Digital Signal Processor: Tone Generation

A tone generator is required for a number of transmission system tests. The tones required include TOUCH-TONE® signaling, multifrequency (MF) signaling, a milliwatt source, centralized automatic reporting on trunks (CAROT) responses and CAROT test tones. In addition, a tone consisting of 21 components each having a settable phase and level is required for a fast Fourier transform (FFT)-based system. In all cases, the tone is to be transmitted on a 4-kHz digital channel, with a sample every 125 jus (Nyquist sampling rate of 8 kHz). Programs have been written for the DSP that enable it to function as a tone generator. The following two methods are used: (i) table look-up, and (ii) Maclaurin expansion. The table look-up method consists of storing in read only memory (ROM) the trigometric values of sin(n0), where 0 n N, and N is determined by both frequency granularity requirements and harmonic distortion considerations. At each sampling instant, the value of the 1655 sample is taken from the appropriate location of the ROM table and scaled for its desired level. If more than one tone is desired, each component is independently determined and then all component values added together to form the sample value. The Maclaurin expansion, /x *2 *6