2.1) What are analog TV broadcasting systems and composite video?

The video display capabilities of the Atari computer are intimately related to
the television (TV) broadcast systems of their time because, in part, consumer
TVs were expected to be the primary display devices used with the system.  The
Atari was designed with the ability to output an analog radio-frequency (RF)
audio/video signal that could be interfaced with a TV's antenna input, the
input normally be used to receive terrestrial TV signals broadcast over the
air.

The analog black-and-white RF television broadcast system standards that
originally emerged throughout the world included System M (1941), System B
(1950s), System I (1962), and System L (1967), plus System G.  Broadcast
signals compliant with these standards carried both audio and luminance ("Y")
video information (plus synchronization information):

     System M: o  525 scan lines per frame
               o  486 scan lines of video per frame including overscan
               o  262 scan lines per field (frame = two interlaced fields)
               o  243 scan lines of video per field including overscan
               o  60 fields per second = 60Hz
                  (refresh rate corresponds to household electricity standard)
               o  Complete frame refresh rate = 30 frames per second (fps)
               o  4:3 aspect ratio
               o  System M was used in most of the Americas and Caribbean,
                  South Korea, Taiwan, Philippines, Brazil, and Laos.
               o  Japan used System J which was nearly identical to System M.
               o  Used with both very high frequencies (VHF channels 2-13) and
                  ultra high frequencies (UHF channels 14-83)

Systems BGIL: o  625 scan lines per frame
               o  576 scan lines of video per frame including overscan
               o  312 scan lines per field (frame = two interlaced fields)
               o  288 scan lines of video per field including overscan
               o  50 fields per second = 50Hz
                  (refresh rate corresponds to household electricity standard)
               o  Complete frame refresh rate = 25 fps
               o  4:3 aspect ratio
               o  While G and I used the same UHF channel frequencies for
                  video carriers, they each used different audio carrier
                  frequencies for the same channels.
               o  B was used with VHF channels, while G was used with UHF
                  channels.  B/G were used together in most of western Europe.
               o  System I was used in the UK over UHF channels only.
                  (The UK used System A over VHF channels until 1985.)
               o  System L was used in France over UHF only until 1984, with
                  VHF switching from System E to System L in 1984.

Later, color (chrominance, or "C", being a combination of hue ("U") and
saturation ("V") information) video encoding standards were adopted for
combined use with the existing underlying RF broadcast system standards.
Three analog video color encoding standards that emerged in different parts of
the world were NTSC ("National Television Standards Committee"; 1953), PAL
("Phase Alternation by Line"; 1967), and SECAM ("Sequentiel couleur avec
memoire"; 1967).  NTSC was used in most countries using the System M broadcast
standard, while PAL was used in countries using Systems B/G or System I, and
SECAM was used in France over System L.  Thus NTSC M, PAL B/G, and PAL I, plus
SECAM L in France, became the most common color TV broadcast systems used
around the world.

NTSC M actually and officially uses a slightly altered System M, where the
frame rate is approximately 59.940 fields per second, or 29.970 frames per
second (fps).  PAL B/G, PAL I and SECAM L frame rates are exactly 25 fps.

In the 1970s a commercial market emerged for video display devices that would
be compatible with existing TV broadcast standard video, but where the RF
modulation/demodulation circuitry for transmitting/receiving audio/video
broadcast signals over the air was omitted.  Such a video signal, containing
both luminance (Y) and (optionally) chrominance (C) information, but no audio,
became known as composite video (often just "video").  A color composite video
signal can be characterized by the color encoding standard used, one of the
same standards invented for broadcast television: NTSC, PAL, or SECAM.

The luminance (Y) and chrominance (C) components that make up a color
composite video signal can also be transmitted as two separate signals.  Such
video is known as Y/C video, or S-video.  Like both analog broadcast TV
signals and color composite video, Y/C video can also be characterized by the
color encoding standard used: NTSC, PAL, or SECAM.

A monochrome composite video signal contains luminance (Y) information but no
chrominance (C) information, and is typically characterized by its refresh
rate: 60Hz (System M compatible) or 50Hz (Systems B/G/I/L compatible).

Each Atari computer version was designed to comply with video system standards
used in the destination target market for that unit.  Atari produced versions
of their computers for NTSC, PAL, and SECAM markets, supporting combinations
of color analog RF broadcast standards (NTSC M, PAL B, PAL G, PAL I), color
composite video standards (NTSC, PAL, SECAM), composite luminance signals
("Y"), and composite chrominance signals ("C"; NTSC or PAL) as follows:

    RF Out  Ch.  ../ Monitor Port \...
    to TV        Composite  "Y"   "C"   Computer Model Versions
    NTSC M  2/3   -          -     -      400,600XL 
    NTSC M  2/3   NTSC      60Hz  NTSC    800,800XL(latest),65XE,130XE
    NTSC M  2/3   NTSC      60Hz   -      1200XL,800XL(most)
    NTSC M  2/3   NTSC       -     -      XEgs 
    PAL B   3/4   -          -     -      400
    PAL B     4   PAL       50Hz  PAL     800,800XL(later),130XE,800XE
    PAL B     4   PAL       50Hz   -      800XL(earlier)
    PAL B     4   PAL        -     -      600XL,XEgs
    PAL G    36   -          -     -      400
    PAL G    36   PAL       50Hz  PAL     800,800XL(later),65XE,130XE,800XE
    PAL G    36   PAL       50Hz   -      800XL(earlier)
    PAL G    36   PAL        -     -      600XL,XEgs
    PAL I    36   -          -     -      400
    PAL I    36   PAL       50Hz  PAL     800,800XL(later),65XE,130XE
    PAL I    36   PAL       50Hz   -      800XL(earlier)
    PAL I    36   PAL        -     -      600XL,XEgs
      *       *   SECAM      -     -      800XL,130XE,XEgs
* monitor port includes provisions for an external in-line RF modulator
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