Specifications

4.2.5 Embedded MP3 decoding technology

The available MP3 decoder technology seems rather simple compared to the networking

solutions. Most of the chips available take data serially or through parallel and output

the analogue audio. Some suitable chips for this project are as follows:

MICRONAS [14] make the MAS 3587F MP3 decoder chip. MP3 data is provided to it

through a serial or parallel interface to a micro-controller and then it decodes it into

digital audio signals. The signals are then converted to analogue audio with its own

digital to analogue converter. It comes in a package with a lot of pins allocated to its

ability to interface with many external devices like flash cards and its 8 bit parallel

interface. Unfortunately its 64 pin package makes it difficult for development.

VLSI [13] make the VS1001k audio decoder chip which has the same basic

functionality as the MICRONAS, but it comes in a larger package with less pins. It also

only has a serial interface and can only interface with a microcontroller and no interface

to flash card or other devices is provided greatly reducing its package size.

Atmel [11] make the AT8xC51SND1A which is an Atmel 8015 microcontroller with a

MP3 decoder on the same package. It comes with more RAM than the normal 8015 and

is in an 80 pin package. It’s a very new product from atmel with very little development

support available at the moment but in coming weeks a development environment is to

be released.

It is important to consider which decoder is best used with the microcontroller (Atmel

ATmega103) chosen. Both the MAC and the VSLI have been shown to work in the

YAMPP [19], but the makers of this YAMMP provide the design and PCB layout for

the VLSI. They also sell what they call the “piggy back board”, which is a small PCB

with the VLSI and its required supporting hardware already constructed. To use this

board it only has to be connected via a bus to the microcontroller. The VLSI contains all

the analogue audio processing (filtering and amplifying) required to get Line Out level

audio. As far as the maker of the YAMMP is concerned, both decoders produce similar

audio quality. It is capable of decoding all possible MP3 bit rates (including variable bit

rate) and sample frequencies. It also uses a simple and familiar SPI interface. The fully

construct piggy back board is shown in figure 7 which was used in the project.