The Engineer's Guide to Standards Conversion - Snell

SECTION 1 - INTRODUCTION TO STANDARDS CONVERSION
1.1 What is a standards converter?
Strictly speaking a television standard is a method of carrying pictures in an
electrical wave form which has been approved by an authoritative body such as the
SMPTE or the EBU. There are many different methods in use, many of which are
true standards. However, there are also signals which are not strictly speaking
standards, but which will be found in everyday use. These include signals specific to
one manufacturer, or special hybrids such as NTSC 4.43.
Line and field rate doubling for large screen displays produces signals which are
not standardised. A practical standards converter will quite probably have to accept
or produce more than just “standard” signals. The word standard is used in the
loose sense in this guide to include all of the signals mentioned above. We are
concerned here with baseband television signals prior to any RF modulation for
broadcasting. Such signals can be categorised by three main parameters.
Firstly, the way in which the colour information is handled; video can be
composite, using some form of subcarrier to frequency multiplex the colour signal
into a single conductor along with the luminance, or component, using separate
conductors for parallel signals. Conversion between these different colour
techniques is standards conversion.
Secondly, the number of lines into which a frame or field is divided differs
between standards. Converting the number of lines in the picture is standards
conversion.
Thirdly, the frame or field rate may also differ between standards. Changing the
field or frame rate is also standards conversion. In practice more than one of these
parameters will often need to be converted. Conversion from NTSC to PAL, for
example, requires a change of all three parameters, whereas conversion from PAL to
SECAM only requires the colour modulation system to be changed, as the line and
field parameters are the same. The change of line or field rate can only be performed
on component signals, as the necessary processing will destroy the meaning of any
subcarrier. Thus in practice a standards converter is really three converters in
parallel, one for each component.
1.2 Types of converters
Fig 1.2.1 illustrates a number of applications in which some form of standards
conversion is employed. The classical standards converter came into being for
international interchange and converted between NTSC and PAL/SECAM.
However, practical standards converters do more than that. Many standards
converters are equipped with comprehensive signal adjustments and are sometimes
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