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The Speech Network
The speech network - also known as the "hybrid" or the "two wire/four
wire network" - takes the incoming signal and feeds it to the earpiece
and takes the microphone output and feeds it down the line. The standard network
used all over the world is an LC device with a carbon microphone; some newer
phones use discrete transistors or ICs.
One of the advantages of an LC network is that it has no semiconductors,
is not voltage sensitive, and will work continuously as the voltage across
the line is reduced. Many transistorized phones stop working as the voltage
approaches 3 to 4 Volts.
When a telephone is taken off the hook, the line voltage drops from 48
Volts to between 9 and 3 Volts, depending on the length of the loop. If another
telephone in parallel is taken off the hook, the current consumption of
the line will remain the same and the voltage across the terminals of
both telephones will drop. Bell Telephone specifications state that three telephones
should work in parallel on a 20 mA loop; transistorized phones tend not to
pass this test, although some manufacturers use ICs that will pass. Although
some European telephone companies claim that phones working in parallel is "technically
impossible," and discourage attempts to make them work that way, some
of their telephones will work in parallel.
While low levels of audio may be difficult to hear, overly loud audio
can be painful. Consequently, a well designed telephone will automatically
adjust its transmit and receive levels to allow for the attenuation -
or lack of it - caused by the length of the loop. This adjustment is called "loop compensation." In
the United States, telephone manufacturers achieve this compensation with
silicon carbide varistors that consume any excess current from a short
loop (see fig. 2). Although some telephones using ICs have built-in loop compensation,
many do not; the latter have been designed to provide adequate volume
on the average loop, which means that they provide low volume on long loops,
and are too loud on short loops. Various countries have different specifications
for transmit and receive levels; some European countries require a higher
transmit level than is standard in the United States so a domestically-manufactured
telephone may suffer from low transmit level if used on European lines without
modification.
Because a telephone is a duplex device, both transmitting and receiving
on the same pair of wires, the speech network must ensure that not too much
of the caller's voice is fed back into his or her receiver. This function,
called "sidetone," is achieved by phasing the signal so that
some cancellation occurs in the speech network before the signal is fed to
the receiver. Callers faced with no sidetone at all will consider the phone "dead." Too
little sidetone will convince callers that they're not being heard and cause
them to shout, "I can hear you. Can you hear ME?" Too much sidetone
causes callers to lower their voices and not be heard well at the other end
of the line.
A telephone on a short loop with no loop compensation will appear to have
too much sidetone, and callers will lower their voices. In this case,
the percentage of sidetone is the same, but as the overall level is higher
the sidetone level will also be higher.
Click on one of the items below to learn more.
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