Audio and Speech Playback

Audio applications that require playback of voice often fall in one of two classes:

• Playback of pre-recorded and stored audio
• Playback of streaming audio

In both cases, the quality of audio depends on both the compression scheme (G.726A, Speex, etc.) used in firmware as well as the hardware peripherals used (Pulse-Width Modulator (PWM), Digital-to-Analog Converter (DAC), etc.) for reproducing the sound. Some applications that require playback of streaming audio may also require encoding of recorded speech data for duplex transmission. In such cases, the processor throughput (MIPS) required by the microcontroller (MCU) or Digital Signal Controller (DSC) is largely dependent on the encoding algorithm. These applications are discussed in detail in the Communications section. In this section, we present the solutions required for playback-only applications in two parts:

• Software Algorithm Libraries
• Hardware Peripheral Options

Note: The MIPS usage statistics shown for algorithms within gray ellipses in the graph above represent the requirements of our implementation of these algorithms on 16-bit PIC MCUs and dsPIC DSCs.

A decision on the algorithm to use is often based on a tradeoff between quality and system cost incurred in saving large amounts of audio data. The table below shows how the various algorithms perform in terms of actual seconds of speech they can store into memory.

Note 1: This is a theoretical maximum value. Since the application program will occupy some of this memory, the actual audio storage capability will be lower than the limits stated here.