Audio data encoding and storing

AN3143

8/13

Doc ID 17011 Rev 1

2.2

External memory used for audio data storing

The micro SD card is used for demonstration purposes in this application note for storing

audio bitstreams. Any other external memory with an interface that is supported by STM8S

can be used instead.

The SD card can be accessed immediately by the operating system if it is formatted by a

supported file system i.e. File Allocation Table (FAT) filing system. If the SD card is FAT

formatted, the microcontroller firmware must also support the FAT file system. Unfortunately,

FAT implementation on an 8-bit microcontroller uses too many resources and the footprint is

too large for a low-cost oriented microcontroller application. Moreover, it is usually not

needed as the audio content of the card is often changed only once. A more convenient way

of putting audio data on the SD card is to use specific software which has an embedded

driver that bypasses the operation and file system drivers, therby allowing direct SD card

access.

A hex editor with such direct access feature can be used for modification of the unformatted

SD card content. One of the available and tested hex editors can be found in Maël Hörz (see

Section 5).

When the compressed ADPCM data is written to the SD card the MCU firmware can directly

read the compressed data samples from the SD card sector by sector. The standard SD

card has fixed sector size of 512 Bytes.