Previous | Table of Contents | Next |
Digital TV still is very much in the early stages of development. Digital TV transmission offers the same advantages as any other form of digital communications, including enhanced bandwidth efficiency and signal quality management and control. Digital video is nothing more than data, offering advantages in terms of processing, storage, and manipulation. These advantages include editing, alteration (morphing), reproduction, compression, transmission, storage, and store and forward capability.
In its native form, video (or TV) comprises synchronized analog voice and analog image information. Digital video requires that the information be digitized through the use of a video codec. Because a broadcast quality video signal is extremely bandwidth- and storage-intensive, the resolution of such digital video requires 640 horizontal pixels and 480 vertical; chrominance and luminance require 24 bits/pixel; and the frame rate is 30 fps. The video signal alone requires 7,372,800 bits per frame. At a frame rate of 30 fps, the bandwidth requirement is 221,184,000 bps. Clearly, it is not within the realm of reason to transmit a broadcast quality video signal requiring 221+ Mbps. It is clear that the storage requirements are incredible and compression is critical if the networks are to support digital TV.
Despite all the advantages of digital technology as applied to television production, storage and transmission, all of the TV sets are analog. A conversion from analog to digital broadcast TV is revolutionary and completely unthinkable, unless there exists a compelling reason for the changeout of untold millions of TV sets. That compelling reason could (but probably wont) take the form of HDTV (High Definition TV).
The definition of HDTV standards caused a debate that raged for a number of years between Japanese analog standards and digital standards proposed by the United States. The U.S. digital standards proposals focused on those offered by the Grand Alliance, comprising AT&T, General Instruments, Zenith, The Massachusetts Institute of Technology (MIT), Thompson Consumer Electronics, Philips Consumer Electronics and the David Sarnoff Research Center. It appears as though the Grand Alliance has won the battle, at least within the United States [14-7]. While the standards have yet to be fully determined, HDTV will be digital, with an aspect ratio of 16:9. The number of scanlines will be either 1,920 or 1,280; resolution will be either 1,080 or 720 pixels per line; and the frame rate will be either 60, 40 or 24 fps. At the time of this writing, the future of HDTV remains very uncertain. Although the quality of the presentation is much improved, the bandwidth required is significant and entirely new equipment is required. Additionally, digital radio spectrum must be cleared in order to support digital TV alongside existing analog broadcast, until such time as the conversion is complete. The FCC has taken the position that the cost of clearing that spectrum will be the burden of the private sector.
The bandwidth required for video transmission is significant, affected by frame rate, resolution, color depth, aspect ratio, and audio. Broadcast-quality TV requires about 6 MHz in analog form, of which the signal occupies about 4.5 MHz. As little can be done to compress an analog signal, analog TV is limited to airwave broadcast or CATV transmission over analog coaxial cablein other words, analog is doomed!
Digital video, on the other hand, can be compressed fairly easily. Because uncompressed, broadcast quality, digital video requires between 90 Mbps and 270 Mbps, compression is critical. Without compression, a 1 Gbps fiber optic network could accommodate no more than 11 digitized NTSC channels. To digitize and compress the video information stream, the analog video and data signal first must be digitized through the use of a codec. Clearly, the amount of bandwidth required to transmit digital video, and the amount of memory required to store it, can be reduced by reducing the frame rate, resolution, or color depth. However, the result is less than pleasing. In order to maintain the quality of the video presentation, therefore, the data must be compressed using an appropriate and powerful data compression algorithm.
Lossless compression allows the video signal to be reproduced faithfully with no data loss and compression rates are in the range of 10:1. However, even the relatively new ITU-R 723 standard reduces the data rate to only 45 Mbps. Lossy compression results in a degraded picture, but allows compression at rates up to 200:1 [14-8]. Actually, compressed video currently can be transmitted with quite acceptable quality at T1 speeds of 1.544 Mbps or less. MPEG uses lossy compression in the form of DCT. There are a number of steps involved in video compression, including filtering, color-space conversion, scaling, transforms, quantization and compaction, and interframe compression.
Previous | Table of Contents | Next |