Adaptive Differential Pulse Code Modulation
ADPCM is a technique which exploits the redundancies present in the speech signal.
Adaptive Differential Pulse Code Modulation (ADPCM):
- Bit rate of 32 kbps, half the standard 64 kbps PCM rate, while retaining the same voice quality
- In differential PCM the output is the difference between the current amplitude value and the previous one.
- In ADPCM, instead of encoding the differences between adjacent samples, a linear predictor isused to predict the current sample
- The difference between the predicted and actual sample called the prediction error is encoded fortransmission
- Prediction is based on the knowledge of the autocorrelation properties of speech
Block diagram of an ADPCM encoder
- This encoder consists of a quantizer that maps the input signal sample onto a 4-bit output sample(see figure 15.3)
- The ADPCM encoder makes best use of the available dynamic range of 4 bits by varying its step size in an adaptive manner
- The step size of the quantizer depends on the dynamic range of the input which is speaker dependent and varies with time.