NEUTRALIZATION SIGNAL DEVELOPING DEVICE FOR AN ECHO SUPPRESSOR

A device for developing a neutralization signal for an echo suppressor is required for the transmission of data over a telephone line having four-wire circuits. Each transmission of data is preceded by an identification signal at a given frequency. An input analog signal including the identification is sampled and quantized so that each sampled value is encoded into a binary codeword composed of a sign bit and a preset number of bits representing the magnitude of the analog signal. The device includes a detecting circuit to detect the identification signal, a level assessing circuit to assess the level of the codeword sequence, and a hangover circuit responsive to the detecting circuit and the level assessing circuit to develop the neutralization signal. The detecting circuit includes a bit pattern generator which produces successive bit patterns from sign bits of the codeword sequence. The bit patterns are in turn used to produce a first detection signal if the bit patterns coincide with a first predetermined bit pattern and a second detection signal if the bit patterns coincide with a second predetermined bit pattern. A counter is responsive to the first and second detection signals to increase its contents by a first number of the first detection signal is generated at any one of predetermined recurrence cycles and to decrease its contents by a second number if the second detection signal is generated. The content of the counter are compared with a predetermined value, and a detection output is produced when the contents of the counter exceed the predetermined value.