First the basic DM and ADM methods are mentioned, followed by the ADPCM method.

»	DM and ADM methods

In the DM (Delta Modulation) method a certain quantity () predetermined for each cycle of sampling is added or subtracted to express a voice waveform. In other words, the addition of a is encoded as 1, while the subtraction of a is encoded as 0. Thus, rapid changes in voice waveform with respect to a step width cannot be covered by this method. Figure 10.1 (a) illustrates this case.

For the ADM (Adaptive Delta Modulation) method, the quantized width () is adjusted according to the rapid changes to improve the response of the voice waveform. Encoding one bit is based on this means. If a value of 1 or 0 continues for a certain period, the quantized width () is enlarged for quicker response. Figure 10.1 (b) illustrates such quick response.

»	ADPCM method

In the ADPCM (Adaptive Differential Pulse Code Modulation) method is such that the basic width of quantization () is adjusted adaptive to the rapid changes for each cycle of sampling to encode each signal as three to four bits of data. This provides higher response of voice waveform.

For example, in the case of four-bit ADPCM, the upper one bit stands for polarity (increase or decrease) data, while the lower three bits determine the multiplier factor to the basic width of quantization (). (The value depends on the correlation with the past data.) This means that about 400 pieces of data [(about 50 pieces) x 8 (for 3 bits)] can be changed at a time. Therefore, approximately 400 pieces of data are available. Figure 10.2 illustrates this case.

Furthermore, in the three-bit ADPCM, the upper one bit stands for polarity (increase or decrease) data, while the lower two bits determine the multiplier factor to the basic width of quantization (). This means that about 200 pieces of data [(about 50 pieces) x 4 (for 2 bits)] can be changed at a time. Therefore, 200 pieces of data are available.

The ADPCM method allows the achievement of high-quality sound in a relatively simple configuration, and the easy creation of voice data.

Note, however, that there is no compatibility between ADPCM data created by different manufacturers.

(Note) When playback starts from part of a phrase, normal waveforms cannot be reproduced because the ADPCM algorithm is employed. See Q10 and A10.

Figure 10.1 DM and ADM Methods
Figure 10.2 4-bit ADPCM