A multiburst waveform is a useful

test pattern A test card, also known as a test pattern or start-up/closedown test, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast (often at sign-on and sign-off). Used since the ear ...

used to quickly ascertain the

frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of s ...

of a

video Video is an Electronics, electronic medium for the recording, copying, playback, broadcasting, and display of moving picture, moving image, visual Media (communication), media. Video was first developed for mechanical television systems, whi ...

system. The name derives from the multiple bursts of

sine wave A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in ...

in the active video section of the waveform which are generated with (usually) increasing

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...

, but identical in

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of a ...

. The test pattern is usually input at the start of the

signal chain Signal chain, or signal-processing chain is a term used in signal processing and mixed-signal system design to describe a series of signal-conditioning electronic components that receive input (data acquired from sampling either real-time phenome ...

and analysed at the end. By measuring the amplitude of each of the 'bursts' of waveform at the end of the video chain, spot measurements of the frequency response of the system can be obtained and

errors An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. The etymology derives from the Latin term 'errare', meaning 'to stray'. In statistic ...

in the response quickly identified.

Importance of frequency response

In a video system, the frequency response needs to be as flat as possible or

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio s ...

in the picture displayed will occur. Analogue video signals contain a frequency content from 25 Hz up to around 5 MHz, and so variations in frequency responses will affect the picture in various ways, depending on whether it is high frequency (>1 MHz) or low frequency (<1 MHz) distortion. Low frequency distortion will cause field-rate impairment, manifesting itself as

luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...

variation between the top and bottom of a picture. High frequency distortion causes problems with

sharpness Sharpness ( ) is an English port in Gloucestershire, one of the most inland in Britain, and eighth largest in the South West. It is on the River Severn at , at a point where the tidal range, though less than at Avonmouth downstream ( typical sp ...

of the picture: high frequency

roll-off Roll-off is the steepness of a transfer function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband. It is most typically ap ...

causes loss of

definition A definition is a statement of the meaning of a term (a word, phrase, or other set of symbols). Definitions can be classified into two large categories: intensional definitions (which try to give the sense of a term), and extensional definitio ...

, while high frequency peaking emphasises edges and adds

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

to the picture.{{cite web, title=Frequency Response Testing, url=http://www1.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/NTSC_Video_Msmt/videotesting2.html, website=Tektronix, publisher=Tektronix, archiveurl=https://web.archive.org/web/20141031003122/http://www1.tek.com/Measurement/cgi-bin/framed.pl?Document=%2FMeasurement%2FApp_Notes%2FNTSC_Video_Msmt%2Fvideotesting2.html, archivedate=31 October 2014, url-status=dead Ideally, each of the frequency bursts should produce a group of distinct, black and white vertical bars on the monitor screen. Due to frequency response roll-off in the signal transmission path, the higher frequency vertical bars tend to blur into a uniform grey. From this, it is possible to get a quick visual estimate of the overall frequency response of the system, since the frequency of each of the bursts is known.

References

Test cards

Importance of frequency response

See also

References