AN2931
Application note
Implementing the ADPCM algorithm
in high-density STM32F103xx microcontrollers
Introduction
This application note describes the ADPCM audio firmware codec, and provides a
demonstration firmware showing how to play ADPCM files using the high-density
STM32F103xx I
2
S feature with an external DAC.
This application note is based on “AN2739: How to use the high-density STM32F103xx
microcontroller to play audio files with an external I
the SPI, I
2
S and the external DAC will be skipped here because they are already explained
2
S audio codec”. The features linked to
in AN2739. It is therefore highly recommended to refer to AN2739 to fully understand this
application note.
April 2009 Doc ID 15349 Rev 2 1/15
www.st.com
Contents AN2931
Contents
1 ADPCM algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 General overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 ADPCM algorithm implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 ADPCM algorithm functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 ADPCM_Encode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 ADPCM_Decode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Implementation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 ADPCM file generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 ADPCM file loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 ADPCM file playing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2/15 Doc ID 15349 Rev 2
AN2931 List of tables
List of tables
Table 1. ADPCM algorithm functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 2. ADPCM_Encode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table 3. ADPCM_Decode function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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List of figures AN2931
List of figures
Figure 1. ADPCM file generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 2. DFU File Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 3. DFU File Manager source browser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 4. DfuSe software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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AN2931 ADPCM algorithm
1 ADPCM algorithm
1.1 General overview
Adaptive differential pulse-code modulation, or simply ADPCM, is an audio algorithm for
waveform coding, which consists in predicting the current signal value from previous values,
and transmitting only the difference between the real and the predicted value.
In plain pulse-code modulation (PCM), the real or actual signal value is transmitted.
The advantage of ADPCM is that the difference between the predicted signal value and the
actual signal value is usually quite small, which means it can be represented using fewer
bits than the corresponding PCM value.
Depending on the desired quality and compression ratio, the differential signal is quantized
using 4 (2 bit), 8 (3 bit), 16 (4 bit) or 32 (5 bit) levels.
Many implementations of the ADPCM algorithm exist. They differ by the quantization and
the prediction patterns.
In this application note, we provide a 4-bit quantization algorithm, developed by the
Interactive Multimedia Association (IMA), IMA ADPCM.
IMA ADPCM was chosen for several reasons:
● it can operate at different sampling rates between 8 kHz and 44.10 kHz
● it guarantees good quality with low CPU usage and memory size requirements
● it has widespread implementations, for instance in the Windows and MAC operating
systems
The IMA ADPCM algorithm is fully described in a document published by the IMA Digital
Audio Focus and Technical Working Groups: “Recommended Practices for Enhancing
Digital Audio Compatibility in Multimedia Systems” revision 3.
1.2 ADPCM algorithm implementation
The IMA ADPCM algorithm provided in this application note is used to encode audio files
that have the following specification:
● Audio format: PCM
● Audio sample size: 16 bits
● Channels: 1 (mono)
● Audio sample rate: from 8 kHz to 44.1 kHz
Each 16-bit PCM sample is encoded into a 4-bit ADPCM sample, which gives a
compression rate equal to ¼.
The implementation of the IMA ADPCM algorithm consists in two functions, one that codes
and the other that decodes the audio samples.
The ADPCM firmware is composed of two files:
a) adpcm.c: it contains the source code of the two ADPCM functions that perform the
coding and decoding.
b) adpcm.h: it is the header file of adpmc.c. It should be included in the files where
the ADPCM functions are called.
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ADPCM algorithm AN2931
1.3 ADPCM algorithm functions
Ta bl e 1 describes the ADPCM functions.
Table 1. ADPCM algorithm functions
Function name Description
ADPCM_Encode Encodes a 16-bit PCM sample into a 4-bit ADPCM sample.
ADPCM_Decode Decodes a 4-bit ADPCM sample into a 16-bit PCM sample.
1.3.1 ADPCM_Encode function
Ta bl e 2 describes the ADPCM_Encode function.
