YouTube Analog TV Doodle

On the homepage of YouTube an animated logo,

"Today marks the final broadcast of analog TV signals in the U.S. To honor this technology as it rides off into the static sunset, here are four classics that helped transform television into the ubiquitous medium it is today."

Analog (or analogue) television encodes television picture and sound information and transmits it as an analog signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal. All systems preceding digital television were analog television systems.

Source: http://s.ytimg.com/yt/img/logo_analog_off-vfl102489.gif

Broadcasters using analog television systems encode their signal using NTSC, PAL or SECAM analog encoding and then modulate this signal onto a VHF or UHF carrier. An analog television picture is "drawn" on the screen an entire frame each time, in the manner of a motion picture (cinematograph) film.
Until the advent of digital television, all television was based on the transmission and reception of analog signals, displayed on a cathode-ray tube. Although a number of different broadcast television systems were in use worldwide, the same principles of operation apply.

The first commercial analog television systems were monochrome; they were enhanced to include color beginning in the 1950s. [Read more on Wikipedia]