Friday, July 13, 2007

In Praise of HDMI

As I said in Aiming Towards a New Living Room TV, I am planning to connect a TiVo Series3 digital video recorder to a 46" flat-panel HDTV from Sony, possibly the model KDL-46XBR2, XBR3, or XBR4. I'll also incorporate a Blu-ray high-def disc player or HD DVD player into the system at some future point. All of the interconnection of components will be via HDMI.

High-Definition Multimedia Interface offers a single-cable connection that carries both video and audio signals. The "HDMI source" can be, say, a DVD player or a DVR. The "HDMI receiver" can be, for example, an HDTV or an audio/video receiver. And the HDMI receiver can send its input HDMI signals on to yet another device. For example, a DVD player can output HDMI signals directly to an A/V receiver, which passes them along to an HDTV. Yet more complex HDMI interconnections use repeaters, distribution amplifiers, and/or switches.

HDMI carries only digital (not analog) signals: high-def video in all available compressed and uncompressed formats; compressed and uncompressed audio in numerous codecs, including the new "lossless" ones on Blu-ray discs and HD DVDs; and other information that allows pressing one button on a remote to control multiple components. This feature is sometimes called "one-touch play," because pressing PLAY on the remote can turn on the TV set and the A/V receiver, select the right inputs on each, and begin playing a DVD.

One of the best things about HDMI is that it eliminates "cable clutter" by becoming the sole cable between any pair of components.

Because HDMI uses two-way communication between source and receiver, a system using HDMI interconnections can automatically configure itself. HDMI devices automatically work together to deliver the most effective video format (480p vs. 720p vs. 1080i/p, 16:9 vs. 4:3) for the source device and the display that it is connected to — eliminating the need for the user to scroll through all the format options on each device to guess what looks best.

The same is true for audio. A source device such as a DVR and an A/V receiver can negotiate to use (say) either Dolby Digital or 2.0-channel linear PCM, depending on the capabilities of each device, and on the nature of the program material.

For example, I now have a Sony KDL-40XBR2 flat-panel HDTV in my bedroom, fed both its audio and its video by a TiVo Series3 DVR, via HDMI. The TiVo is connected to a Comcast cable feed as its input, and uses two CableCARDs to authorize access to the channels I subscribe to. Some of these channels are the old analog kind; their audio is likewise analog. Some channels/programs are digital but lack Dolby Digital 5.1 audio, while other digital channels/programs do have DD 5.1 multichannel audio.

As far as I can tell from the Sony literature, the only type of digital audio the TV can decode is 2.0-channel PCM. It can't handle DD 5.1.

No matter what kind of audio is on the program I am watching, though, it plays fine through the TV.

That means the TiVo must be transcoding the DD 5.1 soundtracks it receives into 2.0-channel PCM. (PCM stands for "pulse code modulation," which is how "linear" or uncompressed digital audio is carried.)

Interestingly, in one of the TiVo's settings menus there is an option that tells the TiVo to transcode Dolby Digital to PCM. In my system, it's unnecessary. Even when it's not enabled, the digital audio sent to the TV gets transcoded anyway!

Why? Because the HDMI two-way interconnection is smart. In effect, the TiVo tells the TV, "Hey, I have a Dolby Digital 5.1 soundtrack I want to send you. Can you handle it?" The TV answers back, "No can do. I can only use 2.0-channel linear PCM." So the TiVo says, "Fine with me, I'll just transcode it so you can deal with it."

However, I cannot tell that my TiVo and TV coordinate video options like aspect ratio and resolution in any way. Pending further research, my assumption is that not all HDMI devices yet take advantage of all the possibilities HDMI offers.

Anyone interested in learning more about HDMI can visit I encourage you to take their quick, 30-minute audio-visual tutorial here.

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