DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 6: Analogue and Digital Signals

Analogue | Digital | Common Terms | Speech Circuits | Problems with telephone circuits | Summary | Test 2

Introduction
In this section, we look at the nature of signals such as voice and data. In data communications, understanding how signals are constructed will help us understand the problems involved in sending them from one place to another.

Objectives
At the end of this section you should be able to

• give an example of an analogue and digital signal
• list the major characteristics of analogue and digital signals
• distinguish between bandwidth, baud rate and bit rate
• explain what happens to a digital signal when it is sent over an analogue telephone circuit

Analogue Signals
The public dial-up service supports analogue signals. Analogue signals are what we encounter every day of our life. Speech is an analogue signal, and varies in amplitude (volume), frequency (pitch), and phase.

The three main characteristics of analogue signals are,

• Amplitude
  This is the strength of the signal. It can be expressed a number of different ways (as volts, decibels). The higher the amplitude, the stronger (louder) the signal. The decibel (named in honor of Alexander Graham Bell) is a popular measure of signal strength.
  
  

  Sound level	Type of Sound
  40db	normal speech
  90db	lawn mowers
  110db	shotgun blast
  120db	jet engine taking off
  120db+	rock concerts

  
  It has been discovered that exposure to sounds greater than 90db for a period exceeding 15 minutes causes permanent damage to hearing. Our ability to hear high notes is affected. As young babies, we have the ability to hear quite high frequencies. This ability reduces as the aging process occurs. It can also be affected by too much noise over sustained periods. Ringing in the ears after being exposed to loud noise is an indication that hearing loss may be occurring.
  

  This diagram shows a single signal of various amplitudes. The base line indicates a steady state, in this example, the signal amplitude rises both above and below the steady state. The measurement of the two extremes is called the peak to peak measurement.

  This diagram illustrates a speech signal, in this instance, the word "Hello". Speech is a very complex signal, and contains many thousands of different combinations of signals all mixed together. Note that it looks much more complicated than the single signal shown above.

  
  
  

• Frequency
  This is the rate of change the signal undergoes every second, expressed in Hertz (Hz), or cycles per second. A 30Hz signal changes thirty times a second. In speech, we also refer to it as the number of vibrations per second. As we speak, the air is forced out of our mouths, being vibrated by our voice box. Men, on average, tend to vibrate the air at a lower rate than women, thus tend to have deeper voices.
  
  

  A cycle is one complete movement of the wave, from its original start position and back to the same point again. The number of cycles (or waves) within a one second time interval is called cycles-per-second, or Hertz.

  
  An example is sitting on the beach, counting the waves as they come in on the shore. If we counted the number of waves that crashes on the beach over a minute period, then divided that number by 60 (because there are 60 seconds in one minute), we would have the number of waves per second... (i.e. frequency!).
  
  Humans can hear from reasonably low frequency tones (about 100Hz) all the way up to about 12KHz. As we get older, our ability to hear the higher notes is lessened, due to the aging process making the bones in the ear harder and less able to vibrate. In addition, human speech contains a great deal of redundant information, and can be compacted within the range of 300Hz to 3400Hz whilst still retaining approximately 80% of its content.
  
  
  

• Phase
  This is the rate at which the signal changes its relationship to time, expressed as degrees. One complete cycle of a wave begins at a certain point, and continues till the same point is reached again. Phase shift occurs when the cycle does not complete, and a new cycle begins before the previous one has fully completed.
  
  

  The human ear is insensitive to phase shift, but data signals are severely affected by it. Phase shift is caused by imperfections in cable media, such as joins and imperfect terminations.

  
  In a practical sense, imagine you are in the bath. If you drop the soap, it forms ripples where the waves travel outwards towards the edge of the bath. When the wave reaches the edge, it hits the wall of the bath and bounces back. This is like phase shift, which is an abrupt change in the signals relationship.

Analogue signals are sent via the PTSN. Digital signals cannot be sent via the PTSN without being first converted to analogue.

Digital Signals
Digital signals are the language of modern day computers. Digital signals comprise only two states. These are expressed as ON or OFF, 1 or 0 respectively. Examples of devices having TWO states in the home are,

• Light Switches: Either ON or OFF
• Doors: Either OPEN or CLOSED

Digital signals require greater bandwidth capacity than analogue signals, thus are more expensive to communicate. This diagram shows a digital signal.