Table 2. ADPCM_Encode function
Function name ADPCM_Encode
Prototype uint8_t ADPCM_Encode(int16_t sample);
Behavior description Encodes a 16-bit PCM sample into 4-bit ADPCM sample
Input parameter Sample: a 16-bit PCM sample
Output parameter None
Return parameter The encoded ADPCM 4-bit sample loaded into a byte
The ADPCM_Encode function returns a byte containing the 4-bit ADPCM sample. The
software has to store every two ADPCM samples into one byte to save memory space.
Example:
//Input: pcm_sample1 and pcm_sample2; two 16-bit PCM samples.
//Output: adpcm_byte; two 4-bit ADPCM samples stored into one byte.
uint8_t code;
/* Encode the first 16-bit sample */
code = ADPCM_Encode(pcm_sample1);
/* Store the first 4-bit sample */
adpcm_byte = code;
/* Encode the second 16-bit sample */
code = ADPCM_Encode(pcm_sample2);
/* Store the second 4-bit sample */
adpcm_byte |= (code << 4);/* The adpcm_byte contains two 4-bit ADPCM
samples */
1.3.2 ADPCM_Decode function
Ta bl e 3 describes the ADPCM_Decode function.
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AN2931 ADPCM algorithm
Table 3. ADPCM_Decode function
Function name ADPCM_Decode
Prototype int16_t ADPCM_Decode(uint8_t code);
Behavior description Decodes a 4-bit ADPCM sample into a 16-bit PCM sample
Input parameter
Code: an 8-bit data item whose 4 LSBs contain the encoded ADPCM
sample
Output parameter None
Return parameter A 16-bit PCM sample
The input of the ADPCM_Decode function is a byte that contains the 4-bit ADPCM sample.
The software has to extract the 4-bit ADPCM data and store them into a byte before calling
the ADPCM_Decode function.
Example:
//Input: adpcm_byte; two 4-bit ADPCM samples stored into one byte.
//Output: pcm_sample1 and pcm_sample2; two 16-bit PCM samples.
uint8_t code;
/* Extract the first ADPCM 4-bit sample */
code = (adpcm_byte & 0x0F);
/* Decode the first ADPCM sample */
pcm_sample1 = ADPCM_Decode(code);
/* Extract the second ADPCM sample */
code = (adpcm_byte >> 4);
/* Decode the second ADPCM sample */
pcm_sample2 = ADPCM_Decode(code);
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Implementation example AN2931
2 Implementation example
2.1 Description
You can run the provided example on the STM3210E-EVAL board. It is a typical audio
application that consists in:
● first encoding a PCM file into the ADPCM format using a PC software (This software is
called muse.exe, and was developed by ST. It implements the same ADPCM algorithm
encoder as the one that runs on STM32F103xx performance line devices. muse.exe is
included in the zipped software package that comes with this application note: please
refer to Section 2.2 for how to use this tool.)
● then loading the encoded file into a Flash memory
● and finally decoding the file and playing it using the high-density STM32F103xx
microcontroller.
On the STM3210E-EVAL board, the 128 Mbit NOR Flash memory stores the ADPCM file
and the firmware decoder that runs on STM32F103xx devices. It then decodes the stream
and, with the I
2
S external DAC, plays back the audio samples.
The hardware environment is based on the application note “AN2739: How to use the highdensity STM32F103xx microcontroller to play audio files with an external I
so please refer to that application note for more information.
To run this demo, three steps are required:
1. generating the ADPCM file
2. loading the ADPCM file into the NOR Flash memory
3. playing the ADPCM file
2.2 ADPCM file generation
The application note package contains a PC software called Muse to encode PCM files into
the ADPCM format.
Figure 1. ADPCM file generator
2
S audio codec”,
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AN2931 Implementation example
If you run the Muse.exe program and press the Source File Browse button, the File
Selection dialog appears, prompting you to select a WAV source file. As mentioned before,
the WAV file should meet the following requirements:
● Audio format: PCM
● Audio sample size: 16 bits
● Channels: 1 (mono)
● Audio sample rate: from 8 kHz to 44.1 kHz
If your WAV file does not satisfy these conditions, you must previously convert it using an
audio converter tool, like sndrec32.exe, which is the Microsoft
records.
