A 16-Bit Microcomputer for Audio Bandwidth Digital Filtering

Small microcomputers using 8-bit machines have been used in teaching digital filtering and discrete-time control. These systems have two major limitations: 1) bandwidth limited to approximately 125 Hz for fourth-order algorithms, and 2) limited precision for representing the coefficients in the algorithm. This paper describes the development and application of a high-speed 16-bit machine which largely overcomes the limitations of previous systems. A special-purpose digital filtering and control computer was built using the MC68000 microprocessor operating at 12.5 MHz. This machine uses 16-bit data transfers to memory and 32-bit arithmetic. The multiply and divide instructions are microcoded in the instruction set, and therefore execute much faster than subroutines used in 8-bit machines. The system has a general-purpose monitor routine and a digital filtering algorithm in firmware, user RAM, two channels of A/D, two channels of D/A, a programmable timer, and an RS232 interface for a CRT. The firmware implements any filtering or compensation algorithm through eighth order in the form of a linear difference equation. The greater precision of the 16-bit machine allows the coefficients in the difference equation to be represented accurately without extensive rationalization procedures. The firmware is written for student use in digital filtering and control laboratories. The computer prompts the user for coefficients and the sampling interval, informing him of any input errors. It then continuously runs the selected algorithm until interrupted. This system has been found to be of much wider bandwidth than previous machines. Signals of 4.