AN2812 - Vocoder demonstration using a Speex audio codec on STM32F101xx and STM32F103xx microcontrollers.pdf - Firmware - kaczor1000

AN2812

Application note

Vocoder demonstration using a Speex audio codec

on STM32F101xx and STM32F103xx microcontrollers

Introduction

This application note describes how to implement the codec Speex software on the

STM32F101xx and STM32F103xx microcontrollers to build a vocoder application.

Speex is a free audio codec dedicated to speech encoding and decoding. It provides a high

level of compression with a good quality of sound. That makes it a high-performance

solution for any application using message playback or a voice recorder, like answering

machines, building and home safety systems, intercoms, smart appliances, voice recorders

or walkie-talkies.

October 2008

Rev 2

1/21

www.st.com

Contents

AN2812

Contents

Speex codec overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The vocoder application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1

Vocoder application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2

Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3

Speex codec implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.1

Speex codec settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.2

Speex codec optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.3

Speex codec requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4

Vocoder firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4.1

Playback application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.2

Record and play application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.3

Loopback application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2/21

AN2812

List of tables

Table 1.

Speex implementation requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Table 2.

Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3/21

List of figures

AN2812

List of figures

Figure 1. STM32F101xx and STM32F103xx vocoder hardware description . . . . . . . . . . . . . . . . . . . . 6

Figure 2. OLIMEX STM32F103-STK audio input/output stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 3. Playback application flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 4. Record & Play block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 5. Main record application flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 6. Main play application flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 7. TIM3_IRQHandler flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 8. Real-time loopback principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 9. Loopback flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 10. TIM2_IRQHandler flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4/21

AN2812

Speex codec overview

The Speex codec is an open-source, patent- and royalty-free software dedicated to speech

compression and decompression.

Speex is based on CELP (code-excited linear prediction) and designed to compress voice at

bitrates ranging from 2 to 44 kbps.

The features of Speex include:

– Narrowband (8 kHz), wideband (16 kHz), and ultrawideband (32 kHz) compression in

the same bitstream

– Intensity stereo encoding

– Packet loss concealment

– Variable bitrate operation (VBR)

– Voice activity detection (VAD)

– Discontinuous transmission (DTX)

– Fixed-point port

– Acoustic echo canceller

– Noise suppression

Note that Speex has a number of features that are not present in other codecs, such as

intensity stereo encoding, integration of multiple sampling rates in the same bitstream

(embedded coding), and a VBR mode.

For more information about the Speex codec, please refer to the Speex website:

www.speex.org .

Note:

This application note was developed with the release 1.2rc1 of the Speex codec.

5/21

AN2812 - Vocoder demonstration using a Speex audio codec on STM32F101xx and STM32F103xx microcontrollers.pdf

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