High-Definition Television

A digital television (and more) broadcasting method to be an enhancement to the current standard NTSC broadcast and cable TV television signals used today. Some features are the following:

• Will initially (for at least 15 years) use the 6 MHz channels that were left unassigned (to reduce interference) between currently assigned TV broadcast channels (that is, if channels 3 and 5 are assigned and used in a coverage area for standard NTSC broadcast television, then channel 4 will be available and will be used for HDTV).
• Provides a 16:9 (horizontal:vertical) aspect ratio, like a movie screen that is showing a standard 35-mm (called Academy format) or 70-mm movie film. This is wider than the 4:3 ratio of current standard televisions.
• Provides up to a 60-frames/s screen writing rate.
• Uses a subset of MPEG-2 data compression (and the discrete cosine transformation method that it specifies) to compress the source information (which has a data rate of 1.2 Gbits/s) by a factor of more than 60:1, to a broadcast data rate of less than 20 Mbits/s per TV channel.
• Using something called digital vestigial sideband modulation (wow, now if that doesn't impress you, they hope the picture quality will), this still fits into the current standard 6-MHz bandwidth television channels.

Several frame rates and formats are defined, as shown in the following table. All yield "square pixels," that is, the number of pixels per inch is the same horizontally and vertically. This facilitates conversions between HDTV formats and also to common PC monitor resolutions (such as VGA, which also use square pixels). NTSC does not have square pixels.

Formats, in number of ...		Scanning	Frame Rates
			(frames/s)
Active samples[a]	Active Lines		HDTV[b]	NTSC[c]
(horizontal pixels)	(vertical pixels)
1,280	720[d]	Progressive	24, 30, and 60	23.976, 29.97, and 59.94
1,920	1,080[d]	Progressive	24 and 30
		Interlaced	30[e] (60 fields per second)
Footnotes: [a] Active means that they are visible (for example, NTSC has 481 active lines out of the total of 525; the remaining 44 occur while the scanning beam is blanked during its retrace to the top-left corner of the screen). [b] Movies are 24 frames per second, computer video and graphics are usually 30 frames per second, and 60 frames per second would be best suited to sports and other fast-action programming. [c] These frame rates are provided in case they simplify using existing NTSC-format programming. [d] Note that the numbers of lines in the two formats are in the exact ratio 3:2, which facilitates conversions between the formats. The ratio between the number of lines in a standard PC's VGA display and the HDTV 720 line format is also 3:2--for the same reason. [e] This single interlaced format is useful for compatibility with existing interlaced program sources.

Noninterlaced (progressively scanned) 60-frame/1,080-line mode may be supported in the future if more bandwidth is available or compression technology advances further.

Uses Dolby Laboratories AC-3 multichannel digital sound system, which provides the following audio channels:

• Left

• Center

• Right

• Left-surround

• Right-surround

• Low-frequency enhancement

Captain Bob says:

All this audio is encoded into a 384-kbits/s bit stream and provides compact-disc-quality audio. Other audio features are provided, such as the following:

• A constant volume level when switching channels

• Using the multiple audio channels for different languages or for services for the visually or hearing impaired

The digitized video, audio, and auxiliary information are packaged in 188-byte packets (only one type of information per packet). A 4-byte header in the packet:

• Includes an 8-bit synchronization byte

• Identifies (using 13 bits) what type of information the 184-byte packet payload carries

• Provides encryption (scrambling) control (to support pay-per-view and other premium services)

Information that is sent periodically in the payload provides synchronization between the different data streams, for example, to ensure lip-sync between the voice and video.