The Engineer's Guide to Standards Conversion - Snell

SECTION 2 - SOME BASIC PRINCIPLES
2.1 Sampling theory
Sampling is simply the process of representing something continuous by periodic
measurement. Whilst sampling is often considered to be synonymous with digital
systems, in fact this is not the case. Sampling is in fact an analogue process and
occurs extensively in analogue video. Sampling can take place on a time varying
signal, in which case it will have a temporal sampling rate measured in Hertz(Hz).
Alternatively sampling may take place on a parameter which varies with distance, in
which case it will have a sample spacing or spatial sampling rate measured in cycles
per picture height (c/p.h) or width. Where a two dimensional image is sampled,
samples will be taken on a sampling grid or lattice. Film cameras sample a
continuous world at the frame rate. Television cameras do so at field rate. In
addition, TV fields are vertically sampled into lines. If video is to be converted to
the digital domain the lines will be sampled a third time horizontally before
converting the analogue value of each sample to a numerical code value. Fig 2.1.1
shows the three dimensions in which sampling must be considered.
Vertical image
axis
Time axis
Horizontal image axis
Fig 2.1.1
The three dimensions concerned with standards conversion. Two of
these, vertical and horizontal, are spatial, the third is temporal.
Vertical and horizontal spatial sampling occurs in the plane of the screen, and
temporal sampling occurs at right angles (orthogonally sounds more impressive).
The diagram represents a spatio-temporal volume. Standards conversion consists of
expressing moving images sampled on one three-dimensional sampling lattice on a
different lattice. Ideally the sample values change without the moving images
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