MAS.632 Conversational Computer Systems - MIT OpenCourseWare

46 VOICE (OMMUNICATION WITH COMPUTERS
Logarithmic Linear
V
Figure 3.11. A comparison of dynamic range of logarithmic and linear
coders for the same number of bits per sample. The logarithmic coder has
a much greater dynamic range than a linear coder with comparable resolution
at low amplitudes.
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Encoding of the differences between samples is called Delta PCM. At each
sampling interval the encoder produces the difference between the previous and
the current sample value. The decoder simply adds this value to the previous
value; the previous value is the only information it needs to retain. This process
is illustrated in Figure 3.13.
The extreme implementation of difference coding is Delta Modulation, which
uses just one bit per sample. When the signal is greater than the last sample, a
"1" bit results; when less, a "0"bit results. During reconstruction, a constant step
size is added to the current value of the signal for each "1" and subtracted for
each "0."
This simple coder has two error modes. If the signal increases or decreases
more rapidly than the step size times the sampling rate, slope overload results
as the coder cannot keep up. This may be seen in the middle of Figure 3.14. To