Digital Video for AV Integrators
When you transform AV into digital bits and bytes, it opens up a world of possibilities?and a few challenges. Here is everything you need to know about moving digital video around an AV installation.
SOME ASSEMBLY REQUIRED
Having created and encoded a digital video (and audio) stream, it’s time to do what AV pros do best: get those bits from point A to point B. Unlike analog video systems, digital video travels in a single cable, with the color, luminance, sync, audio, and data all flowing together. That’s an efficient way to move things around, but how do you separate the different bits from each other at the receiving end?
The answer is to brand each string of video bits as a packet, and to give each packet a unique serial number, or packet ID (PID). In a digital TV broadcast, for instance, there will be three kinds of PIDs: video, audio, and clock (officially known as the Program Clock Reference, or PCR). The clock is merely the synchronizing information, but it is critical to reassembling the parts of the program.
OK, so now we have a bin full of packets that appear identical in length (188 bytes), each stamped with a unique serial number, expressed as a hexadecimal code. And we can move that bin as a stream of packets from a transmitter to a receiver at some predetermined speed, or bit rate. How do we sort out the packets and rebuild them back into a program?
The solution is to provide a parts list and assembly instructions to the digital video receiver. In the language of MPEG, these are known as the Program Association Table (PAT) and Program Map Table (PMT), respectively.
The PAT lists all the MPEG packet IDs that are contained in a bit stream, including video, audio, and clock PIDs. The PMT tells the receiver which PIDs go together to make up a complete program. All the MPEG decoder has to do is sort the PIDs into the appropriate bins at high speed, fast enough to deliver uninterrupted video and audio to your TV set.
If you think about it, the PID system is a tremendous improvement over analog video. You can combine different audio PIDs with a single video PID to support multiple language tracks. Or you can provide a separate video PID to support closed captions, or standard- and high-definition versions of the same program. Or you can have both stereo and surround-sound audio tracks in the same stream. Just change the PIDs and you’re enjoying 5.1 or 7.1 surround sound effects.
It’s important to note that the bit rate doesn’t change, nor does the channel width, as you mix and match PIDs. You simply create room for more video programs by parceling out the available bits. The trade-off comes in image quality: You can’t expect to jam two HDTV programs and two SDTV programs into the same stream and expect any of those programs to look very good. (But that doesn’t stop DTV service providers from trying.)
PICK A PIPE, ANY PIPE
Because the structure of a digital video is simple (it’s just a stream of data), you can use just about any type of cabling to pipe the signals where you want them to go. Coaxial cable works just fine; so does Cat-5 cable. Optical fiber is even better.
The key is to ensure you have sufficient bandwidth to pass the signals. Depending on the bit rate, you may need bandwidth in the range of several hundred megahertz and possibly a gigahertz to get the job done. That’s a radical departure from designing a video system the old-fashioned analog way. With digital, you’re not so concerned about handling and switching different video signal formats (they’re all component, by the way) as you are with bandwidth and bit rates.
One commonly used digital video transport standard is the Serial Digital Interface (SDI). SDI was developed to work with ITU BT.601 video (for encoding interlaced analog video in digital form) and has a maximum data rate in the range of 360 Mbps. High-definition video requires more bits, so a variant known as HD-SDI came into existence with a data rate of about 1.3 gigabits per second (Gbps).
For production and transport of 1920x1080 progressive-scan video at a frame rate of 60 Hz, a dual-link HD-SDI connection is normally used. This connection can pass data at up to 3 Gbps. And these days, a new single-wire 3G-SDI system is set to replace dual-link. Needless to say, 3G connections make heavy use of optical fiber cable whenever possible.