Bit patterns, analogue and digital

Overview

Recall questions on how one bit pattern can mean many things, analogue against digital data and signals, sampling, and the error quantisation introduces.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Explain how a single bit pattern can represent many different forms of data.

   Model answer

   A bit pattern has no meaning in itself; its meaning comes entirely from how the software chooses to interpret it. The byte 01000001 is 65 as an unsigned binary number, the character 'A' in ASCII, a shade of grey in an 8-bit image, one sample of a sound waveform, part of a machine code instruction, or a colour component — and nothing in the pattern itself says which. The context and the file format tell the program which interpretation to apply.

2. 2

   Describe how bit patterns are used to represent a graphic and a sound.

   Model answer

   A graphic is divided into a grid of pixels and each pixel's colour is recorded as a binary number, so the image becomes a long sequence of bit patterns read row by row. A sound is measured at regular intervals and the amplitude of each measurement is recorded as a binary number, so the sound becomes a sequence of sample values. In both cases a continuous real-world quantity has been turned into a list of numbers held as bit patterns.

3. 3

   What is the difference between analogue data and digital data?

   Model answer

   Analogue data varies continuously and can take any value within its range, with infinitely many possible values between any two points — temperature, air pressure and the loudness of a sound are analogue. Digital data is discrete: it can only take a finite set of separate values, because each value is represented by a fixed number of bits.

4. 4

   What is the difference between an analogue signal and a digital signal?

   Model answer

   An analogue signal is a continuously varying voltage or current whose value at any instant represents the data, so its waveform is a smooth curve. A digital signal takes only a small number of discrete levels — normally two, representing 0 and 1 — so it appears as a train of square pulses switching between those levels.

5. 5

   Why is it important to distinguish analogue and digital data from analogue and digital signals?

   Model answer

   Because the data is the information and the signal is only the means of carrying it, and the two need not match. Digital data can be carried on an analogue signal, as when a modem modulates a carrier wave; analogue data such as speech can be converted and then carried as a digital signal. Confusing the two makes questions about conversion impossible to answer, since a converter changes the form of the signal while preserving the information.

6. 6

   Why are digital signals preferred to analogue signals over long distances?

   Model answer

   Both pick up noise as they travel, but a digital signal only has to be recognised as one of two levels, so it can be regenerated exactly at each repeater, giving a clean copy of the original. An analogue signal can only be amplified, which magnifies the accumulated noise along with the signal, so the degradation is permanent and gets worse with distance.

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