What type of display shows a signal both above and below the sweep line in ultrasonic testing?

A radio frequency (RF) display is the correct choice because it represents the ultrasonic signals in a way that allows for visual analysis of both the positive and negative amplitudes of the echo signals relative to the baseline or sweep line. In this context, the sweep line typically indicates the time base on the display, and the RF display plots the amplitude of the received ultrasonic echoes both above and below this line.

This dual representation is essential for evaluating the characteristics of the signals, such as their strength, timing, and phase relationships. The ability to see both positive and negative signals helps technicians identify flaws or discontinuities in the material being tested, as they can observe not just the presence of echoes but also their relative strengths and patterns.

Other display types, such as video displays, are primarily used for visual imaging and may not show such detailed amplitude information, while audio displays output sound rather than a visible signal graph. Frequency modulated displays pertain to changes in frequency rather than amplitude interpretation and do not provide a representation of both above and below the sweep line in the same context as RF displays.