According to the exemplary embodiments, a voltage or current-controlled
oscillator is controlled in frequency by the signal (i.e., voltage or
current) from a microphone. The frequency modulated signal is applied to a
direct digital discriminator that produces a digital representation of the
instantaneous frequency at the desired speech sampling rate. The digital
discriminator may be formed, for example, by applying the oscillator
signal to a direct phase digitizing circuit along with a reference
frequency and calculating a sequence of instantaneous phases of the
oscillator relative to the reference frequency. The phase sequence is then
applied to a digital phase locked loop (or otherwise numerically
differentiated) to generate a sequence of binary words representative of
instantaneous frequency and therefore representative of the speech
waveform. Since the low-level speech waveform substantially does not enter
the integrated circuit except as a high-level frequency-modulated carrier,
the technique is immune to noise caused by high speed random logic
circuits such as microprocessors and DSPs operating on the chip.
