Information Technology Reference
In-Depth Information
Digitised speech signal rate = 8
×
8 kbps = 64 kbps
Table G.7 outlines the main compression techniques for speech. The G.722 standard allows
the best-quality signal, as the maximum speech frequency is 7 kHz rather than 4 kHz in nor-
mal coding systems; and has the equivalent of 14 coding bits. The G.728 allows extremely
low bit rates (16 kbps).
Table G.7
Speech compression standards
ITU standard
Technology
Bit rate
Description
G.711
PCM
64 kbps
Standard PCM
G.721
ADPCM
32 kbps
Adaptive delta PCM where each value is
coded with four bits
G.722
SB-ADPCM
48, 56 and 64 kbps
Subband ADPCM allows for higher-
quality audio signals with a sampling rate
of 16 kHz
G.728
LD-CELP
16 kbps
Low-delay code excited linear prediction
for low bit rates
G.7 Exercises
G.7.1
What is the function of a B-channel in ISDN:
(a)
It transmits data
(b)
It sends control information
(c)
It creates a network
(d)
It emulates a modem
G.7.2
What is the function of a D-channel in ISDN:
(a)
It transmits data
(b)
It sends control information
(c)
It creates a network
(d)
It emulates a modem
G.7.3
What is the bit rate of a single ISDN B-channel:
(a)
16 kbps
(b)
64 kbps
(c)
128 kbps
(d)
256 kbps
G.7.4
What is the bit rate of the an ISDN D-channel:
(a)
16 kbps
(b)
64 kbps
(c)
128 kbps
(d)
256 kbps
G.7.5
What is the maximum bit rate of an ISDN connection:
(a)
16 kbps
(b)
64 kbps
(c)
128 kbps
(d)
256 kbps
