Hold your fire
There’s no need for the conflict that’s brewing over how to connect digital AV devices.
Lee Ratliff, 1394 Trade Association

Fortunately, there's no real basis for misunderstanding, let alone a standards death match a la Beta vs VHS. A technical examination of 1394 and DVI reveals that they serve distinctly separate purposes. What's more, 1394 emerges as the clearly superior choice for linking the devices in the digital living rooms of the near future.pp

1394, commonly referred to as i.LINK or Firewire, was developed as a high-speed serial bus to address the needs of the PC and consumer-electronics markets. The PC market needs a fast and easy-to-use interface that can provide high performance, yet achieve unparalleled ease-of-use for an increasingly non-technical customer. Legacy serial and parallel ports just don't cut it anymore.

Most importantly, 1394 serves as the bridge between the PC and AV worlds; it makes convergence a reality. For evidence of this merger, we need look no further than the overwhelming success of one of the first true convergence applications—digital video editing. 1394 enables DV editing with your PC and camcorder, turning those potentially monotonous home movies into exciting mini-masterpieces complete with graphics and special effects.

Job specialization

DVI is a very-high-speed point-to-point connection used to connect PCs to digital monitors. DVI's extremely high bandwidth enables transmission of uncompressed graphics with resolution at the UXGA level (1600 by 1200 pixels) and beyond. As a single-purpose interface, DVI was designed to do one thing and do it fast. As a result, DVI is like a cheetah—capable of blazing speed, but not very adaptable to situations requiring flexibility.

However, DVI is very good at what it was designed to do—transmitting uncompressed computer graphics to a monitor. For PCs, uncompressed graphics are important. Some PC applications, such as games, are sensitive to the latency that compression and decompression can introduce. Also, graphics tend to suffer from compression artifacts—visible imperfections introduced during compression—more than video does. For these reasons, graphics rarely get compressed. Video, however, is a different story.

Some have touted DVI's ability to transmit uncompressed video as an advantage over 1394. While it's true that DVI can transmit uncompressed video, the flip side of this argument is that DVI is unable to transmit compressed video. And this turns out to be a big problem.

Digital video is always received as MPEG-2 compressed streams, whether the source is terrestrial broadcast, cable, satellite, DVD, or otherwise. The normal model is to keep the video compressed until it reaches the sink device (the receiving device). It is convenient to keep it compressed as long as possible to save bandwidth, to save storage space on recording media, and to avoid the need for recompression.

As an example, suppose you're receiving a movie broadcast from a digital satellite service. You want to watch it on your DTV and simultaneously record it on your D-VHS. This will be a typical application in the digital home of the not-so-distant future, and it must be comprehended in any user model.

Figure 1 shows the accepted method. The satellite set-top tunes the signal, performs MPEG transport demultiplexing to pick the desired program from the transport stream, and then transmits the MPEG-compressed video via 1394. The DTV receives the compressed bit stream, decodes it, and displays the video. The D-VHS deck is daisy chained to the DTV via 1394. It receives the compressed bit stream and records it without decoding it.

The model in Figure 2 really gets ugly if you want to enable video playback from the D-VHS—and of course, you must. The DVI model might work as shown if you can feed playback via 1394 through the set-top, but that implies that VCR playback is dependent on having a set-top box. Alternatively, you can send playback through DVI, but you'd have to add an MPEG-2 decoder and a DVI transmitter to the VCR, not to mention another DVI receiver in the DTV, since DVI is a point-to-point interface, not a bus. Other ways of enabling playback without 1394 exist, but none are as simple as the model shown in Figure 1.

Embracing diversity

1394 is a packet-based bus that is completely independent of data type. It can transmit dozens of different data types simultaneously using the same interface. Video, audio, control, and graphics are not a problem. What's more, it's futureproof, enabling transmission of data types that haven't even been conceived of yet.

DVI, on the other hand, is designed for graphics transmission and graphics transmission only. It is not packetized. DVI uses one or two signal pairs for each primary color—red, green, and blue. Data is transmitted in a raster-scanned format, that is, one pixel after another according to their spatial location on the screen. DVI has no provisions for other data types, most importantly audio.

Bring the noise

Let's use our previous example to explore the consequences of adding a few more data types. Naturally, you'll want to hear the movie you're watching, so we need to consider how audio is handled in each case.

