DIGITAL TRANSMISSION ON AN ANALOG CHANNEL
Typical differential delay across a voice channel, with FDM equipment back-to-back.
``Back-to-back'' means that the transmit and receive portions of the equipment are placed on a test
bench and appropriately interconnected.
Phase or delay distortion is the major limitation of modulation rate on the voice channel.
The shorter the pulse width (the width or period of 1 bit in binary systems), the more
critical will be the EDD parameters. As we discuss in Section 10.9.5, it is desirable to
keep delay distortion in the band of interest below the period of 1 baud.
Attenuation Distortion (Frequency Response)
. Another parameter that
seriously affects the transmission of data and can place definite limits on the modulation
rate is attenuation distortion. Ideally, all frequencies across the passband of a channel of
interest should undergo the same loss or attenuation. For example, let a
enter a channel at any given frequency between 300 and 3400 Hz. If the channel has
13 dB of flat attenuation, we would expect an output at the distant end of
-23 dBm at
any and all frequencies in the band. This type of channel is ideal but unrealistic in a real
In Rec. G.132 (Ref. 9), the CCITT recommends no more than 9 dB of attenuation
distortion relative to 800 Hz between 400 Hz and 3000 Hz. This figure, 9 dB, describes
the maximum variation that may be expected from the reference level at 800 Hz. This
variation of amplitude response is often called attenuation distortion. A conditioned chan-
nel, such as a Bell System C-4 channel, will maintain a response of
-2 dB to +3 dB
from 500 Hz to 3000 Hz and
-2 dB to +6 dB from 300 Hz to 3200 Hz.
Considering tandem operation, the deterioration of amplitude response is arithmeti-
cally cumulative when sections are added. This is particularly true at band edge in view
of channel unit transformers and filters that account for the upper and lower cutoff char-
acteristics. Figure 10.11 illustrates a typical example of attenuation distortion (amplitude
response) across carrier equipment back-to-back. Attenuation distortion, phase distortion,
and noise were introduced in Section 3.3.
. Another important consideration in the transmission of data is noise.
All extraneous elements appearing at the voice channel output that were not due to the
input signal are considered to be noise. For convenience, noise is broken down into
four categories: (1) thermal, (2) crosstalk, (3) intermodulation, and (4) impulse. Thermal