Doc ID16895 Rev 2

19/24

AN3126

Application examples

23

2.2.2

Audio wave file specifications

This application assumes that the .WAV file to be played has the following format:

•

Audio Format: PCM (an uncompressed wave data format in which each value

represents the amplitude of the signal at the time of sampling)

•

Sample rate: may be 8000, 11025, 22050 or 44100 Hz

•

Bits Per Sample: 8-bit (Audio sample data values are in the range [0-255])

•

Number of Channels: 1 (Mono)

2.2.3

.WAV file format

The .WAV file format is a subset of the Resource Interchange File Format (RIFF)

specification used for the storage of multimedia files. A RIFF file starts with a file header

followed by a sequence of data chunks. A .WAV file is often just a RIFF file with a single

"WAVE" chunk consisting of two sub-chunks:

1.

a fmt chunk specifying the data format

2.

a data chunk containing the actual sample data.

The WAVE file format starts with the RIFF header: it indicates the file length.

Next, the fmt chunk describes the sample format, it contains information about: Format of

the wave audio : (PCM/...), Number of channels (mono/stereo), sample rate (number of

samples per seconds : e.g., 22050), and the sample Data size (e.g. 8bit/16bit). Finally, the

data chunk contains the sample data.

2.3

Audio wave player implementation

The Audio wave player application is based on the SPI, DMA, TIM6, and DAC peripherals.

At start up, the application first uses the SPI to interface with the MicroSD card and parses

its content, using the DOSFS file system, looking for available .wav files in the USER folder.

Once a valid .wav file is found, it is read back though the SPI, and the data are transferred

using the CPU to a buffer array located in the RAM. The DMA is used to transfer data from

RAM to DAC peripheral. TIM6 is used to trigger the DAC which will convert the Audio digital

data to an analog waveform.

Before the audio data can be played, the header of the WAV file is parsed so that the

sampling rate of the data and its length can be determined.

The task of reproducing audio is achieved by using sampled data (data contained in the

.WAV file) to update the value of the DAC output, this data is coded in 8 bits (with values

from 0 to 255),

The DAC Channel 1 is triggered by TIM6 at regular interval specified by the sample rate of

the .WAV file header.

The .WAV files are read from the MicroSD Memory using a DosFS file system.

In the Demo code, code files handling the waveplayer demo are:

waveplayer.c

waveplayer.h

The wave player demo is called using WavePlayerMenu_Start() function which has the

flowchart shown in Figure 16.