Slide1: Communications and Control II
Department of Engineering,
University of Hull,
Slide2: Overview Communications & Control II
Kevin Paulson Control
Ming Hou & Ron Patton Digital Modulation Information Theory Slide3: Modulation If you have a signal containing only low frequencies e.g.speech, and you need to transmit it over a communications channel that works at much higher frequencies e.g. radio, then you use modulation. Signal Carrier Wave Modulated Signal Modulation Demodulation Slide4: Modulation A carrier wave can be modulated by changing its:
In the coming weeks we will revisit AM, FM and PM and look at their application to digital signals Slide5: Digital Signals Analogue signals are continuously variable i.e. they can take any value over some range, and change all the time.
Digital signals can only take discrete values (often only two) and change at specific times.
There are three stages in the digitisation of a signal: Sampling: the signal value is sampled at specific times
Quantisation: the signal value is approximated by the nearest allowed value
Coding: the approximation is expressed as a sequence of a small number of digits.
Slide6: Digital Signals Periodic sampling of an analogue signal Slide7: Digital Signals Quantisation of a sampled signal Slide8: Example The analogue signal is sampled at times , quantised to the nearest integer and then expressed as a 3-bit binary. The digital signal to transmit would then be 001010011100. Slide9: Problem The analogue signal is sampled at times , quantised to the nearest integer and then expressed as a 3-bit binary. The digital signal to transmit would then be ??????110??? Slide10: Problem The analogue signal is sampled at times , quantised to the nearest integer and then expressed as a 3-bit binary. The digital signal to transmit would then be 000011110111. Slide11: Nyquist-Shannon Theorem The Nyquist-Shannon Theorem states that an analogue signal with a highest frequency B can be completely recreated from its sampled form provided it is sampled at a rate s equal to at least twice this frequency. We will prove this in a few weeks and use this result a lot. Slide12: Example Question
An 8-bit ADC is used to sample speech (maximum frequency 4KHz) at the Nyquist rate. What is the data rate?
The Nyquist rate is twice the highest frequency i.e. 8KHz.
At 8-bits per sample the data rate is 8x8 KHz = 64 Kbps Slide13: Problem What communications networks could you be connected to in your home? Slide14: Problem Slide15: Convergence In the very near future all communications: voice, audio, video, data, documents, commands etc, will be carried over the same networks in the same format.
TV, phone, stereo, computer, diary may all be the same piece of equipment.
One government organisation overseas communications: Ofcom. Slide16: Communication Channel Slide17: High fidelity. The discrete nature of digital signals makes their distinction in the presence of noise easy. Very high fidelity transmission and regeneration are possible.
Time independence. A digitised signal is a stream of numbers. Once digitised a signal may be transmitted at a rate unconnected with its recording rate.
Source independence. The digital signals may be transmitted using the same format irrespective of the source of the communication. Voice, video and data may be transmitted using the same channel.
Signals may be coded. The same transmitted message has an infinite number of meanings according to the rule used to interpret it.
Digital Advantages Slide18: Increased complexity