LEARNING DECIBELS AND THEIR APPLICATIONS
(VF channel). A voice channel conjures up in our minds an analog channel, something
our ear can hear. The transmit part (mouthpiece) of a telephone converts acoustic energy
emanating from a human mouth to electrical energy, an analog signal. At the distant
end of that circuit an audio equivalent of that analog energy is delivered to the receiver
(earpiece) of the telephone subset with which we are communicating. This must also hold
true for the all-digital network.
When dealing with the voice channel, there are a number of special aspects to be
considered by the transmission engineer. In this section we will talk about these aspects
regarding frequency response across a well-defined voice channel. We will be required to
use dBs, dB-derived units, and numeric units.
The basic voice channel is that inclusive band of frequencies where loss with regard to
frequency drops 10 dB relative to a reference frequency.
There are two slightly different
definitions of the voice channel, North American and CCITT:
North America: 200 Hz to 3300 Hz (reference frequency, 1000 Hz).
CCITT: 300 Hz to 3400 Hz (reference frequency, 800 Hz).
We sometimes call this the nominal 4-kHz voice channel; some others call it a 3-kHz
channel. (Note: There is a 3-kHz channel, to further confuse the issue; it is used on HF
radio and some old undersea cable systems.)
To introduce the subject of a "flat" voice channel and a "weighted" voice channel
we first must discuss some voice channel transmission impairments. These are noise and
We all know what noise is. It annoys the listener. At times it can be so disruptive that
intelligent information cannot be exchanged or the telephone circuit drops out and we get
a dial tone. So we want to talk about how much noise will annoy the average listener.
Amplitude distortion is the same as frequency response. We define amplitude distortion
as the variation of level (amplitude) with frequency across a frequency passband or band of
interest. We often quantify amplitude distortion as a variation of level when compared to
the level (amplitude) at the reference frequency. The two common voice channel reference
frequencies are noted in the preceding list.
To further describe amplitude distortion, let us consider a hypothetical example. At
a test board (a place where we can electrically access a voice channel) in New York
we have an audio signal generator available, which we will use to insert audio tones at
different frequencies. At a similar test board in Chicago we will measure the level of
these frequencies in dBm. The audio tones inserted in New York are all inserted at a level
-16 dBm, one at a time. In Chicago we measure these levels in dBm. We find the
level at 1000 Hz to be
+7 dBm, our reference frequency. We measure the 500-Hz tone
+3 dBm; 1200-Hz tone at +8 dBm; 2000-Hz tone +5 dBm, and the 2800-Hz tone at
0 dBm. Any variation of level from the 1000-Hz reference value we may call amplitude
distortion. At 2800 Hz there was 7 dB variation. Of course, we can expect some of the
worst-case excursion at band edges, which is usually brought about by filters or other
devices that act like filters.
The human ear is a filter, as is the telephone receiver (earpiece). The two are in tandem,
as we would expect. For the telephone listener, noise is an annoyance. Interestingly we
find that noise annoys a listener more near the reference frequencies of a voice channel
than at other frequencies. When using the North American 500-type telephone set with
average listeners, a simple 0-dBm tone at 1000 Hz causes a certain level of annoyance.
This value applies when looking toward the subscriber from the local serving exchange. Looking into the
network from the local serving exchange the value drops to 3 dB.