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AVR32709: AVR32 UC3 Audio Decoder Over USB - page 2 / 29

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4.

Theory of Operation

4.1

Overview

Today, embedded MP3 decoders are everywhere for consumers listening to audio content on mobile devices.

4.1.1

MP3

MPEG-1 Audio Layer 3, more commonly referred to as MP3, is a digital audio encoding format using a form of lossy data compression. Several bit rates are specified in the MPEG-1 Layer 3 standard: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320Kbit/s, and the avail- able sampling frequencies are 32, 44.1 and 48KHz. A sample rate of 44.1KHz is almost always used. 128 Kbit/s bitrate files are slowly being replaced with higher bitrates like 192Kbit/s, with some being encoded up to MP3's maximum of 320Kbit/s.

A tag in a compressed audio file is a section of the file that contains metadata such as the title, artist, album, track number or other information about the file's contents.

The chosen MP3 decoder here is MAD (libmad), a high-quality MPEG audio decoder. It currently supports MPEG-1 and the MPEG-2 extension to Lower Sampling Frequencies, as well as the so-called MPEG 2.5 format. All three audio layers (Layer I, Layer II, and Layer III a.k.a. MP3) are fully implemented. MAD does not yet support MPEG-2 multichannel audio (although it should be backward compatible with such streams).

4.2

Block diagram

The following block diagram describes the UC3 interfacing the USB stick and output the audio stream from the key to the external DAC. The user can control the player using a keypad, run- ning a customisable Human-Machine Interface (HMI).

O

12 MHz

S

C

11.2896 MHz

Figure 4-1.

Block diagram

USB

Clocks

SDRAM / SRAM

EBI

ABDAC

Amplifier

Head

SSC/I2S

DAC

set

SPI

AT32UC3

GUI

Controller

2

AVR32709

7817D–AVR32–05/11

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