Once you have selected the file to be converted, the New File Selection dialog appears,
prompting you to select the save location of your ADPCM converted file.
You can retrieve the PCM format of the encoded audio file by ticking the Make a recovered
PCM file check box. In this case, the generated ADPCM file is decoded and formatted into a
WAV file. This WAV file is called Recover.wav and it is generated in the same directory as
your WAV source file. In this way you can listen to both the original WAV file and the
recovered one to compare the sound quality.
2.3 ADPCM file loading
Use the DfuSe software to load the ADPCM file into the NOR Flash memory. To do so, two
steps are necessary:
1. generating the .dfu image
2. loading the .dfu image into the NOR Flash memory
Generating the .dfu image
To generate the .dfu image, you have to use the DFU File Manager software provided within
the DfuSe package. For that, open the DFU File Manager program and choose the DFU file
generation option. The window shown in Figure 2 opens. You have to select 2 for the target
ID, meaning that the file is loaded into the NOR Flash memory. Then, click on the Multi
BIN... button to open the source browser and select your ADPCM file.
®
default program for audio
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Implementation example AN2931
Figure 2. DFU File Manager
The NOR Flash memory is mapped on the FSMC interface from address 0x6400 0000,
which is the start address of the ADPCM file storage area. So, you have to set the address
of the .dfu image to 0x6400 0000 (see Figure 3). Finally, click on Add to list and then
Generate to finish the DFU file generation.
Figure 3. DFU File Manager source browser
Loading the DFU image
To load the DFU image into the NOR Flash memory you have to:
1. connect the STM3210E-EVAL board to a PC using a USB cable
2. load and run the device firmware upgrade (DFU) demonstration on the STM3210EEVAL board. You can find this firmware in the STM3210xxx USB development kit
3. start the DfuSe software on the PC (see Figure 4)
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AN2931 Implementation example
Figure 4. DfuSe software
Select the NOR Flash memory as target, then click on the Choose button located in the
Upgrade or Verify Action area to select your ADPCM DFU image. Finally, click on
Upgrade to perform the download.
2.4 ADPCM file playing
At this point, you have succeeded in loading the ADPCM file into the NOR Flash memory. It
is now time to start the playback demo. Go to the application note package and open the
ADPCM_AN folder. Open the folder project, select the suitable project workspace and open
it in your toolchain. Build all the source files, load the project image and then run the
program.
The LCD on the STM3210E-EVAL board will display the audio commands while the joystick
and the key buttons will be used to control the demo. The allowed audio commands are:
play, pause, stop, forward, rewind and volume setting.
You can select the audio output interface, which can be the speaker provided on the
STM3210E-EVAL board, or headphones. Better audio quality is achieved with headphones.
You can connect them to the STM3210E-EVAL board via the audio jack.
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Implementation example AN2931
Note that to rewind and forward through the audio file, you have to use the joystick buttons
as follows:
● push the joystick to the right to forward
● push the joystick to the left to rewind
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AN2931 Conclusion
3 Conclusion
This application note describes how to implement the IMA ADPCM firmware audio codec
dedicated to STM32F103xx devices. It also provides an example of use that consists in
decoding an ADPCM-encoded audio stream stored in a Flash memory, and playing back the
decoded samples.
The high-density STM32F103xx was used in this application note because it features an I
interface that allows communication with the external DAC available on the STM3210EEVAL evaluation board.
It is however possible to run this ADPCM firmware codec and play audio files with other
STM32F10xxx devices that do not feature an I
2
S peripheral, by using the PWM output.
In this case, you simply need to update the provided decoding example for the STM3210BEVAL evaluation board, and use the PWM output for audio playback.
2
S
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Revision history AN2931
4 Revision history
Table 4. Document revision history
Date Revision Changes
04-Mar-2009 1 Initial release.
30-Apr-2009 2
In code, u8 and s16 updated to uint8_t and int16_t,
respectively.
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AN2931
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