How accessDTV Works ...

The excitement around digital television (DTV) has been growing steadily for several years. If you have been to any of the major electronics stores recently, you have probably noticed shelves filled with digital television sets.

At the same time, television stations have been quietly launching their digital transmitters. The stations and the networks have been outfitting their studios and trucks with the equipment they need to shoot, record and edit with purely digital signals. Almost all prime-time shows and sporting events are now digital.

In most major cities, you can receive digital broadcasts. For example, in San Jose, CA, you can receive about a dozen DTV broadcasts. Even in a relatively small city like Raleigh, NC, you can receive four stations. More than 100 million Americans are able to receive at least one digital broadcast, but far fewer than a million currently do. The main barrier has been the price and complexity of home DTV equipment.

AccessDTV Features

The accessDTV card has a number of interesting features: It allows you to receive all of the DTV stations in your area on your PC. It allows you to display the picture you receive in a window on your computer monitor's screen, full on your computer monitor's screen or externally on a DTV display.

It provides personal video recording that allows you to record DTV broadcasts onto your hard disk for later viewing. It provides a service that gives you a program guide, listings for digital broadcasts in your area and links to content-related Web sites. DTV technology and programming is advancing quickly, and this card lets you experience it all.

DTV

If you have read How Digital Television Works, then you are familiar with the world of DTV. Here is a quick summary of the important points: Broadcasters in your area have each been allocated a new channel for their DTV broadcasts. The broadcasters each transmit a 19.39-Mbps stream of digital data. This signal contains television programs compressed using the MPEG-2 compression system.

DTV shows can be broadcast at several different resolutions: 480p - The picture is 704x480 scan lines sent at 60 complete frames per second. 720p - The picture is 1280x720 scan lines sent at 60 complete frames per second. 1080i - The picture is 1920x1080 scan lines sent at 60 interlaced frames per second (30 complete frames per second).

Broadcasters can transmit either a single 1080i high-definition channel that consumes the entire 19.39-Mbps stream, or several different sub-channels by encoding multiple programs at 480p resolution and lower bit rates. For example, the DTV station 53 can have sub-channels named 53.1, 53.2 and 53.3. accessDTV can record and play back the sub-channels.

The accessDTV Digital Media Receiver Solution consists of hardware and software. The hardware is a PCI card that you install inside your PC. The software controls the card and allows you to tune in and view DTV broadcasts in your area, using either your computer monitor or an external HDTV display.

AccessDTV Card Components

The following figure shows you a block diagram of the components on the accessDTV PCI card:

The tuner receives the signal from the antenna and tunes in a single channel. The demodulator retrieves the 19.39-Mbps digital stream from the channel. The MPEG decoder decompresses the MPEG encoding and separates subchannels. The signal then goes to either the connectors on the board that connect to a DTV monitor, or to the computer's video card directly. MPEG signals and sound information can go through the PCI bus to the hard disk and sound card, respectively.

The two most important components on the accessDTV card are:

The digital tuner
The MPEG-2 decoding system

By connecting a standard UHF/VHF antenna to the accessDTV card, you can tune in any of the 69 DTV channels. (In a typical city, there will be from three to 10 DTV channels on the air.) The tuner pulls the 19.39-Mbps data stream off the channel you choose. The MPEG-2 decoder circuit decodes this data stream and separates any sub-channels so that you can view them. This is the most important part of the card because it offloads all of the MPEG-2 decoding from your CPU.

The 19.39-Mbps stream is so complex that it would totally consume a Pentium 4 processor running at 1.5 gigahertz (GHz). The accessDTV card contains a custom processor specifically tuned for MPEG-2 decoding. With the accessDTV card handling decompression, only about 5 percent of your computer's CPU power is spent displaying the digital image on the screen. From your computer monitor, you can watch a DTV broadcast in one window and do anything you want in other windows without even knowing that the card is running.

The card also contains a cable-ready and NTSC off-the-air-ready analog tuner. You can connect the coax from your cable system or a standard TV antenna and receive analog channels 2 through 83 as you would on any normal TV. You can also view these channels in a window on your computer screen.

Connectors The accessDTV card comes with a collection of connectors that you use to accept video input and generate audio/video output.

Here's what these connectors do:

Analog in - This accepts analog video input. The input can be an analog-TV antenna, a feed from the cable company, or a channel-3 input from something like a VCR or DVD player.

Digital in - This accepts digital video input. Typically, this would be the Yagi antenna collecting the digital broadcasts in your community, but it could also be a cable from a digital satellite receiver.

Dolby Digital Surround Sound (AC-3) output - This is the output for digital sound. Typically, you would connect this to your 5.1-channel home-theater sound system (see How Home Theater Works).

PC Video passthru in

Video output - This looks like a standard VGA connector. You can use a cable to connect this to the component video input of any supported digital display, including DTV sets. In this case, the card acts as the display's digital receiver. There are two typical ways that you might connect the card in your home: for computer-only viewing or for HDTV-display viewing.

Viewing Methods

Computer-only Viewing
Let's say that you do not own an HDTV display right now, and you simply want to watch HDTV broadcasts on your computer's monitor. You would do the following:

Connect a standard UHF/VHF antenna to the digital video input on the accessDTV card. Connect the cable that comes with the accessDTV card between the card and your normal video card. Run the accessDTV application, choose your channel and enjoy the broadcast. You can watch DTV in a window or full-screen on your computer monitor.

HDTV-display Viewing Let's say that you own an HDTV display and a home-theater sound system, and you want to watch HDTV broadcasts on your HDTV display. This means that you want to use your accessDTV card as the digital receiver for your HDTV display. You would do the following:

Connect a standard DTV antenna to the digital video input on the accessDTV card. Connect the cable that comes with the accessDTV card between the card and your normal video card. Connect the external HDTV display to the XVGA connector on the accessDTV card using a standard XVGA-to-RGB cable. Connect the home-theater sound system to the serial-digital output on the accessDTV card. Run the accessDTV application, choose your channel and enjoy the broadcast. You can watch DTV on the external HDTV display, or in a window or full screen on your computer monitor.