Fast rise time square waves cannot be used for testing bandwidth-limited systems because attenuation and phase shift of out-of-band components will cause ringing in the output pulse. These outof- band distortions can obscure the in-band distortions that are of interest.

Sine-squared pulses are bandwidth limited and are useful for testing bandwidth- limited television systems. ‘’’Description of the Pulse:’’’ Sine-squared pulses look like one cycle of a sine wave as shown. Mathematically, a sinesquared wave is obtained by squaring a half-cycle of a sine wave.

Physically, the pulse is generated by passing an impulse through a sinesquared shaping filter.

‘’’T Intervals:’’’ Sine-squared pulses are specified in terms of half amplitude duration (HAD) which is the pulse width measured at the 50% pulse amplitude points. Pulses with HADs which are multiples of the time interval T are used to test bandwidth limited systems. T, 2T, and 12.5T pulses are common examples. T is the Nyquist interval, or 1/2 fc where fc is the cutoff frequency of the system to be measured. For NTSC, fc is taken to be 4 MHz, thus T is 125 nsec.

‘’’T Steps:’’’ The rise times of transitions to a constant luminance level (such as the white bar) are also specified in terms of T. A T step has a 10% to 90% rise time of nominally 125 nsec, while a 2T step has a rise time of 250 nsec. Refer to the figure below.

‘’’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/(2 HAD) is one-half of the amplitude at zero frequency.