where Dm is the digit detected for frame m, m 0, 1, 2, . . . is the frame index, C is the
index of column frequencies which has been detected, and R is the index of row
frequencies which has been detected. For example, if two frequencies 750 Hz and
1219 Hz are detected, the valid digit is computed as
Dm 2 42 À 1 6:
This value is placed in a memory location designated D(m). If any of the tests fail,
then `À1' representing `no detection' is placed in D(m). For a new valid digit to be
declared, D(m) must be the same for two successive frames, i.e., Dm À 2 Dm À 1.
If the digit is valid for more than two successive frames, the receiver is detecting the
continuation of a previously validated digit, and a third digit D(m) is not the output.
There are two reasons for checking three successive digits at each pass. First, the
check eliminates the need to generate hits every time a tone is present. As long as the
tone is present, it can be ignored until it changes. Second, comparing digits Dm À 2,
Dm À 1, and D(m) improves noise and speech immunity.
9.4 Adaptive Echo Cancellation
Adaptive echo cancellation is an application of adaptive filtering to the attenuation of
undesired echo in the telecommunication networks. This is accomplished by modeling
the echo path using an adaptive filter and subtracting the estimated echo from the echo-
path output. The development of echo canceling chips and advances in DSP processors
have made the implementation of echo cancelers at commercially acceptable costs.
Beginning from canceling the voice echo in long-distance links and now being applied
to control acoustic echo in hands-free telephones, adaptive echo cancelers have also
found wide use in full-duplex data transmission over two-wire circuits such as high
speed modems. In addition, echo canceling techniques are used in providing the digital
data stream between the customer premise and serving central office. Since the require-
ments for voice and data echo cancelers are quite different, this section emphasizes on
introducing voice echo cancelers for long-distance networks.
9.4.1 Line Echoes
One of the main problems associated with telephone communications is the generation
of echoes due to impedance mismatches at various points in telecommunication net-
works. Such echoes are called line (or network) echoes. If the time delay between the
speech and the echo is short, the echo is not noticeable. Distinct echoes are noticeable
only if the delay exceeds tens of milliseconds, which are annoying and can disrupt a
conversation under certain conditions. The deleterious effects of echoes depend upon
their loudness, spectral distortion, and delay. In general, the longer the echo is delayed,
the more echo attenuation is required. Echo is probably the most devastating degrad-
ation for long-distance telecommunications, especially if the two parties are separated
by a great distance with a long transmission delay.
ADAPTIVE ECHO CANCELLATION