AN3143

ADPCM codec

Doc ID 17011 Rev 1

3/13

1

ADPCM codec

1.1

Adaptive differential pulse coded modulation (ADPCM)

ADPCM is a fixed length codeword audio codec which reduces redundant information from

audio waveforms. The information core is separated from the correlated waveform samples

by encoding differences between the current samples and the predicted ones. As the

correlation between consecutive audio samples is generally high, this method is reasonably

effective and preserves good audio quality. The ADPCM codec, which is based on coding

waveforms in time domains, is much less complex than codecs based on voice perception

(vocoders) that operate in frequency domains. For these reasons, the ADPCM codec is the

best choice for the audio output on 8-bit microcontrollers.

The ADPCM algorithm has been developed for speech coding. It is implemented in several

steps and is used in telephone systems like ITU-T G.726 (covering CCITT G.721 and G.723

standards). G.726 and similar telecommunication codecs, offer several advanced features

which are necessary for successful transmission and streaming. Such features include

synchronous coding adjustment, tone detection to carry data modem signals, adaptated

speed control, and recovery from transmission error conditions. These features lead to

additional algorithm complexity which either requires the use of a digital signal processor

(DSP) or lowering the encoding/decoding sample rate. Fortunately, such features do not

bring many additional benefits to the microcontroller for coding/decoding the audio or

speech signals. Less complex alternatives can be found.

1.2

Interactive multimedia association (IMA) ADPCM

different computers platforms, for example in Microsoft® Sound Recorder or Apple®

QuickTime®. It was originally offered by Intel/DVI® as an open standard for use by the IMA.

The reference algorithms and recommended formats were initially developed by the digital

audio technical working group (DATWG) and refined by the digital audio focus group (DAFG)

of the IMA. These groups are no longer active.

The IMA DATWG reference algorithm is less complex than the G.726 algorithm. The number

of encoding/decoding CPU cycles needed is reduced by using fixed prediction and by

replacing complex floating point mathematical operations by look-up tables. The

implementation of the ADPCM codec presented in this application note is compatible with

the IMA reference algorithm published by the IMA DATWG/DAFG in the “Recommended

Practices for Enhancing Digital Audio Compatibility in Multimedia Systems”, revision 3.00

(see References).

a.

IMA is a computer/audio/video industry trade association that has worked to promote multimedia application

development.