By Jeffrey Krauss, Interface Speculator and President of Telecommunications and Technology Policy

Here comes another interface dispute. The previous dispute dealt with a baseband interface for carrying compressed digital video. Now we are looking at two contenders for a digital baseband interface with a data rate high enough to carry uncompressed HDTV from a set-top box to a display. This is important to the cable industry because it supports full screen bitmapped graphics, necessary for high quality electronic program guides.


In the beginning, there was the RF interface. A set-top box produced an output signal on the frequency of channel 3 or 4, and that was connected by cable to the antenna input terminal of the TV. In an analog environment, the FCC never assigned both channel 3 and 4 to TV stations in a city, so one of them was always free. But in a digital broadcast world, there are some cities where both channel 3 and channel 4 will be used, which creates the risk of ingress interference. Another disadvantage is that this is a dumb one-way interface, without any interactive ability between the TV set and the set-top box. And finally, there is no copy protection method defined for this interface.

So attention shifted to a baseband interface, in particular IEEE 1394, also known as "Firewire." But 1394 is insufficient because it is merely a physical layer specification, and a command language was needed so that the set-top box and TV set could talk to one another. After a dispute between Sony and Thomson was settled with an agreement to use the Sony AV/C command language rather than the CAL language supported by Thomson, the resulting interface was adopted as the EIA-775 standard. The cable industry tacked on a copy protection method known as "5C," and this became the basis for the agreement earlier this year between the cable and consumer electronics industries. Digital TV sets with the copy-protected EIA-775 interface can be labeled "Digital TV—Cable Interactive," but digital sets with only an RF interface will be labeled, "Digital TV—Cable Connect."

But EIA-775 only works up to 400 Mbps, not fast enough for some bitmapped graphics needs.

Now cometh Digital Visual Interface (DVI) and Low Voltage Differential Signaling (LVDS). These are both high data rate, short distance interfaces that support uncompressed digital video using low voltage differential signaling.

DVI uses chipsets developed by Silicon Image, and is supported by the Digital Display Working Group (DDWP) which includes Intel, Compaq, IBM, Fujitsu, NEC and Hewlett Packard (see DVI supports transfer rates as high as 9.6 Gbps (easily fast enough for uncompressed 720p and 1080i HDTV formats) on multi-wire cables of five to 10 meters in length. DVI has already been incorporated into numerous computer displays, some projection TVs, a JVC digital VCR, and will be incorporated into Echostar set-tops later this year.

DVI includes a copy protection method called HDCP that has been endorsed by motion picture studios Warner Brothers, Disney and Fox (more details at

LVDS is based on chips made by National Semiconductor. It is widely employed in laptop computers, in the link through the hinge between the keyboard/processor and the LCD display. Depending on clock speed, cable length and number of conductors, LVDS supports data rates of 655 Mbps up to 5 Gbps, at distances of 15 meters or more, and display formats up to 2048 × 1536 (QXGA). Two different flavors of LVDS have been standardized as ANSI/TIA/EIA-644 and IEEE 1596.3. LVDS does not have copy protection capability, but National Semiconductor claims that HDCP could be "bolted on."

It's too early to tell whether a big fight is brewing. Both DVI and LVDS are being used in computer and industrial markets, and silicon chips exist for both. But, as with the step from the IEEE 1394 standard to EIA-775, some work is needed to create standards that meet the needs of the consumer TV market.

Both DVI and LVDS proponents have made presentations to the consumer electronics standards bodies. Will these standards groups adopt one, or perhaps both, as digital TV standards? Will the cable industry adopt one? Will it be the same as the consumer electronics standard? At this point, although it is too early to tell for sure, it seems that DVI has more support than LVDS among both cable and consumer electronics interests.

This high data rate interface will support more functions and capabilities than EIA-775. But will consumers understand it, or will they be confused by too many interfaces? The consumer electronics industry has already adopted several slightly different flavors of the EIA-775 interface, one for camcorders, one for broadcasting, one for DirecTV, one for Echostar and one for cable TV, all using the same connector, but perhaps not fully interoperable. The marketing people will have to work hard to overcome any resulting confusion. Adding another interface, using a different connector, and supporting different capabilities is not necessarily a step toward marketplace success.