11.20.2007 — The display data channel (DDC) line continues to be, in many cases, one prime area for HDMI-related problems to exist.

One major attribute of the DDC line is its load impedance. It is pretty high, as much a 4.7K Ohms.

When dealing with load impedance that high, any significant loading on the buss can reduce its dynamic range or corrupt the data that is being transported over it.

The loading is not so much resistive, but capacitive, which means that it distorts the data wave form. If you consider what a square wave looks like, a capacitive loading problem can exist on the leading edge of the wave form.

If there is too much leading edge distortion, the receiver will not be able to understand the source instructions. These are critical for the system to work.

These instructions, which include audio enabling, screen native rate arbitration and HDCP authentication, must be communicated or the damn thing won’t work!

Although these loading issues are, in fact, covered in the HDMI spec, if production products go out the door with higher or close-to-the-edge capacitive loading, and are coupled to other devices that are not much better, data will be lost.

You’ll be left scratching your head with that ugly thought of replacing products.

This is not necessary. The system works. Source, sink and cable firms must follow the rules and everyone will have a great day.

Jeff Boccaccio, principal of InVisions Technology, can be reached by e-mail at .