An attempt is made to quantify the circuit complexity and mean
circuit speed of linearly quantized straight PCM video encoding techniques.
Any significant reduction in circuit complexity (i.e. the number of active
and passive devices to be integrated) is considered important since this
determines: chip area and yield if the encoder is to be fully integrated.
Analysis indicates that the complexity of the more highly developed straight
PCM video encoders can be reduced by typically a factor 3 using either non-programmed
sequential encoding, pulse width modulator encoding or programmed
sequential encoding (closed loop successive approximation).
The encoder studied in this work is an 8-bit pulse width modulator
video encoder using a 2-step production line technique and a detailed design
procedure for a prototype encoder is given. This encoder is considered to
achieve 7-bit resolution at a sampling rate of 13.3MHZ.
A mathematical model of the encoder-decoder system is developed for
numerical evaluation of the effect of encoder errors and white Gaussian noise
upon a coded and decoded video signal. A triangular wave test is applied to
examine the effect of encoder errors upon the statio transfer characteristic
of the encoder. Dynamic errors are investigated by simulating colour subcarrier
at the model input and observing the phase and gain errors at the filtered
codec output. Using differential phase and gain, an attempt is made to determine
a circuit design and alignment criterion such that most practical codecs will
fall within specific bounds on these parameters (taken as ±6° and ±6%
respective1y). In the absense of dither, Monte Carlo analysis indicates that
the maximum voltage error incurred by each encoder error source should have a
high probability (95%) of being less than a half quantum if 85 - 90% of codecs
measured are to fall within the above bounds. If white Gaussian noise is used
as a simple dither signal then the probability of a codec falling within the
above bounds may increase to about 95%.
Improvements to the encoder are discussed, including several
automatic error correction techniques which combat instrumental errors and
give a more robust PWM encoder. Also, by predetermining the most significant
bit for each set of 4 coded bits it is possible to halve the encoder clock
frequency (to 133MHZ) without significantly changing the encoder complexity.

Date of Award | 1975 |
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Original language | English |
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Awarding Institution | |
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DIGITAL ENCODING OF TELEVISION SIGNALS USING THE PULSE WIDTH MODULATOR

WADE, J. G. (Author). 1975

Student thesis: PhD