Test Signal Generators

A test signal generator generates test patterns, and other useful test signals, for troubleshooting and analzying television systems. These devices are generally intended for offline use (test patterns are seldom broadcast, unless a station is not operating properly or is off the air at the time), as they output complete television signals. Examples of signals output by such a device include:

Color bars, one of several test signals used to verify the proper reproduction of a TV system's color gamut, and/or that a television signal or plant is compliant with the appropriate analog transmission standards
Flat fields, a signal consisting of nothing but a specific color (typically white, black, a shade of gray, or one of the primary colors (red, green, and blue) at maximum saturation). A red field is especially important in PAL applications, as it is the "red difference" portion of the chroma signal whose phase alternates every line; the red field should appear as a solid block of color, with no visible "bands" going across the screen.
Multibursts, sweeps, and pulse signals, used to test the frequency response of a television system
Ramp signals and staircase signals are used to check the voltage linearity of a television system
Convergence patterns, used to check the alignment and linearity of a receiver
The bowtie signal, used to check the relative (inter-channel) timing of a component video signal.
A few specialized signals are used in digital environments:

The PLL test signal is a pathological test signal used to stress the phase-locked loop of a serial digital receiver; this is done by outputting a bit pattern which, after passing through the linear feedback shift register used to scramble serial digital signals, resulting (with a high degree of probability) in a long strings of zeroes or ones, followed by a long string of the opposite polarity, on the digital (NRZI) signal; an issue which can cause poorly-designed PLLs to unlock.
The Equalizer test signal is another pathological signal, consisting of a long string of zeroes or ones, followed by a single bit of the opposite polarity. It can cause poorly-designed cable equalizers to malfunction.
The SDI Checkfield signal, standardized by SMPTE RP178 (for SD) and RP198 (for HD), is a test signal which contains one of the above signals in the upper portion of the video, and the other in the lower portion of the video.
In addition, sophisticated signal generators may allow modification of the video timing, adjustment of the gains of the various components (including out of range), the introduction of jitter or bit errors (into digital signals), the introduction of motion, or other effects