Reading an Eye Diagram
The eye diagram is a valuable tool for analyzing the quality of a digital signal. It consists of the signal’s oscilloscope display, repetitively sampled to present an “at-a-glance” depiction of its behavior. It is particularly useful in evaluating how well an interconnect maintains signal integrity over distance.
As with any interconnect, signal attenuation and interference in an HDMI cable will increase over distance, until at a certain length the signal-to-noise ratio becomes unacceptable. As the signal degrades, the “eye” begins to collapse. Once the eye collapses into the mask (the area that describes the minimally acceptable opening for a signal to be recovered by the receiver of the signal – the HDTV or projector), then the signal is not strong enough to pass HDMI compliance.
The eye diagram on the left shows a robust, undistorted digital signal that does not encroach in the mask (gray area in the center). Notice the large area of undisturbed black space in the center of each sample, indicating that the signal is strong and noise is low. Note how signal noise and attenuation has closed down the “eye” and the signal has started to encroach on the mask.
Maximum length can vary widely based on the cable’s materials (i.e., copper or fiber), the quality of its construction, the quality of the HDMI receiver chip, and the presence or absence of active electronics in the transmission path. As well, a cable ability to successfully pass an HDMI signal depends on its bandwidth capability. The illustrations below demonstrate how twice the bandwidth (1080i versus 1080p) can affect a cable’s performance.