PAL SYSTEM TELEVISION MEASUREMENT : APPENDIX B — SINE-SQUARED PULSES

APPENDIX B — SINE-SQUARED PULSES

Testing Bandlimited Systems.
Fast rise time square waves cannot be used for testing bandlimited systems because attenuation and phase shift of high-frequency components cause ringing in the output pulse. These out-of-band distortions can obscure the inband distortions of interest.
Sine-squared pulses are themselves bandwidth limited, and are thus useful for testing bandwidth limited systems.
Description of the Pulse.
The sine-squared pulse looks like one cycle of a sine wave (see Figure 110).
Mathematically, a sine-squared  pulse is obtained by squaring a half-cycle of a sine wave.
P h y s i c a l l y, the pulse is generated by passing an impulse through a sine-squared shaping filter.

Figure 110. 2T pulse and 1T pulses for PAL systems.

T Intervals. 
Sine-squared pulses are specified in terms of half amplitude duration (HAD), which is the pulse width measured at 50% of the pulse amplitude.
Bandwidth limited systems are tested with pulses having an HAD that is a multiple of the time interval T. T, 2T, 10T and 20T are common examples. T is the Nyquist interval, or 1/2fc, where fc is the cutoff frequency
of the system to be measured. For PAL systems, fc is usually taken to be 5 MHz and T is therefore 100 nanoseconds. Most PAL test signals use this default value for T, even though the system under test may have a bandwidth of 5.5 or 6 MHz.
T Steps. 
The rise times of transitions to a constant luminance level (such as a white bar) are also specified in terms of T. A T step has a 10%-to-90% rise time of nominally 100 nanoseconds (see Figure 111). A 2T step has a rise time of nominally 200 nanoseconds.
Mathematically, a T step is obtained by integrating a sinesquared pulse. (This is why the T step has a rise time that is only nominally equal to T. The integral actually yields a rise time of 0.964T for a T step.) Physically, it is produced by passing a step through a sinesquared shaping filter.

Figure 111. T rise time step.

Energy Distribution. 
Sine-squared pulses possess negligible energy at frequencies above f = 1/HAD. The amplitude of the envelope of the frequency spectrum at 1/(2HAD) is one-half of the amplitude at zero frequency.
Energy distributions for a T pulse, 2T pulse, and T step are shown in Figure 112.

Figure 112. Frequency spectra of T pulse, 2T pulse, and T step.

Lihat juga

Table of Contents

Preface	3
Good Measurement Practices	4
EQUIPMENT REQUIREMENTS	4
CALIBRATION	6
INSTRUMENT CONFIGURATION	6
DEMODULATED RF SIGNALS	8
TERMINATION	8
DEFINITION OF THE PAL TELEVISION STANDARD	8
PERFORMANCE GOALS	8
Waveform Distortions and Measurement Methods	9
I VIDEO AMPLITUDE AND TIME MEASUREMENTS	9
Amplitude Measurements	10
Time Measurements	12
SCH Phase	15
II LINEAR DISTORTIONS	18
Chrominance-to-Luminance Gain and Delay	19
Short Time Distortion	24
Line Time Distortion	26
Field Time Distortion	28
Long Time Distortion	30
Frequency Response	31
Group Delay	36
K Factor Ratings	38
III NONLINEAR DISTORTIONS	41
Differential Phase	42
Differential Gain	46
Luminance Nonlinearity	50
Chrominance Nonlinear P h a s e	5 2
Chrominance Nonlinear Gain	5 3
Chrominance-to-Luminance Intermodulation	54
Transient Sync Gain Distortion	55
Steady-State (Static) Sync Gain Distortion	56
IV NOISE MEASUREMENTS	57
Signal-to-Noise Ratio	58
V TRANSMITTER MEASUREMENTS	6 0
ICPM	61
Depth of Modulation	63
GLOSSARY OF TELEVISION TERMS	64
APPENDICES
APPENDIX A - PAL COLOUR BARS	67
APPENDIX B - SINE-SQUARED PULSES