V IDEO Q UALITY I NDEX

The overall of video quality is influenced at a number of index: amplitude, timing, linear distortion, nonlinear distortion and noise in Table 2-1.

Index Description Influence How to measure

Amplitude

NTSC defines

peak-to-peak amplitude 1 volt (140 IRE)

Too light or too dark 100 IRE white level

Timing

Horizontal and vertical synchronization pulse widths fall within specified limits

Picture breakup Any composite signal

Linear Distortion

modulation, T rise time white bar, 18 us 100 IRE bar, window or field square wave colors not reproduced,

Modulated staircase, unmodulated 5 step staircase, modulated pedestal

Noise

Noise random or coherent from natural and

man-made sources

Snowy, grainy, sparkles Any line with constant level

Table 2-1 Video Quality Index

Amplitude

Video amplitudes are most frequently measured in order to verify the conformity to nominal values. Measurement of the peak-to-peak amplitude is 1 voltage for NTSC systems. 1 IRE is defined into an absolute unit equal to 1/140 of 1 voltage. Signal amplitude can be measured with a waveform monitor. The amplitude of gain errors causes the picture to appear too light or too dark.[22]

Timing

Both RS-170A and the FCC provide the recommended limits for these timing parameters. However, the two standards have different definitions for various time

intervals. FCC specifies sync width between the 90% points of the two transitions as shown in Figure 2-6. RS-170A specifies sync width between the 50% points as shown in Figure 2-5. The RS-170A requirements are generally more stringent.[22]

Figure 2-6 RS-170A Defined Timing Standard

Figure 2-7 FCC Defined Timing Standard

Linear Distortions

All systems require a specific bandwidth to transmit all significant luminance and chrominance information. Table 2-2 summarizes significant linear distortions and gives a short description of the test concept.[8]

Parameter Definition Test signal Test method

Gain versus frequency response

Gain variation over band from 500kHz to maximum frequency

Multiburst or sweep Measure signal level at selected frequencies in dB wrt level at 500kHz Long-time centerer of bar excluding first and last 250 µs centerer of bar excluding first and last 1 µs

K factor Quantified subjective

impairment rating

2T sine-squared pulse Read K rating in % on Special graticule Chrominance to

luminance gain

Change in ratio of chrominance versus

Change in timing of chrominance versus in ns wrt to the luminance component

Table 2-2 Summary of Linear Distortions and Test Methods

Nonlinear Distortions

Table 2-3 summaries and defines the nonlinear distortions and gives a short description of the measurement concept.[5] The most common measurable nonlinear distortions are luminance nonlinearity, differential gain and differential phase.[8]

Parameter Definition Test signal Test method

Luminance nonlinearity

Output/input amplitude

proportionality change of a small unitstep function as the step level is shifted from blanking level to white lecel. The average picture level is kept constant.

Luminance staircase

Measure % of largest step amplitude variation

proportionality change as the subcarrier amplitude is varied from a minimum to a maximum

specified value. Luminance and average picture levels are kept constant. reference to step 2

Chrominance phase nonlinearity

Chrominance subcarrier phase variation when the subcarrier amplitude is varied from a minimum to a maximum specified value. Luminance and average picture levels are kept constant.

Modulated pedestal

Measure largest phase difference(in degrees) of steps 3 or 2 with reference to step 1

Chrominance to luminance intermodula- tion

Variation of luminance signal amplitude resulting from the superimposition of a specified amplitude chrominance signal. The average picture level is kept constant.

Modulated pedestal

Measure % of largest luminance level change due to chrominance level

Differential gain

Amplitude change of a constant small-amplitude chrominance subcarrier superimposed on a luminance signal level that changes from blanking to white. The average picture level is kept constant.

Modulated ramp or staircase

Measure % of largest chrominance subcarrier level change with reference to burst amplitude

Differential phase

Change in phase of a constant small-amplitude chrominance subcarrier without phase modulation superimposed on a luminance signal level that changes from blanking to white. The average picture level is kept constant.

Modulated staircase or ramp

Measure largest chroma phase change in degrees with reference to burst phase

Table 2-3 Summary of Nonlinear Distortions and Test Methods

Noise

Table 2-4 summaries the types of noise encountered in studio environment and gives a short description of the test method.[23] The test signal for the measurement of noise is a flat field at the black level.[8]

Parameter Definition Test signal Test method

Continuous random noise

Ratio, expressed in dB, of the nominal amplitude of the luminance signal (714.3 or 700 mV) to the RMS amplitude of the noise in a frequency band extending from 10 kHz to the upper frequency of the video band (4.2,5,5.5 or 6 MHz).

Flat field Feed test signal to input of equipment under test and measure SNR at output with specialized RMS-reading instrument. Alternately, use an oscilloscope preceded by a band-limiting filter and synchronized to display single line.

Hum Ratio, expressed in dB, of the nominal amplitude of the luminance signal (714.3 or 700 mV) to the peak-to-peak amplitude of the noise after band-limiting to 10 kHz.

Flat field Feed signal to input of equipment under test and measure SNR at output with specialized peak-to-peak reading instrument. Alternately, use an oscilloscope synchronized to display a single field and measure peak-to-peak hum amplitude.

Other Ratio, expressed in dB, of the Flat field Feed test signal to equipment

periodic noise

nominal amplitude of the luminance signal(714.3 or 700 mV) to the peak-to-peak amplitude of the noise in a frequency band extending from 1 kHz to the upper frequency of the video band(4.2,5,5.5, or 6 MHz).

under test and use an

oscilloscope to isolate, identify, and measure peak-to-peak amplitude of interfering single in the specified frequency band.

Crosstalk Ratio, expressed in dB, of the nominal amplitude of the luminance signal (714.3 or 700 mV) to the peak-to-peak amplitude of the interfering signal.

Sweep signal and flat field

Feed sweep to unwanted channel and flat field to wanted channel.

Normalize gains and measure the peak-to-peak value of the interfering signal at the wanted channel output.

Table 2-4 Summary of Noise and Test Methods