ST AN2931 Application note

AN2931

Application note

Implementing the ADPCM algorithm in high-density STM32F103xx microcontrollers

Introduction

This application note describes the ADPCM audio firmware codec, and provides a demonstration firmware showing how to play ADPCM files using the high-density STM32F103xx I2S feature with an external DAC.

This application note is based on “AN2739: How to use the high-density STM32F103xx microcontroller to play audio files with an external I2S audio codec”. The features linked to the SPI, I2S and the external DAC will be skipped here because they are already explained in AN2739. It is therefore highly recommended to refer to AN2739 to fully understand this application note.

April 2009

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Contents	AN2931

Contents

1	ADPCM algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	1.1	General overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.2	ADPCM algorithm implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.3	ADPCM algorithm functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6

1.3.1 ADPCM_Encode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2 ADPCM_Decode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2	Implementation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 8
	2.1	Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.2	ADPCM file generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	2.3	ADPCM file loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
	2.4	ADPCM file playing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
3	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
4	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14

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AN2931	List of tables

List of tables

Table 1. ADPCM algorithm functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 2. ADPCM_Encode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 3. ADPCM_Decode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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List of figures	AN2931

List of figures

Figure 1. ADPCM file generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 2. DFU File Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 3. DFU File Manager source browser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 4. DfuSe software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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AN2931	ADPCM algorithm

1 ADPCM algorithm

1.1General overview

Adaptive differential pulse-code modulation, or simply ADPCM, is an audio algorithm for waveform coding, which consists in predicting the current signal value from previous values, and transmitting only the difference between the real and the predicted value.

In plain pulse-code modulation (PCM), the real or actual signal value is transmitted.

The advantage of ADPCM is that the difference between the predicted signal value and the actual signal value is usually quite small, which means it can be represented using fewer bits than the corresponding PCM value.

Depending on the desired quality and compression ratio, the differential signal is quantized using 4 (2 bit), 8 (3 bit), 16 (4 bit) or 32 (5 bit) levels.

Many implementations of the ADPCM algorithm exist. They differ by the quantization and the prediction patterns.

In this application note, we provide a 4-bit quantization algorithm, developed by the Interactive Multimedia Association (IMA), IMA ADPCM.

IMA ADPCM was chosen for several reasons:

●it can operate at different sampling rates between 8 kHz and 44.10 kHz

●it guarantees good quality with low CPU usage and memory size requirements

●it has widespread implementations, for instance in the Windows and MAC operating systems

The IMA ADPCM algorithm is fully described in a document published by the IMA Digital Audio Focus and Technical Working Groups: “Recommended Practices for Enhancing Digital Audio Compatibility in Multimedia Systems” revision 3.

1.2ADPCM algorithm implementation

The IMA ADPCM algorithm provided in this application note is used to encode audio files that have the following specification:

●Audio format: PCM

●Audio sample size: 16 bits

●Channels: 1 (mono)

●Audio sample rate: from 8 kHz to 44.1 kHz

Each 16-bit PCM sample is encoded into a 4-bit ADPCM sample, which gives a compression rate equal to ¼.

The implementation of the IMA ADPCM algorithm consists in two functions, one that codes and the other that decodes the audio samples.

The ADPCM firmware is composed of two files:

a)adpcm.c: it contains the source code of the two ADPCM functions that perform the coding and decoding.

b)adpcm.h: it is the header file of adpmc.c. It should be included in the files where the ADPCM functions are called.

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