The TMS470 family of ARM7 microcontrollers contains a RISC coprocessor referred to as the High-End Timer (HET). The HET can implement complex I/O operations independent from the CPU, one of which is the capability to generate PWM signals. This application report describes using this PWM capability, along with an open source voice algorithm called Speex, to encode 8-bit voice messages without the need for extra hardware.
There are many instances in which voice can be used with a microcontroller; e.g., security systems, smoke alarms, and automatic door controls can issue helpful warning messages rather than just annoying beeps. Metering and measuring systems can issue verbal instructions and transmit verbal information back to the users. This document is focused toward organizations that require the power of an ARM7 along with the ability to transmit voice.
This method of digitizing and encrypting voice into a narrow voice-band channel is referred to in this report as a vocoder. Unlike Peter Frampton’s Talk Box or Homer Dudley’s Vocoder, the TMS470 method generates clear and concise sounding voice messages that do not sound computer generated or robot-like.