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This programme is introduced by Dr. Dave Crecraft who then hands over to Glyn Martin and a demonstration of a gross-correlation flowmeter. This is similar to the sewage flowmeter discussed in the t...ext, but in this case the flow of water containing air bubbles is measured. The resistivity across the tube is continuously measured by two pairs of electrodes mounted on the inside walls and separated by a distance equivalent to a few diameters. The rest of the programme is about three types of modulation which can be used to carry signals from transducers, either directly along wires or indirectly by radio. An inductive displacement transducer is shown measuring the eccentricity of a shaft. It is connected to an a.c. bridge and produces an amplitude modulated output. A force transducer with a vibrating wire, whose frequency depends on the applied force, is an example of frequency modulation. John Monk then shows how the signal information is recovered by demodulation. A.M. and F.M. waveforms are displayed both in the time domain (oscillogram) and the frequency domain (spectrum). Finally a television picture signal is used to provide an example of a P.C.M. system. The complex signal is processed by the P.C.M. encoder, transmitted along a channel (i.e. wire) and then decoded and displayed on a TV monitor. The effect of adding noise to the channel demonstrates the insensitive nature of P. C. M. to noise whose amplitude is below a certain threshold level.
Metadata describing this Open University video programme
Module code and title: T291, Instrumentation
Item code: T291; 09
First transmission date: 12-08-1974
Published: 1974
Rights Statement:
Restrictions on use:
Duration: 00:23:00
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Producer: Tony Jolly
Contributors: David Crecraft; Glyn Martin; John Monk
Publisher: BBC Open University
Keyword(s): AM/FM; Cross-correlation flowmeter; De-modulation; Oscillogram; PCM; Signalling; Spectrum; Television signal; Transducers
Footage description: Introduction by David Crecraft. The programme will examine transducers which produce modulated output signals and how these signals are de-modulated to recover the original modulating signal. Glyn Martin with a flow meter apparatus which measures liquid flow velocity by calculating time taken by bubbles in a liquid to pass between two electrodes. Martin explains how the apparatus works. The changes in conductivity caused by the bubbles as they pass the electrode are shown as traces on an oscilloscope screen. Martin uses an animated diagram to explain how the cross correlation function for two simple pulse signals is calculated. Martin then uses an electronic correlator to work out the correlation function for the signal waves from the two electrodes in the flow meter above. The result is shown as a wave form on the oscilloscope screen. Martin explains how flow rate is calculated from this information. Martin explains how the electronic correlator calculates the correlation function. He shows a correlation function wave form being built up on the oscilloscope screen. David Crecraft with an inductive displacement transducer. He explains how it works and displays the transducer's output on an oscilloscope screen. Crecraft next shows a torque transducer. He explains how it works. (Both transducers generate amplitude modulated output signals). Crecraft with a transducer which produces a frequency modulated output signal. This transducer measures force. Crecraft explains how it works. The signal is displayed on an oscilloscope screen. John Monk uses a rectifier and filter to demonstrate how signals can be de-modulated. The results are shown on an oscilloscope screen. David Crecraft with, a large display board on which are drawn various wave forms. He explains how the amplitude modulated wave is first rectified to produce a rectified wave form and is then filtered. Examples of each wave form are on the board. Crecraft then explains how a frequency modulated signal is de-modulated. He again uses the board to aid his discussion. John Monk with a de-modulating apparatus for frequency modulated wave signals. He explains how it works and demonstrates. Monk then shows the spectrum for both an amplitude modulated and a frequency modulated wave signal on the oscilloscope. He explains the components of the spectra. David Crecraft begins a discussion on pulse code modulation (P.C.M.). He has with him an apparatus which produces pulse code modulated television signals. Crecraft demonstrates the apparatus. Glyn Martin explains the fundamentals of pulse code modulation. A signal voltage is converted into a binary number and this number is then converted into a sequence of pulses which are transmitted to a de-modulator where they are re-constructed into a voltage signal. Martin demonstrates the process with some experimental apparatus. He shows how quantisation noise occurs and then demonstrates the effect of quantisation noise on the output signal. Crecraft explains how pulse code modulation produces a television picture output and discusses the effect of noise on the system. He demonstrates by adding noise to a P.CM. television system. The demonstration shows the noise combating effectiveness of P.C.M. systems. Crecraft sums up the programme.
Master spool number: 6HT/71340
Production number: 00525_5125
Videofinder number: 1070
Available to public: no