Speech Eroder

Nowadays, the speech
quality on our telephone systems is generally very good, irrespective of
speech quality of yesteryear. The eroder circuit accepts an acoustic
(via an electret micro-phone) or electrical signal. The signals are
applied to the circuit inputs via C1 and C2, which block any direct
voltage. The input cables should be screened. The signals are brought to
(about) the same level by variable potential dividers P1-R1-R4 and
P2-R2-R3, and then applied to the base of transistor T1. The level of
the combined signals is raised by this preamplifier. The preamplifier is
followed by an active low-pass filter consisting of T2–T4, C3, C4,
R6–R8, and P4.

Circuit diagram:

Speech Eroder Circuit

Speech Eroder Circuit Diagram

Although, strictly speaking, P3 serves merely to adjust the volume
of the signal, its setting does affect the filter characteristic. Note,
by the way, that the filter is a rarely encountered current-driven one
in which C3 and C4 are the frequency-determining elements. It has a
certain similarity with a Wien bridge. Transistors T3 and T4, and
resistors R8 and P4 form a variable current sink. The position of P4
determines the slope of the filter characteristic and the degree of
overshoot at the cut-off frequency. The low-pass filter is followed by
an integrated amplifier, IC1, whose amplification is matched to the
input of the electronic circuits connected to the eroder with P5. The
final passive, third-order high-pass filter is designed to remove
frequencies above about 300 Hz. The resulting output is of a typical
nasal character, just as in telephones of the past.

Author: T. Giesberts
Copyright: Elektor Electronics

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