Audio Engineering

Representation of Audio Signals 439

electrically driven pen wrote the same shape onto a second drum [ Figure 15.2(a) ]. In this
method we would be subject to the nonlinearity of both the reading pen and the writing
pen at the far end. We would also have to come to terms with the noise that the line, and
any amplifi ers, between us would add to the signal describing the plot. This additive
property of noise and distortion is characteristic of handling a signal in its analogue
form and, if an analogue signal has to travel through many such links, then it can be
appreciated that the quality of the analogue signal is abraded irretrievably.

As a contrast consider describing the shape of the curve to your colleague by measuring
the height of the curve at frequent intervals around the drum [ Figure 15.2(b) ]. You’ll need
to agree fi rst that you will make the measurement at each 10-min mark on the drum, for
example, and you will need to agree on the units of the measurement. Your colleague
will now receive a string of numbers from you. The noise of the line and its associated
amplifi ers will not affect the accuracy of the received information since the received
information should be a recognizable number. The distortion and noise performance
of the line must be gross for the spoken numbers to be garbled and thus you are very

Linear meter

movement

Send Receive

(a)

Figure 15.2 : Sending tidal height data to a colleague in two ways: (a) by tracing out the

curve shape using a pen attached to a variable resistor and using a meter driven pen at the

far end and (b) by calling out numbers, having agreed what the scale and resolution of the

numbers will be.

Time (hr)Height (m)

0000

0010

0020

0030

0.3

0.32

0.34

0.37

• 
  


• 
  


• 
  

  (b)

  
  

Time

Height