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Local Echo Canceller Disclosure Number: IPCOM000101114D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 77K

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Kobayashi, Y: AUTHOR [+1]


An analog circuit for efficient local echo cancellation is proposed.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 62% of the total text.

Local Echo Canceller

       An analog circuit for efficient local echo cancellation
is proposed.

      A local echo, sometimes referred to as a near-end echo or a
side- tone, is an echo that bounces back from a 4-2 wire conversion
circuit of a telephone line interface, and is a wide band signal. It
causes degradation of communication quality, especially in a
full-duplex communication through a telephone line, such as a
full-duplex MODEM.

      In order to suppress these local echoes, local echo cancellers
(LECs) are widely used on boards such as MODEM boards. Usually LECs
are made up of impedance bridge circuits, which are tuned to a
typical frequency, such as 1 KHz. However, since an impedance bridge
can be tuned only to a pin-point frequency, these LECs are unable to
cover the whole telephone band (300 Hz - 3.5 KHz).

      The LEC, which is proposed here, covers the entire telephone
band, by simulating impedance characteristics of both a line
transformer and a telephone line. As shown in Fig. 1, it consists of
two portions:
1) Active filter
  This active filter is a multi-feedback-type band-pass filter. It
simulates total transfer characteristics of the line transformer and
the telephone line in the frequency range of 100 Hz through 10 KHz,
which is about three times as wide as the telephone band width and
generates the expected (transmit) signal (ES).
2) Differential amplifier
   This differential amplifier compares ES against the actual signal