by Jeff Fisher
It Doesn't Work!
Why Doesn't It Work?
- Some of you may remember that some combinations of early HDMI products wouldn't work together at all. While much blame was initially placed on cables, a couple of manufacturers eventually admitted that they didn't meet the spec and subsequently fixed their problems. Some of that equipment is still around. If you're having trouble with HDMI between a particular source device and a display, and the source device is relatively old (in HDMI terms, pre-v1.3 / pre-2006), the source device may be to blame.
- It turns out that all of these "active" HDMI devices contain chips that run complicated software (technically, "firmware"). This firmware is written to meet a specific version of the HDMI specification, one that may be different than the other equipment in the chain. The firmware probably assumes that it is talking directly to the source device(s) on one end and the display device(s) on the other. Switches, splitters, and extenders are not transparent! Each time you insert another HDMI device in the chain it adds another firmware layer and increases the likelihood of an incompatibility. A device can work perfectly in the simple configurations the manufacturer tests it in, and it can meet spec, but it can still fail when used with other interconnecting devices because of version differences.
- The HDMI specification doesn't adequately address splitters and routers. If it did, many of the existing incompatibilities could be eliminated. When designing switchers, splitters, and extenders, manufacturers make many decisions affecting the compatibility with other interconnecting devices with little guidance. Often manufacturers purchase chips with the firmware already installed. So the chip manufacturer is making these decisions. More than once I've seen a splitter and switch from the same company that was incompatible because the manufacturer used chips from different sources in each.
- The source device puts a voltage on the HDMI cable. The display device and intermediate active devices can pull current from the source. The resulting voltage drop can cause all kinds of problems. Since "self-powered" active devices, such as an extender without a separate power supply, get their power from this line, when the voltage on that line drops too far, they can quit working. Which device draws and/or supplies power? Do they draw or supply the power all the time or just when they are turned on? This problem alone makes it impossible to guarantee that a complex HDMI configuration will work.
- HDMI keeps changing. It's very complex, very high speed, and equipment is sold at commodity prices. (Sometimes I wonder how it works at all.) While it's a good thing that HDMI is keeping up with consumer demand for new features, it makes it difficult for manufacturers of all the "glue" equipment (chips, cables, splitters, switches, extenders, etc.) to keep up. And every time a new version comes out, it at least doubles the number of combinations and permutations of HDMI versions that may be used in a single installation.
- Lastly, the biggie: The HDMI protocol includes a negotiating process between the source device and the sink (usually display) device. The two devices talk together to decide all kinds of things, like the ultimate dimensions of the video stream, and whether copy protection could be violated. The copy protection failure is the primary reason for the "flashing on and off" and "picture for about 10 seconds then nothing" video problem. Imagine that there are multiple displays connected to a single source, and multiple splitters/switches/extenders in-between them. This negotiation process automatically includes all these active devices and can be quite unpredictable. Even when everything works, keep in mind that the negotiation will result in a video feed selected to work on the lowest definition monitor. A higher definition display in this configuration would be presented with a lower resolution image than it is capable of displaying.
To complicate matters further, you can imagine that the equipment power-on sequence can cause different results! As well as the current power on, power off (standby) and unplugged state of each piece.
What Will Work
We carry HDMI combination devices that combine a switch and splitter, and devices that combine a splitter and extenders. Even though these products are sometimes more expensive than the cheapest of the separate versions, at least you can expect that these will work for you!
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