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Solutions for Applications Requiring 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 (PWM, 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 MCU or 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

The choice of software algorithm used to playback audio and speech signals depends on the compression algorithm used to encode the raw speech data prior to storage. The graph below compares a variety of compression schemes in use today. Microchip provides software libraries to support algorithms that require no payment of royalties, such as IMA ADPCM for 8- and16-bit PIC® MCUs, G.711Speex and G.726A for PIC24 MCU and dsPIC® DSCs, and G.711Speex and ADPCM for PIC32 MCUs.

Software Algorithm Libraries BD
Mean Opinion Score

Note: The MIPS usage statistics shown for algorithms within gray ellipses in the graph above represent the requirements of Microchip’s 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.

AlgorithmG.711G.726ASpeex
Memory needed to store 1 second of encoded speech8 KB2,3,4 or 5 KB1 KB
How does this affect your choice of MCU, DSC or Memory component?
To store N seconds of speech in on-chip Flash memory using
the following encoding algorithms:
Example devices and their code/audio storage capabilityG.711G.726ASpeex
25XX1024 Serial EEPROM (1024 KB of storage)16 Sec25 to 64 sec128 sec
PIC24FJ256GA or PIC24HJ256GP610 (256 KB of storage)32 sec52 to 128 secn/a
dsPIC33EP512MU810 (256 KB of storage)64 sec102 to 256 sec512 sec
PIC32MX360F512L (512 KB of storage)64 sec102 to 256 sec512 sec

Hardware Design Options

A few variations are available for implementation in hardware based on trade-offs on quality of audio versus system cost, simplicity versus system-integration etc. Popular methods supported by Microchip’s PIC MCUs and dsPIC DSCs are shown below:

  • Use on-chip A/D Converter
  • Use off-chip Codecs or A/D Converters
  • Use off-chip Codecs or D/A Converters

Hardware Option 1: Use off-chip Codecs or D/A Converters

Playback option3
This option can be prototyped using the Explorer 16/32 development board and the Speech Playback PICtail™ Plus accessory. Schematics and source code for a demo are available in the zip file provided with the board.

Hardware Option 2: Use on-chip PWM followed by external low-pass filtering

Playback option2
The option shown above can be used with many of Microchip’s 8-, 16- & 32-bit PIC® MCUs and dsPIC® DSCs. PWM peripherals are used to drive speakers. The PWM output is filtered prior to amplification. AN5xx describes this technique in greater detail. This option can be prototyped using the Explorer 16/32 development board and the Speech Playback PICtail™ Plus accessory or Audio PICtail Plus Daughter Board.

Hardware Option 3: Use on-chip A/D Converter

Playback option1
In the option shown here, all hardware except for the speaker and analog amplification circuitry resides within the MCU or DSC. A 16-bit Audio D/A converter peripheral is available in some dsPIC Digital Signal Controllers.