Telecommunication Channels
Senior 3 · Physics
Telecommunication Channels
Lesson 2 of 4
25 min
When music was copied from one cassette tape to another, every copy hissed a little more than the one before, and a copy of a copy of a copy was hard to enjoy. Today a song or a voice note forwarded from phone to phone a hundred times sounds exactly like the first. The difference is not better tape or better phones. It is the kind of signal: the cassette held an analogue signal, the phone holds a digital one.
Definition
Analogue signal: a signal that varies continuously and can take any value within a range. Example: the voltage from a microphone, which follows the shape of the sound wave.
Digital signal: a signal that can take only a few separate values, usually two (0 and 1). Example: the pulses of light in an optical fibre carrying internet data.
Bit (binary digit): one 0 or 1, the smallest piece of digital information.
The analogue voltage on the left can have any value at any instant, like the height of the sound wave it copies. The digital signal on the right is either high (1) or low (0); its information is in the pattern of bits, here 1 0 1 1 0 0 1 0.
Sound, light and temperature are naturally analogue, so a phone or computer must convert them. An analogue-to-digital converter (ADC) measures the signal at equal, very short time intervals (sampling), rounds each measurement to the nearest of a fixed set of levels (quantising), and writes each level as a binary number. A digital-to-analogue converter (DAC) in the receiver turns the numbers back into a varying voltage for the loudspeaker.
In the diagram, 3 bits give levels, 0 to 7, so level 6 is sent as 110 and level 1 as 001. Rounding to the nearest level loses a little detail (the gaps between the dots and the bars). Using more bits gives more levels and a more faithful copy:
Worked example 1: levels and codes
(a) How many levels can a 4-bit ADC use? (b) Write level 6 and level 13 as 4-bit codes.
(a) levels, numbered 0 to 15.
(b) , so 6 is 0110. , so 13 is 1101. (The place values of 4 bits are 8, 4, 2 and 1.)
Worked example 2: the data rate of a phone call
Standard digital telephone speech is sampled 8000 times a second, with 8 bits per sample. How many bits are sent each second, and how many levels are used?
Worked example 3: sending a photo
A photo file holds 3.2 megabytes (1 byte = 8 bits; 1 megabyte = bytes). How long does it take to send over a link carrying bits per second?
Every channel attenuates a signal and adds noise. On a long route, the signal must be boosted many times by repeaters along the way.
An analogue repeater can only amplify what arrives, so it amplifies the noise along with the message, and noise picked up on each section adds up. A digital repeater only has to decide, at each step, "is this a 0 or a 1?". As long as the noise is not big enough to flip a bit, it sends out perfect new pulses: the signal is regenerated. That is why the forwarded voice note is a perfect copy.
| Analogue signals | Digital signals | |
|---|---|---|
| Advantages | Simple, cheap equipment; need less bandwidth (a narrower range of frequencies) for the same message; no conversion needed for sound | Can be regenerated, so noise does not build up; errors can be detected and corrected; easy to store, copy, compress and encrypt; computers can process them; many signals can share one channel |
| Disadvantages | Noise builds up and cannot be removed; every copy is a little worse; hard to keep private | Need ADCs and DACs; need more bandwidth; rounding to levels loses some detail; a weak signal fails suddenly (the picture freezes) instead of fading gently |
Definition
Regeneration: rebuilding a weak, noisy digital signal into clean pulses by deciding whether each part is a 0 or a 1.
Bandwidth (of a channel): the range of frequencies it can carry; a wider bandwidth carries more information each second.
Warning
Common mistakes
Tip
Ramp or staircase. An analogue signal is a ramp: every height is possible. A digital signal is a staircase: you can only stand on the steps. More bits means smaller steps.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the three diagrams, the table, worked examples and practice exercises. The telephone figures (8000 samples a second, 8 bits) and the CD figures (44 100 samples a second, 16 bits, two channels) are the standard values for those systems.
