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.
Questions of this topic
In an adaptive PCM system for speech coding, the input speech signal is sampled at 8 kHz, and each sample is represented by 8 bits. The quantizer step size is recomputed every 10 ms, and it is encoded for transmission using 5 bits. Compute the transmission bit rate of such a speech coder. What would be the average and peak SQNR of this system?