Waveform Quantization and Compression - Digital Signal Processing: Fundamentals and Applications - page 515

TABLE 11.7 Discrete Function WðIÞ

7 6 5 4 3 2 1 0

jIðnÞj

70.13 22.19 12.38 7.00 4.0 2.56 1.13 0.75

WðIÞ

The fast and slow scale factors are then combined to compute the scale factor:

yðnÞ¼a l ðnÞy u ðn 1 Þþð 1 a l ðnÞÞy l ðn 1 Þ (11.10)

Next the controlling parameter 0 a l ðnÞ 1 tends toward unity for speech signals and toward zero for
voice band data signals and tones. It is updated based on the following parameters: d ms ðnÞ , which is the
relatively short-term average of FðIðnÞÞ ; d ml ðnÞ , which is the relatively long-term average of FðIðnÞÞ ;
and the variable a p ðnÞ , where FðIðnÞÞ is defined as in Table 11.8 .

TABLE 11.8 Discrete Function F ðIðnÞÞ

7 6 5 4 3 2 1 0

jIðnÞj

7 3 1 1 1 0 0 0

FðIðnÞÞ

Hence, we have

d ms n ¼ 1 2 5 d ms n 1 þ 2 5 F I n

(11.11)

and

d ml n ¼ 1 2 7 d ml n 1 þ 2 7 F I n (11.12)

while the variable a p ðnÞ is given by

<
:

1 2 4 a p n 1 þ 2 3

d ml n jd ms ðnÞd ml ðnÞj 2 3

if

1 2 4 a p n 1 þ 2 3

if yðnÞh 3

1 2 4 a p n þ 2 3

a p ðnÞ¼ (11.13)

if t d ðnÞ¼ 1

1 if t r ðnÞ¼ 1

1 2 4 a p n

otherwise

a p ðnÞ approaches 2 when the difference between d ms ðnÞ and d ml ðnÞ is large and approaches 0 when the
difference is small. Also a p ðnÞ approaches 2 for an idle channel (indicated by y(n) < 3) or partial band
signals (indicated by t d ðnÞ¼ 1). Finally, a p ðnÞ is set to 1 when a partial band signal transition is
detected ( t r ðnÞ¼ 1).
a l ðnÞ , which is used in Equation (11.10) , is defined as