Adpcm Decoder

 •  AVR Application Decodes ADPCM Signal in Real-Time
•  Supports Bit Rates of 16, 24, 32 and 40 kbit/s
•  More Than One Minute Playback Time on ATMEGA128 (at 16 kbit/s)
•  Decoded Signal Played Using Timer Counter in PWM Mode
Introduction
Adaptive Differential Pulse Code Modulation, or ADPCM, is a digital compression
technique used mainly for speech compression in telecommunications. ADPCM is
a waveform codec that CAN also be used to code other signals than speech, such
as music or sound effects. ADPCM is simpler than advanced low bit-rate voice
coding techniques and doesn’t require as heavy calculations, which means
encoding and decoding CAN be done in a relatively short time.
ADPCM is usually used to compress an 8 kHz, 8-bit signal, with an inherent flow
rate of 64 Kbit/s. When encoded at the highest compression ratio, using only 2 bits
to code the ADPCM signal, the flow rate is reduced to 16 Kbit/s, i.e. 25% of the
original. Using 4-bit coding, the flow rate  is 32 Kbit/s, i.e. 50% of the original, and
the quality of the signal is fine for most applications.
This application note focuses on decoding the ADPCM signal. It uses the on-chip
timer Counter to create a Pulse-Width Modulated (PWM) output signal, which is
then passed through a simple, external Filter The Filter consists of just a few
external components and turns the digital signal into Analog suitable for
connecting to speakers. 
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