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The Digital Cliff and HDMI

Filed under: audio-video systems design,video — 2012-12-12 by Richard L. Hess — Last Edit 2012-12-12 by Richard L. Hess

I was at a church today where I struggled to get a video to play properly in advance of an event in a few days. There were issues with the application of a TRS phone plug (another story).

The church had a PC and two 60-inch diagonal video monitors plus a smaller monitor for the choir. There appeared to be a powered splitter/amplifier at the balcony console to split the HDMI to the two big monitors, with one cable running down each side of the church. I believe there must have been a second two-way splitter (hopefully another powered splitter/amplifier) to tap off the feed from the right main monitor for the choir monitor.

When I arrived, the PC was set up for 1024 x 768 display with the HDMI output mirroring the laptop built-in display. The laptop built-in display was capable of 1366 x 768. The slide show was a 16:9 WMV file. I tried expanding the display to the widescreen resolution and it was fine on the PC, but the monitors would not sync.

I tried my PC at 1280 x 720 and the left monitor with the shorter cable would sync and the right monitor would not.

When I set the church’s PC back to 1280 x 768  it had somehow been changed to 66 Hz refresh rate and the right monitor was tearing and sometimes breaking up.

This is the feared “digital cliff.” Once bitrates increase beyond a certain point for a certain cable run, the performance goes from excellent to unusable. In this case, it was a 10% change in data rate on the right channel.

So we left the PC set up to run at 1280 x 768 with a 60 Hz refresh rate and we’re going to have a black border around the video plus we’ll have people looking wider than they are as the image is stretched at the monitor. The church’s chair of the property committee indicated that the aspect ratio change was normal in their installation. They did not want to fuss with changing things for one event and I’m not convinced we could have found a fully workable solution considering where the digital cliff was situated. We needed to be on the other side of the digital cliff and it wasn’t going to happen.

When I installed a monitor in the narthex (lobby) of my church a few years ago, I went to great lengths to transmit full 1920 x 1080 HDMI signals over a longer distance than in the church today, but I used two CAT6 cables and a pair of MuxLab adapters (one at each end). We used this powered extender but I would certainly look at this passive one and see if that met my needs if I had to do this again. We came out of the PC’s DVI output with a DVI to HDMI adapter cable into the MuxLab. That fed two lengths of CAT 6 cable (we pulled three just in case) to the second MuxLab unit which then fed the display via an HDMI to HDMI cable (which ran down the inside of the pipe mount).

When I looked into this a couple of years ago, the extender and CAT 6 cable was about the same price (and much lower risk) than long-length HDMI cables. We did have an issue where we attempted to terminate the CAT 6 cable to plugs and would have to say DON’T DO THAT! Terminate the CAT 6 cable to jacks which are designed for field wiring and use CAT 6 jumpers between the jacks and the MuxLab units on both ends. Yes, it’s more connections, but it was a nightmare attempting to crimp the CAT 6 cable into plugs. This task is much better done at a factory where that is all they do.

When working with the data rates of HDMI (or even SDI) video, there is no substitute for actually engineering the link. Guesswork or a sales person’s word just won’t cut it. As I was installing the monitor in my church the electrician who was doing the work that required a license (hanging the monitor in a public space, providing electric power) said he had done this type of a system with a single coax, he couldn’t understand what I was fussing about until he saw the 1920 x 1080 P screen light up. It’s gorgeous. There is no comparison to NTSC!

Please, when designing video systems, do the engineering design for the distribution and get it all to work on paper (with a margin of safety) before pulling a single cable. My comment above about the risk with long HDMI cables is that they tend to hit the cliff many metres before the CAT 6 extender solution AND if you place a staple through a long HDMI cable, you’ve just ruined a multi-hundred dollar cable. I would (and did) pull a third CAT 6 cable for each run, to be safe.

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