Talk

Basics of Television

Television works at a rate of 30 frames / second. Rasterization is the process of converting a picture into individual pixels each with their own intensity and color. Combined with a vertical and horizontal sync the rows of pixel represent a composite picture signal

Broadcast channels in the US are in the frequencies of 2 - 13 VHF and 14 - 83 UHF. Picture is transmitted in the AM band and sound in the FM band.

Digital satellite and cable systems are not real digital television. For something to be digital everything from capture to transmission to display must be digital.

Resolution defines the clarity/crispness of a picture. Analog tv tends to be interlaced half the lines are drawn in 1/60th of a second and the other half in the next 1/60th of a second. The human eye cannot discern this change and this allows transmission to send less information. A tv displays at about 512x400 (contrast this with computer monitors that look hideous at 640x480).

Digital transmission is enabled by broadcasting in a channel that has a througput of 19.39 Mb(Mega-bits)s. Broadcasters can divide the channel into subchannels. The following encoding formats are available.

480p 704x480 - SD - Standard Definition as this is close to the Analog tv resolution. (Excepting the progressive scan)
720p 1280x720 - HD - High Definition.
1080i 1920x1080 - HD - High Definition.

Interlaced vs Progressive As discussed above in interlaced the image is updated half the lines at a time every 1/60th of a second. In Progressive the entire image is updated. Why 1/60th?

Lastly the aspect ratio for standard tvs is 4:3 essentially 4 units width to 3 units height. HD tvs have a 16:9 aspect ratio (think movie screens)

Compression MPEG-2 is the widly used standard for compression. It allows you to pick the bit - rate and screen size. This has the benefit of allowing the broadcaster for using different settings for different programs. A talk show with little movement and mostly static data a 480p display will be quite watchable. A roller-coaster scene however has rapidly changing details and would require maybe a 1080i stream.

All of this is from How stuff works. I have merely condensed it for my own notetaking. If you havent been to HSW yet I strongly urge you to go there and support it. One of the best side effects of the exuberant venture funding is HSW