Can Category Cable Replace HDMI?
An inter-skew test on an oscilloscope reveals the gap in time accuracy among three sets of 25-foot twisted pairs representing three video channels (red, blue and green lines) vs. the clock channel (yellow). In this test, the blue cable performed the worst. Click here to enlarge image.
Since there is no said standard for testing ATDs and the cable they use, DPL had to develop a new set of standards that would emulate the same types of testing limits as HDMI’s current cable specification. There are a series of tests to guarantee that HDMI cable transmission lines function in harmony with all HDMI products. Some of the key evaluations are:
Video integrity: Determined by sending high-speed data through the cable and examining what it looks like as it exits the transmission line.
Supply voltage: There are certain limits of loss over the length of an HDMI cable. These voltage limits are very small and must be measured down to a tenth of a volt.
Near-field and far-field crosstalk: This can be the silent HDMI killer that many don’t know exists and in many cases gets passed over as part of the operational specification. There are well-defined tests to determine the integrity of each video channel’s crosstalk values.
HotPlug detect: This function is also mandatory for the interface to work and supplies the necessary trigger to start HDCP and EDID. These tests are much lower in frequency and in many ways stay under the radar from many firms.
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But the one we will focus on here is for Category 5/6 HDMI cable substitutes: timing.
Timing is one of the mandatory attributes needed for HDMI to work. There are two types of timing functions that need to be within certain performance limits: inter-pair skew and intra-pair skew, and they’re taken from the red, blue and green video channels and the clock channel.
Inter-pair skew is the time accuracy between two separate twisted pairs. In this case, the clock channel and all three video channels. Each video channel within the HDMI interface is differential, which means it uses two wires instead of one: a non-inverting wire matched with an inverting wire. In most cases these two wires are twisted pairs, and in an effort to reduce crosstalk these twists must be perfect in symmetry for each wire within the pair to be equal in length. The length differences between each pair cannot be more than 1.7ns (nanoseconds).
Intra-pair skew is similar in that it is the measurement of lengths between the two wires that make up a twisted pair. There are a total of four video channels that use this configuration and all of them must be below 112ps (picoseconds).
Has anyone taken the time to measure Cat 5/6 using the HDMI specification as the base? We did, and the findings were pretty astonishing. We used four brands of Cat 5/6 and cut each brand to 25, 50 and 100 feet. They were then terminated with RJ45s to achieve accurate in-field results.
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There was no question that the longer we went, the more skew errors there were. Fig. 1 demonstrates what the interpair skew test looks like on our oscilloscope. The yellow is the clock trace from which we reference. Then there is a trace for red, blue and green. The difference between the clock channel from each of the other color channels is the measurement we are examining. When measured for 25 feet, the inter-pair skew was reasonable at 920ps, or .920ns, from the worst channel (in this case blue). This was at least below the spec limit of 1.7ns.
Then we moved to the 50-foot category cable. With the longer cable, the inter-pair skew climbed from .920ns to 1.8ns, breaching the HDMI maximum skew specification.
As you would imagine, it only got worse when we sampled our 100-foot cable. This sample produced an inter-pair skew of a whopping 2.5ns, almost twice as much over the rated minimum. And these samples were the best of the bunch.
Lesson learned? If you’re going to use Cat 5 and 6 you best seek out these products that are built more along HDMI’s performance limits and not Ethernet’s. Then, of course, you have to take into consideration the RJ45 installation and the reality of wiring a home through studs and walls. There is no question that a well-manufactured HDMI cable is the way to go. Add the necessary EQ to the cable and you’re off and running.