The Engineer's Guide to Standards Conversion - Snell

andwidth of the input signal. The HDTV display is being used as a spatially
oversampling conventional definition display. The subjective results of viewing an
oversampled display which has come from an oversampled camera are very close to
those obtained with a full HDTV system, yet the signals can be passed through
existing SDTV channels.
2.3 Interlace
Interlace was adopted in order to conserve broadcast bandwidth by sending only
half the picture lines in each field. The flicker rate is perceived to be the field rate,
but the information rate is determined by the frame rate, which is halved. Whilst the
reasons for adopting interlace were valid at the time, it has numerous drawbacks
and makes standards conversion more difficult. Fig 2.3.1a) shows a cross section
through interlaced fields. In the terminology of standards conversion it is a
vertical/temporal diagram. It will be seen that on a given row, the lines only appear
at frame rate and in any given column the lines appear at a spacing of two lines. On
stationary scenes, the fields can be superimposed to give full vertical resolution, but
once motion occurs, the vertical resolution is halved, and in practice contains
aliasing rather than useful information. The vertical/temporal spectrum of an
interlaced signal is shown in Fig 2.3.1b).
Field 2 Field 1
Vertical
distance
Time
Fig 2.3.1 a) In an interlaced system, fields contain half of the lines in a frame as
shown in this vertical/temporal diagram.
It will be seen that the energy distribution has the same pattern as in the
vertical/temporal diagram. In order to convert from one interlaced standard to
another, it is necessary to filter in two dimensions simultaneously.
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