Whatever its relative merits and demerits may be,
HDMI is here to stay as the audio/video (A/V)
interface of the foreseeable future. As the number
of consumer and professional devices using
HDMI to deliver audio and HD video grows, the pressure mounts
on A/V manufacturers to ensure that their devices’ Extended Display
Identification Data (EDID) is accurate (Fig. 1).
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 usually in just a couple of hundred bytes of
data, is all of the information about the video and audio formats
the device will handle.
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 (Fig. 2). 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, frustrating
manufacturers and end-users alike.
EDID ISSUES
Generally speaking, the specification of HD A/V equipment
isn’t always as clearly expressed as it could be. This is particularly
the case in the consumer domain, where labeling is everything.
In the U.S. and Europe, for example, if you buy a TV that says
“1080p” in large letters on the box, it’s not always a guarantee
that the screen can show video with a vertical resolution of 1080
progressively updated lines. Sometimes, the label only indicates
that the device can receive incoming video in the 1080p 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. It doesn’t contain information
about how a particular format will be reproduced.
If the EDID says a device will accept audio at a sample rate of
96 kHz, 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
48 kHz internally beforehand.
However, can it honestly be
said that the EDID is accurate
if the 96-kHz audio is
received by the device but
is reproduced full of dropouts and pops (as was the case with an
HD television recently tested by Audio Precision’s engineers)?
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.
In other instances, off-the-shelf EDID information produced
by third parties is being used, and it doesn’t correctly describe
the product. This might be due to oversight; perhaps the companies
concerned bought the third-party EDID as a starting point
to describe their product, but they never got around to adapting
it for their device. Others may be producing EDID information
for a top-of-the-range product, and then they use 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.
WHAT’S THE PROBLEM?
There’s no doubt that producing accurate HDMI EDID requires
care. Add to that the increasing number of available formats for HD
video and audio delivery, and the task becomes even more complex.
It could be that the situation will both improve and worsen
over time. Manufacturers may become more appreciative of the
importance of producing accurate EDID information and more
accustomed to the task as time goes by. But new formats will
continue to develop, which means it won’t be easy to stay on top
of the situation.
It’s been interesting to watch the growing popularity of the
now-regular PlugFest events organized by HDMI manufacturers.
This can be interpreted as a measure of just how difficult it can
be in this digital, metadata-driven age to create products that are
compatible with a wide range of industry hardware. For those
unfamiliar with PlugFests, they’re essentially opportunities for
manufacturers to meet and bring along their hardware and software,
sometimes at an early stage in the development of a new
product, and try connecting it to other manufacturers’ equipment.
At this year’s Spring PlugFest, the Audio Precision team was
able to observe what was happening down at the metadata level.
Some of the HDMI devices at the show were connected together,
thanks to a recently
added feature in
Audio Precision’s
APx analyzer that
presents metadata
in a view like a logic
analyzer (Fig. 3).
One example
that was observed
involved two
HDCP content-protected devices. When hooked together, they have to decide
whether they’re going to allow digitally protected content to pass
from one to the other. They’re supposed to have a short “digital
handshake,” authenticate each other, and if successful, exchange
encryption keys and begin encrypted transfer.
Yet some of the hardware for this process was authenticating,
starting encryption, de-authenticating, re-authenticating, starting
again... The devices took a lot longer than normal to settle down
and get on with the transfer.
With consumer equipment pairings like HDMI-equipped Bluray
players and TVs, authentication errors like this could lead to a
noticeable delay of several seconds between a consumer pressing
“Play” and playback beginning on screen. That’s the kind of thing
consumers would notice and would compare unfavorably to old
home video systems, even analog ones like VHS.
CONSUMER PRESSURE
Some companies argue that they attend PlugFest to pick up on
these kinds of problems and that further action is unnecessary.
It’s true that some consumers will never encounter problems, depending on the devices they hook up via HDMI and the formats
they use. Certainly, none of this is likely to matter if a manufacturer
of broadcast technology produces devices that respond to
well-established standard audio and video formats and correctly
flag this fact in their EDID.
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