DPL Labs has created an extensive suite of tests covering all relevant aspects of HDMI™ cable performance. DPL Labs founder Jeffrey Boccaccio found that previously existing testing processes were not sufficiently broad-based enough in scope to accurately reveal true performance.
In order to fully test any HDMI™ cable’s performance, Boccaccio and his engineering team created a custom suite of tests that fully expose a cables strengths and weaknesses. To test to the level of resolution required, DPL Labs team had to create custom testing algorithms that are exclusive to the DPL testing process.
DPL Labs testing regime includes, amongst other things, testing of the following items:
Eye Pattern Mask Margin
A popular measure, the eye diagram or pattern is created when a digital data signal is repetitively sampled and applied to the vertical input of an oscilloscope while the data rate is used to trigger the horizontal sweep. This process results in a series of lines or traces that create a pattern on the scope’s screen that resembles an eye – hence the name.
The HDMI™ organization set the specification for the mask (flat blue hexagon at center of the screen) to indicate the minimum performance standard. If any trace touches the mask, the cables performance fails.
DPL tests and measures the percentage of headroom above the mask margin. More headroom means better performance. In the images shown here, the first example is virtually a perfect eye. You will note the symmetry of the eye image as well as the significant amount of space over the mask.
In the second image, you will see that the trace touches the mask. We call this “in the dirt” and this cable has failed DPL Labs tests and will be rejected.
Jitter refers to any time shift in the characteristic of any periodic signal, especially as it relates to the reference clock source. HDMI™ signals are at extremely high bandwidths and data rates. Excessive jitter degrades system performance and in extreme situations can cause system failure.
The HDMI™ specification sets the bandwidth; DPL engineers confirm the cable’s performance relative to this spec.
Cable impedance is one of the factors that impact the cable’s overall performance. DPL Labs engineers check for any negative impact as a result of impedance issues.
As you might suspect, excessive noise transmission in any cable will have a negative impact. Excessive noise could cause a performance failure, so DPL engineers check any cable’s noise performance.
5-Volt Supply Reliability
The HDMI™ specification provides for a +5-volt supply voltage. This voltage must not vary by more than .3 volts or serious problems can occur. DPL tests cables for this critical operating voltage.
Different cable designs have different current handling profiles. DPL engineers test each submitted cable for its individual current characteristics.
Inter-Pair Skew Test
A key factor in HDMI™ cable performance is inter-pair skew. Inter-pair skew refers to minute differences in cable lengths creating timing issues between two pairs of wires within the HDMI™ cable.
In the illustration here, the yellow trace is measuring the highly precise clock signal. Notice the separation between the clock trace and the blue channel trace. This is the very visible electronic impact of inter-pair skew.
Square Wave Rise Time
Square wave rise time is another critical element in the performance of the cable as it relates to the Digital Display Channel (DDC). While ideally, the result should be a perfect square wave, it is not uncommon to see designs with poor rise time, which can flatten out the leading edge.
In the example shown here, you see a slight rounding of the leading edge of the square wave. This rounding is the result of capacitive loading. In this case, the impact is not severe enough to cause problems.
In the second example, however, you can see severe rounding of the front edge of the square wave. This is a very poor rise time and would fail DPL Labs test for this parameter.
The capacitance of the DDC line is directly proportional to the square wave rise time. The higher the cable capacitance the greater the negative impact on the square wave rise time.