Unfortunately, as the analog signal was regenerated, the repeater device was unable to differentiate between the voice traveling over the wire and line noise. Each time the repeater regenerated the voice, it amplified the line noise as well. Thus, the more times a phone company regenerated a signal, the more distorted and difficult to understand the signal became.
The second difficulty encountered with analog connections was the sheer number of wires the phone company had to run to support a large geographical area or a business with a large number of phones. Because each phone required two wires, the bundles of wire became massive and difficult to maintain (imagine the hassle of a single pair of wires in the bundle breaking). A solution to send multiple calls over a single wire was needed. A digital connection is that solution.
Converting Analog to Digital Signals
Simply put, digital signals use numbers to represent levels of voice instead of using a combination of electrical signals. When someone talks about “digitizing voice,” they are speaking about the process of changing analog voice signals into a series of numbers (shown in Figure 1.7), which you can use to put the voice back together at the other end of the line.
To convert an analog signal into digital format, the converting device goes through a four-step process:
1. Sample the signal.
2. Quantize the signal.
3. Encode the quantized value into binary format.
4. Optionally compress the sample to save bandwidth.
The following sections describe each part of the process in turn.
Sample the Signal
To convert an analog waveform into a numeric value, the digitizing device must sample it many times as the analog signal changes, as shown in Figure 1.8. This sampling process is known as pulse-amplitude modulation (PAM).
In 1927, Dr. Harry Nyquist, an engineer at Bell Laboratories at the time, found that by sampling a signal at twice the number as the highest electrical frequency of the signal per second, he could regenerate the voice with acceptable audio quality levels. Human speech typically uses frequencies up to 9000 Hz, which would result in 18,000 samples each second. Sending this many samples per second would result in a large bandwidth requirement for each voice call, so Nyquist cut the sampling frequency range for human voice to 4000 Hz (resulting in 8000 digital samples per second). Although limiting the frequency range like this did cut down on the quality of voice, this frequency range is sufficient to allow you to identify the remote caller and sense their mood.
Sending Multiple Calls over a Single Line
Now, let’s come back to the original problems of analog connections:
- The signal degrades over long distances.
- You can’t send multiple calls over a single line (resulting in massive cabling requirements).
Digitizing voice solves the first problem because you can easily transmit a numeric value any distance a cable can run without any degradation or line noise. Time-division multiplexing (TDM) solves the second problem.
TDM allows voice networks to carry multiple conversations at the same time over a single, four-wire path. Because the multiple conversations have been digitized, the numeric values are transmitted in specific time slots (thus the “time-division”) that differentiate the separate conversations. Figure 1.10 illustrates three separate voice conversations sent over a digital connection.
