boosts the amplitude of the high-band frequencies as referenced to the low-band frequencies. The answer tone section of the circuit is accomplished by sharing portions of the FSK section since the two tones are embedded in the FSK frequencies. A digital DAC is provided for converting the modulated signal (parallel) to a serial bit stream. The modulated signal in analog form is available after a low cost low pass filter.