Interface picture clears up
|By Jeffrey Krauss, |
Interface Interrogator and President of Telecommunications and Technology Policy
If you look at interface standards developed by the Consumer Electronics Association, you will find four that can carry a high definition video signal from one device to another. They are:
- EIA/CEA-775-A, which tells how to use the IEEE-1394 "Firewire" digital bus to carry compressed MPEG video packets;
- EIA/CEA-861-B, which tells how to use the computer industry's Digital Visual Interface (DVI) to carry uncompressed digital video;
- EIA/CEA-770.3-A, which defines a wideband analog component interface (R,G,B or Y, Pr, Pb); and
- EIA/CEA-761-A, a remodulation standard that allows a set-top box to create an 8-VSB RF signal on channel 3 or 4 for a wired connection to a digital TV antenna port.
HD digital TV receivers today come as two separate pieces, a monitor and a set-top box. The set-top box contains a digital 8-VSB tuner and MPEG decompression circuitry. The monitor is just a dumb display. Some may be smart enough to convert between 1080i and 720p picture formats, but others require the set-top box to do that conversion. The Best Buy Web page shows about 40 of these HD monitors, and they all say "requires an optional set-top box to receive over-the-air digital TV broadcasts." But I could never find the set-top boxes at www.bestbuy.com. (I did, however, find set-top boxes for receiving satellite HD video.)
And that's because people have been buying the HD monitors to watch DVDs. The HD monitors usually have the 770.3 wideband analog component interface (and most also have an S-video interface which supports a standard definition picture). The wideband component video interface is well-suited for high-end DVD players with 480p progressive scan video output. The sales of set-top boxes to receive TV broadcasts have been miniscule.
The 770.3 wideband interface has a problem, however. There is no associated copy protection method that has been developed and is acceptable to the Hollywood studios. In contrast, the DVI interface has an associated copy protection method called HDCP (see http://www.digital-cp.com). And the 1394 interface has a copy protection method called DTCP (see http://www.dtcp.com), also known as 5C. (The 8-VSB remodulation interface does not have a copy protection method, but that doesn't matter because, unlike the other three, it isn't directly recordable. And anyway, nobody uses it.)
The 770.3 wideband interface and the DVI interface have similar applications. They carry uncompressed video, so they are well-suited for connecting from a set-top box that does the MPEG decompression to a dumb monitor or display.
In contrast, the 1394 interface carries compressed MPEG packets. It is now becoming clear that its use will be primarily between a set-top box and a digital video recorder. If you are going to record a digital HD program, you want to record the compressed signal at the rate of 19 Mbps, not the uncompressed rate of several Gbps. At 19 Mbps, a 90-minute HD movie takes up about 13 GBytes of hard drive space, well within the capacity of today's 80 GByte hard drives. At 2 Gbps, it would take 1350 GBytes, which is impractical.
The cable industry, in a recent submission to the SCTE Digital Video Subcommittee, is in the process of adopting a standard for a digital set-top box. The set-top box (called a host device) handles the QAM demodulation and the MPEG decompression. It feeds uncompressed digital video to the display over a copy-protected DVI interface. It feeds compressed digital video to the recorder over a copy-protected 1394 interface. The satellite industry has adopted a similar model–the latest satellite set-top boxes also have a DVI interface to feed the display. And DVI interfaces have begun to appear on the latest HD video displays.
Who wins? Consumers who waited for the confusion to subside will now have the certainty that all the pieces will play together. Who loses? The early adopters who bought HD digital monitors that have only the 770.3 wideband component interface may never be able to watch copy-protected HD broadcast TV or cable or satellite–unless they rush out today to Best Buy and track down a legacy set-top box with a wideband component output. If they can still find one.
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