Solutions for Applications requiring Audio & 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
● Hardware Peripheral Options
Software Algorithm Libraries
The choice of software algorithm used to playback audio & 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 &16-bit PIC® MCUs, G.711, Speex & G.726A for PIC24 MCU & dsPIC
DSCs & Speex & ADPCM for PIC32 MCUs.
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.
Algorithm
Memory needed to
store 1 second of
encoded speech
G.711
8 KB
G.726A
2, 3, 4, or 5 KB
Speex
1 KB
How does this affect your choice of MCU, DSC or Memory component?
Use the device below:
To store N seconds of speech in on-chip Flash memory using the
following encoding algorithms:
Device
G.711
G.726A
Speex
25xx1024 Serial
EEPROM
PIC24FJ256GA106 or
PIC24HJ256GP610
dsPIC33FJ256GP710
PIC32MX360F512L
16 sec
25 to 64 sec.
128 sec
32 sec.
52 to 128 sec.
n/a
32 sec.
64 sec.
51 to 128 sec.
102 to 256 sec.
256 sec.
512 sec.
Listed below are several application notes and software libraries useful in audio playback applications:
Audio Functions
MP3 Playback
Playback
Resource Type
App. Note &
Source Code
App. Note &
Source Code
App. Note &
Source Code
Title & Link
AN1367 – Porting the Helix MP3 Decoder onto Microchip’s PIC32MX
32-bit MCUs
AN 538 – Using PWM to generate Analog Output
Supported Families
PIC32 MCU
ALL
Record & Playback
Library
SW300026 -dsPIC/PIC24 G.711 Speech Encoding & Decoding
library
PIC18, PIC24F, PIC24H,
dsPIC30F, dsPIC33F, PIC32
PIC18, PIC24F, PIC24H,
dsPIC30F, dsPIC33F,
Record & Playback
Library
SW300090 : dsPIC G.726A Speech Encoding/Decoding Library
dsPIC30F, dsPIC33
Record & Playback
Library
SW300070: Speex Speech encoding/decoding Library
dsPIC30F, dsPIC33
Record & Playback
Library
dsPIC DSC Equalizer Library
Record & Playback
Library
Audio Library for PIC32MX - supports ADPCM and WAV formats
ALL
Library
dsPIC DSC Automatic Gain Control Library
dsPIC30F,dsPIC33F
ALL
GUI
dsPIC® DSC Speech and Audio Fast Forward (SAFF) tool
dsPIC30F, dsPIC33F
ALL
GUI
Analog Filter & Amplifier Design
Record & Playback
AN643-Adaptive Differential Pulse Code Modulation
dsPIC30F, dsPIC33F
PIC32 MCU
Analog Design
ALL
GUI
Digital Filter Design
dsPIC30F, dsPIC33F
Hardware Peripheral Options
A few variations are available for implementation in hardware based on trade-offs on quality of audio versus system cost, simplicity versus systemintegration etc. Popular methods supported by Microchip’s MCUs and DSCs are shown below:
1. Use on-chip D/A Converter
2. Use on-chip PWM followed by external low-pass filtering
3. Use off-chip Codecs or D/A Converters
Hardware Option 1: Use on-chip D/A Converter
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. Learn more about the 16-bit Audio DAC peripheral in our web
seminar.
Hardware Option 2: Use on-chip PWM followed by external low-pass filtering
The option shown above can be used with many of Microchip’s 8, 16 & 32-bit MCUs and 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
development board and the Speech Playback PICtail Plus accessory.
Hardware Option 3: Use off-chip Codecs or D/A Converters
A representative design exercising this option can be found in the dsPICDEM 1.1 Plus Development Board. Schematics and source code for a
demo are available in the zip file provided with the board.