Datasheet

11
Software and Hardware Solutions for the 32-bit Designer
ADPCM and Speex (Audio) Library for
PIC32 Microcontrollers
Summary
The audio library for PIC32 MCUs consists of APIs for
Pulse Code Modulation, Adaptive Differential Pulse Code
Modulation and Speex encoding and decoding algorithms.
Speex is a Code Excited Linear Prediction (CELP) based,
open source, patent-free audio compression format designed
for speech. The ADPCM algorithm takes advantage of the
high correlation between consecutive speech samples,
which enables future sample values to be predicted.
Key Features
■ Free software/open source, patent- and royalty-free
■ Portable across all PIC32 microcontrollers
■ Supported encoding formats: PCM
(raw, uncompressed), IMA ADPCM, Speex
■ Implements an audio player behavior with play, record,
pause, stop functionality
■ Supports standard input/output stream formats:
Wave, Ogg for Speex, as well as a raw format containing
just data
■ Supports various sampling rates in both play and
record mode
■ Supports narrowband (8 kHz) and wideband (16 kHz)
bit-streams for Speex
■ Provides information about the missed samples
■ User-selectable ratio between the sampling frequency
and the play/record frequency
Applications
■ Answering machines
■ Building and home safety systems
■ Intercoms
■ Smart appliances
■ Voice recorders
■ Walkie-talkies
■ Toys and robots
■ Any application using message playback
Sample Rate Conversion Library
Summary
The Sample Rate Conversion (SRC) library, which can be
used with Microchip’s PIC32 and dsPIC33E families of
devices, provides the ability to upconvert the sampling rate
of real-time 16-bit stereo audio data. At run-time, the input
sampling rate can be selected between 32 kHz or
44.1 kHz, with a fixed output sample rate of 48 kHz.
Up-sample FIR Filter
Down-sample
Polynomial
Interpolation
A
udio
Data In
Poly-phase Filtering
SRC Audi
o
Data Out
Key Features
■ Stereo 16-bit audio sample rate conversion library
■ Two common audio modes:
• 32 kHz to 48 kHz conversion
• 44.1 kHz to 48 kHz conversion
■ Low MIPS and resource requirement
■ Both PIC32 MCUs and dsPIC33E DSCs supported
■ Can be used with low-cost DAC with limited sample
rate capability
■ Designed for streaming audio applications
Performance and Resource Consumption for PIC32
Library Modes
SRC
Library
Version
SRC Mode MIPS
Code Size
(bytes)
Data Size
(bytes)
SNR
(dB)
Lite
Version
32k to 48 kHz 27.4
5684 1284
82
44.1k to 48 kHz 30.1 82
Full
Version
32k to 48 kHz 33.5
5760 1364
84
44.1k to 48 kHz 36.7 82
Note: Tested with a 1 kHz full-scale sinusoidal signal.