PRODUCT HOW-TO: Dealing with the risks inherent in HDMI EDID testing

This article looks at the risks inherent in an ad hoc approach to testing EDID with a selection of random HDMI sources and sinks, and advocates a more systematic approach to HDMI test (Figure 1, below).

There's no doubt that HDMI, whatever its relative merits and demerits, is here to stay as the A/V interface of the foreseeable future.

And as the number of consumer and professional devices using HDMI to deliver audio and HD video grows, so pressure is increasing on A/V manufacturers to ensure that their devices' Extended Display Identification Data (EDID) is accurate.

Figure 1: If written such that it correctly describes its parent device, this unfriendly looking hex readout—the code at the heart of the EDID—can make HDMI equipment testing and troubleshooting much easier.

The idea of EDID is a powerful one, analogous to the spec sheet of an HDMI device in electronic form. Contained in one simple ROM, and in typically just a couple of hundred bytes of data, is all of the information about the video and audio formats the device can receive and do business with (Figure 2, below).

In the world of broadcast manufacturing and test, it's a welcome innovation, because if you want to know what formats a certain piece of hardware can cope with over HDMI, all you have to do is read its EDID. Or that's the theory. In practice, there seems to be quite a lot of equipment out there with EDID that doesn't accurately describe the device's capabilities—a frustrating state of affairs for manufacturers and end-users alike.

Figure 2: Like EDID, HDMI is a great idea in principle—a high-bandwidth format capable of carrying audio and video through a single connector.

Getting the labeling right
Generally speaking, the specification of HD AV equipment is not always as clearly expressed as it could be, especially in the consumer domain, where labeling is everything.

In the United States and Europe, for example, if you buy a TV that says 1,080p in large letters on the box, it's not always a guarantee that the screen can show video with a vertical resolution of 1,080 progressively updated lines.

Sometimes all the label does is indicate that the device can receive incoming video in the 1,080p format, which it then decimates before displaying it at a lower resolution.

There are similar ambiguities with EDID information, as EDID is concerned only with describing the formats that a device can successfully recognize and receive, and contains no information about how a particular format will be reproduced.

If the EDID says a device will accept audio at a sample rate of 96kHz, and it plays the audio back correctly, the EDID is accurate, even if the device actually achieves playback by sample-rate converting the audio down to 48kHz internally beforehand.

However, can it honestly be said that the EDID is accurate if the 96kHz audio is received by the device, but is reproduced full of dropouts and pops, as was the case with an HDTV Audio Precision's engineers were testing recently? It's not clear what causes the mistakes in HDMI EDID, but it's possible to speculate, based on some of the errors that crop up regularly.

Some manufacturers seem to be uncertain how to write the required information in accordance with the EDID specification. Some appear to be using off-the-shelf EDID information produced by third parties, which doesn't correctly describe their product. This might be due to oversight; perhaps the companies concerned have bought the third- party EDID as a starting point to describe their product, but they never get around to adapting it for their device.

Others may be producing EDID information for a top-of-the-range product, and then using that same data for all of the products in the range, without considering that the EDID is inaccurate for the entry-level product in the same range.