DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 424

FIGURE 10.54. Block diagram of the ADPCM encoder.
Encoder
The subband adaptive differential pulse code-modulated (SB-ADPCM) encoder
consists of a transmit quadrature mirror filter that splits the input signal into a low-
frequency band, 0 to 4 kHz, and a high-frequency band, 4 to 8 kHz. The low- and
high-frequency signals are encoded separately by dynamically quantizing an adap-
tive predictor's output error. The low and high encoder error signals are encoded
with 6 and 2 bits, respectively. As long as the error signal is small, a negligible amount
of overall quantization noise and good performance can be obtained. The low- and
high-band bits are multiplexed, and the result is 8 bits sampled at 8 kHz for a bit
rate of 64 kbits/s. Figure 10.54 shows a block diagram of an SB-ADPCM encoder.
Transmit Quadrature Mirror Filter
The transmit quadrature mirror filter (QMF) takes a 16-bit audio signal sampled
at 16 kHz and separates it into a low band and a high band. The filter coefficients
represent a 4-kHz lowpass filter. The sampled signal is separated into odd and
even samples, with the effect of aliasing the signals from 4 to 8 kHz. This aliasing
causes the high-frequency odd samples to be 180° out of phase with the high-
frequency even samples. The low-frequency even and odd samples are in phase.
When the odd and even samples are added after being filtered, the low-frequency
signals constructively add, while the high-frequency signals cancel each other, pro-
ducing a low-band signal sampled at 8 kHz.
The low subband encoder converts the low frequencies from the QMF into an
error signal that is quantized to 6 bits.
Decoder
The decoder decomposes a 64-kbits/s signal into two signals to form the inputs to
the lower and higher SB-ADPCM decoder, as shown in Figure 10.55. The receive
QMF consists of two digital filters to interpolate the lower- and higher-subband
ADPCM decoders from 8 to 16 kHz and produce output at a rate of 16 kHz. In the
higher SB-ADPCM decoder, adding the quantized difference signal to the signal
estimate produces the reconstructed signal.
Components of the ADPCM decoder include an inverse adaptive quantizer,
quantizer adaptation, adaptive prediction, predicted value computation, and recon-