1. A non-linear differential pulse code modulation signal processing apparatus comprising: delta modulator means for converting an analog input signal to a delta modulated signal having a clock frequency determined by clock pulses supplied thereto; digital filter circuit means for receiving the delta modulated signal, removing quantizing noise components that fall outside a predetermined transmission band from the delta modulated signal, and for converting the delta modulated signal into a differential pulse code modulation signal; a direct feedback pulse code modulation encoder that operates at a sampling frequency equal to the clock frequency divided by a positive integer for converting the differential pulse code modulation signal into a non-linear differential pulse code modulation signal, including subtractor means for determining the difference between a decoded digital signal and the differential pulse code modulation signal from the digital filter means, a digital integrator means for integrating the output of the subtractor means, a digital encoder means for converting the output of the integrator means to a non-linear differential pulse code modulation signal, and a digital decoder means for converting the non-linear differential pulse code modulation signal to the decoded digital signal supplied to the subtractor means; and means for supplying clock pulses to the delta modulator means, the digital filter means, and the direct feedback pulse code modulation encoder.