SOME COMMON TERMS

Baud Rate
Baud rate is the reciprocal of the shortest signal element (a measure of the number of line changes which occur every second). For a binary signal of 20Hz, this is equivalent to 20 baud (there are 20 changes per second). For telephone cables, the limiting factor in speed is the number of line changes per second. A line change is defined as switching from one state to another, for instance, switching from a 1 to a 0, or from a 0 to 1 for a digital signal. If the number of line changes per second are exceeded, errors occur and the signal at the receiving end cannot be reliably reconstructed.

Bits Per Second
This is an expression of the number of bits per second. Where a binary signal is being used, this is the same as the baud rate. When the signal is changed to another form, it will not be equal to the baud rate, as each line change can represent more than one bit (either two or four bits).

Digital signals sent via the PSTN need to be converted to analogue first (by using a device called a modem). Digital signals can be sent via the ISDN unmodified.

Bandwidth
Bandwidth is the frequency range of a channel, measured as the difference between the highest and lowest frequencies that the channel supports. The maximum transmission speed is dependant upon the available bandwidth. The larger the bandwidth, the higher the transmission speed. A nominal voice channel has a bandwidth of 3.1KHz. In reality this equates to about 1200bps maximum for a binary digital signal.

Dial Up Speech Circuits provided by Telephone Communication Companies
The voice channel was designed to handle analogue voice in the range 300Hz to 3.4Khz. The nature of voice traffic is

• periodic - talk and then listen
• varying in intensity - talking softly, shouting

In addition, the voice channel was implemented using two way amplifiers, which meant special devices were used to prevent echoes or unwanted oscillations (the circuit suffering from feedback). These devices are called echo suppressors, and affect the signal by reducing the available bandwidth of the channel. The amplifiers are designed to boost low level signals and attenuate high level signals, with the intention of trying to maintain an average signal level on the channel.

Data signals are digital in nature and are

• continuous
• constant in intensity
• require greater channel bandwidth than analogue signals

This causes several problems. The two way amplifiers tend to get overloaded, with the net result of putting too much signal level on the channel. This overflows onto other channels, affected the signals on them also (called crosstalk). The second problem is the signals are affected by the bandwidth of the channel, such that only some of the original signal will appear at the other end. This effect becomes more pronounced as the speed of the data signal is increased.

Problems of using Voice Channels for Digital Transmission
A digital signal is comprised of a number of signals. Specifically, the signal is represented as follows,

signal = f + f3 + f5 +f7 +f9 +f11 +f13 ....f(infinity)

This means a digital signal has a base frequency, plus another at three times the base frequency, plus another at five times the base frequency etc. f3 is called the third harmonic, f5 the fifth harmonic and so on. The third harmonic is one third of the amplitude of the base frequency (called the fundamental frequency), the fifth harmonic is one fifth the amplitude of the fundamental and so on.

In order to send a digital signal across a voice channel, the bandwidth of the channel must allow the fundamental plus third and fifth harmonic to pass without affecting them too much. As can be seen, this is what such a signal looks like, and is the minimum required to be correctly detected as a digital signal by the receiver.

Lets consider sending a 2400bps binary digital signal down a voice channel rated with a bandwidth of 3.1KHz. The base frequency of the digital signal is 1200Hz (it is always half the bit rate), so the fundamental frequency will pass through the channel relatively unaltered. The third harmonic is 3600Hz, which will suffer attenuation and arrive severely altered (if at all). The fifth harmonic has no chance of passing the channel.

In this case it can be seen that only the base frequency will arrive at the end of the channel. This means the receiver will not be able to reconstruct the digital signal properly, as it will require f3 and f5 for proper reconstruction. This results in errors in the detection process by the receiver.

Summary
The Public Switched Telephone Network has, for a long time, provided users with dial up telephone connections on demand. Each connection has supported analogue speech in the voice range of 300Hz to 3400Hz. The signals provided by computers are digital, and are not designed to travel across the dial up telephone connections. In order for digital signals to be sent across a telephone connection, they must be converted to analogue voice tones within the frequency range 300Hz to 3400Hz (this is done by a modem).

Analogue signals have three main characteristics which define them, being amplitude (a measure of how loud they are), frequency (a measure of how often they change) and phase. Speech is an example of an analogue signal. Analogue signals can be sent across a telephone connection. Digital signals comprise two states, and must be converted to an analogue form before being sent across a telephone connection.

Test 2 [JavaScript]
Now it is time to review this section and discover how much you have absorbed. Click on the hyperlink above to begin the test.

© Copyright B Brown. 1995-2000. All rights reserved.