What Is Analog TV?

Analog TV, or analog television, is a broadcast and reception system in which picture and sound information are transmitted as continuously varying electrical signals modulated onto a radio-frequency carrier. The brightness of each point in the image is encoded as an amplitude value, color information is carried as a subcarrier phase and amplitude offset, and audio accompanies the video on a separate frequency-modulated subcarrier. Analog television systems dominated terrestrial, cable, and satellite broadcasting from the mid-twentieth century until a global transition to digital standards accelerated in the 2000s.

The technical foundations of analog television draw from amplitude modulation theory, cathode ray tube optics, and synchronization electronics developed in the 1930s and 1940s. Each frame of video is constructed by scanning a beam across the screen in horizontal lines, a process called raster scanning, with the receiver synchronized to the transmitter through vertical and horizontal sync pulses embedded in the signal. The ITU Radiocommunication Sector standardized the defining parameters of analog broadcast systems internationally in 1961, assigning letter designations (Systems A through N) that captured differences in scan line count, channel bandwidth, and video-to-audio frequency separation.

Signal Encoding and Transmission

Analog television transmits video using vestigial sideband modulation, a variant of amplitude modulation in which one full sideband and a remnant of the other are broadcast. This approach reduces the channel bandwidth required without discarding the low-frequency information needed for picture stability. A standard 6 MHz channel (as used in North America) or 8 MHz channel (as used in much of Europe) carries the luminance signal, the color subcarrier, the audio subcarrier, and the synchronization pulses within a single allocated slice of the radio spectrum.

The synchronization structure is critical: the transmitter inserts horizontal sync pulses at the end of each scan line and a vertical sync pulse at the end of each field to allow the receiver to lock its own scanning circuits to the broadcast rate. Any loss of horizontal lock produces a slanted or torn picture; loss of vertical lock causes the image to roll. This tight coupling between transmitter timing and receiver hardware made analog television receivers mechanically complex compared to their digital successors.

Color Standards

Three principal color encoding systems were developed for analog television. The NTSC (National Television System Committee) standard, first adopted by the United States Federal Communications Commission in 1953, encodes color as a 3.58 MHz subcarrier modulated with two color-difference signals, I and Q, in quadrature. PAL (Phase Alternating Line), developed at Telefunken in West Germany and patented in 1962, corrects for phase errors that degrade NTSC hue accuracy by inverting the phase of the color subcarrier on alternate lines. SECAM (Sequential Color and Memory), the French standard, transmits the two color-difference signals on alternate lines using frequency modulation rather than amplitude modulation. The European Broadcasting Union coordinated the adoption and harmonization of PAL across Western Europe, establishing technical norms for the 625-line, 25-frame-per-second system.

Transition to Digital Broadcasting

Governments and broadcasters worldwide began migrating from analog to digital terrestrial television in the 1990s, motivated by spectrum efficiency and picture quality. The United States completed its transition on June 12, 2009, when all full-power analog broadcasts ceased under FCC mandate. Luxembourg became the first country to switch entirely to digital in 2006. The FCC's digital television transition documentation describes how the shift freed up spectrum in the 700 MHz band for mobile broadband while requiring existing viewers to obtain converter boxes or replace analog receivers.

Applications

Analog TV technology, while superseded for broadcast, remains relevant in several areas, including:

• Legacy cable infrastructure, where analog channels continued on many systems alongside digital tiers
• Closed-circuit television using older coaxial distribution equipment
• Amateur radio television (ATV), which still uses analog video modulation on certain bands
• Historical and archival preservation of analog broadcast recordings on videotape formats such as VHS, Betamax, and U-matic