MEASUREMENTS

Good Measurement Practices
Figure 1. Tektronix TSG-271 PAL Test Signal
Generator.
• EQUIPMENT REQUIREMENTS
Television system performance is evaluated by sending test signals
with known characteristics through the signal path. The signals
are then observed at the output (or at intermediate points) to
determine whether or not they are being accurately transferred
through the system. You need two basic types of television test
and measurement equipment to perform these tasks: test signal
generators provide the stimulus, and specialised oscilloscopes
known as waveform monitors and vectorscopes provide
displays for evaluating the response.
Test Signal Generators
Television signal generators typically provide a variety of test
signals and synchronisation signals, ranging from colour black
to more complex signals designed for evaluation of specific
distortions. Each generator model offers a different selection of
signals, so be sure to choose one that provides all the signals
you need for characterising your system. It is also important to
take a close look at the output signal specifications — generator
performance is critical to accurate system testing. You need to
be confident that any distortions you observe are introduced
by the system under test and not by the generator itself.
Analogue generators, such as the Tektronix 1411 PAL Test
Signal Generator, have traditionally provided highly accurate,
flexible signal generation. While analogue generators are still
widely used today, digital generators are increasingly being
chosen for modem facilities. Digital generators are very stable,
and tend to be less expensive than their analogue counterparts.
The Tektronix TSG-271 PAL Television Generator is an
example of a general-purpose digital test signal generator.
Most generators provide test signals which occur on every active
line of the video field. These signals, known as full-field signals,
can be used only for out-of-service testing: the system cannot
simultaneously pass programme material and full-field test
signals. In-service tests can be performed if the test signals
appear only in the vertical interval of the programme signal.
Several blank lines are available in the vertical interval for this
purpose, and generators such as the VITS 201 PAL Insertion
Generator perform the insertion function.
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