The 1394 example already comprehends audio without any additional hardware. The audio gets embedded in the MPEG-2 transport stream and delivered to the sink devices along with the video. The same 1394 and MPEG components that already exist to handle the video handle the audio. You don't even need to add audio-specific connectors and cables. The Dolby Digital (formerly known as AC-3) decoder that already exists in the DTV (for terrestrial broadcasts) decodes the audio.

The DVI scheme lacks any audio support. You'd have to send the audio to the DTV using another interconnect method, presumably analog. The set-top would have to have a two-channel Dolby Digital decoder and analog stereo outputs for the DTV, requiring two more connectors and cables. For home-theater sound, you'd need a separate Dolby Digital output to connect to an external six-channel decoder. Optionally, the entire six-channel decoder could go in the set-top. Either way, you're adding costs and complexity.

One of the most exciting features of 1394 is its ability to transmit commands from device to device. This makes the elusive goal of a universal remote control a reality. The 1394 AV/C protocol defines a set of universal commands for the various functions of many types of consumer electronics. For instance, using AV/C commands, your set-top could command your DTV to change the volume. Or your DVD recorder could command your set-top to tune to a desired channel for automatic program recording. This is a much more sensible and robust alternative than today's IR blasters.

HAVi devices automatically exchange drivers, which exist as platform-independent Java applets (or "havlets"). Using devices that support HAVi, consumers will never have to worry about device drivers, obscure technical standards, or interoperability. HAVi products will just work. Together.

Some future DVI transmitters will integrate a master I2C (inter-integrated circuit bus) controller to facilitate authentication and key exchange for content protection. However, while I2C may be suitable for simple, low-bandwidth control within a system, it lacks the sophisticated and robust nature of 1394 asynchronous control, which is required for a multi-node home network.

In use

What is the industry actually implementing? Without question, the answer is 1394. Already, the consumer PC industry has embraced 1394 with an installed base of 28 million units by the end of this year. Digital camcorders have also adopted 1394 as a standard feature, with approximately 12 million units sold and an annual run-rate in excess of 6 million units. Clearly, 1394 has already found a place in the home. However, the upcoming applications are even more exciting.

Sony has announced five models of digital TVs with 1394. These TVs complement Sony's other announced 1394-equipped products, such as an OpenCable set-top box that will be purchased by Cablevision and distributed to more than 4 million homes in New York City, the nation's largest media market. Another Sony product, the Playstation II game console, includes 1394 and is projected to sell 7 million units this year and 15 million units in 2001.

Panasonic recently announced another 1394-ready DTV, to join the DTV, D-VHS, and terrestrial set-top that the company already has in production with 1394. Mitsubishi announced support for 1394 in its line of digital televisions, as well as a 1394 upgrade module for many of the DTVs it has sold since 1998.

In addition, 1394 has received considerable support from the standards community and industry associations. The cable industry has made 1394 a requirement in the OpenCable specification. Boxes will be hitting store shelves this year. The DVD Forum is finalizing work on the 1394 specification for DVD players and recorders. You can expect to see these in production in mid 2001. HAVi has the backing of nearly every A-list player in the consumer-electronics industry. HAVi products will begin to appear later this year and ramp up in 2001. Finally, the specification for Europe's Multimedia Home Platform requires 1394. No consumer-electronics standards endorse DVI.

What's right

Clearly, 1394 is a requirement for digital recording. The source and sink devices must use MPEG, so 1394 is the obvious choice. DVI could be used for the DTV, but you'd have to add a redundant interface to the set-top box. Besides, the DTV will probably become a source device for digital recording, especially if it has a terrestrial tuner.

Ultimately, the question of 1394 vs DVI comes down to where the intelligence resides—set-top or DTV. DVI doesn't eliminate the need for decompression, graphics support, or an audio decoder—it just moves this functionality from the DTV to the set-top. More realistically, it will duplicate these blocks in both boxes, since the DTV will need this functionality for terrestrial reception anyway. 1394 enables the lowest system cost by eliminating the need for redundancy. It also allows for the optimal mix of intelligence between the set-top and the television.

The next few years will see a lot of experimentation as manufacturers explore the new digital paradigm, test the market with new features, and cope with legacy support of analog electronics. 1394 is flexible enough to handle this rapid pace of change, while enabling the most compelling applications and the most efficient system design well into the future.

Author information

Lee Ratliff is an active member of the 1394 Trade Association and has eight years of experience in the field of digital consumer electronics.