Embedded Systems Design, 2nd Edition

Interfacing to the analogue world 181

The diagram shows how this encoding works. The ana-

logue value is sampled and the previous value subtracted. The

result is then encoded using the required sample size and allowing

for a plus and minus value. With an 8 bit sample size, 1 bit is used

as a sign bit and the remaining 7 bits are used to encode data. This

allows the previous value to be used as a reference, even if the next

value is smaller. The 8 bits are then stored or incorporated into a

bitstream as with a PCM conversion.

To decode the data, the reverse operation is performed. The

signed sample is added to the previous value, giving the correct

digital ...