Specifications

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scan lines are drawn as follows: 240* odd lines of picture information, 22* black lines,
240* even lines of picture information, 23* black lines. This is all repeated 30+ times a
second. The "black" lines contain information to keep the TV set scanning in
synchronism with the broadcast and allow time to get the electron beam back to the top of
the screen. They also contain coded information for closed caption text, commercial
identification and tracking, etc.
To create picture detail the electron beam makes tiny dots and dashes as it draws each
scan line. With 525 scan lines redrawn 30 times a second, millions of these dots and
dashes are needed every second. Electronically the NTSC broadcast video signal uses
frequencies up to 4.2 million cycles per second (MHz). Every cycle has an "up" that
represents a dark dot and a "down" that represents a light dot, thus 4.2 MHz can represent
8.4 million dots per second. To make coarser detail (dashes) the cycles are stretched out,
or represent lower frequencies. To make shades of gray the ups and downs are not so
high or deep. At this rate we can get a maximum of 533 dots per scan line. Not all of
these dots can be used for picture detail. In the early days of TV, the last seventeen
percent of each scan line had to be made black to hold synchronizing information and to
get the electron beam back to the left side to draw the next line. The remaining 83% of a
scan line holds 442 dots which is the maximum amount of horizontal detail we can put
into the picture.
Applying the circle rule, 330 of the dots fit across the largest circle that in turn fits in the
screen, thus giving the commonly published horizontal resolution of 330 lines for NTSC
broadcast TV programs. While the TV set may be capable of higher resolution (making
even smaller sized dots as needed for the best playback of DVD) the most resolution that
will be seen for received TV broadcasts is 330 lines.
Trying to broadcast more dots on a scan line would make parts of the video signal go
above 4.2 MHz and interfere with the sound portion of the TV channel which is
positioned at 4.5 MHz. The circuits (filters) that make sure the video signal does not
contaminate the audio signal unfortunately usually weaken the portion of the valid video
signal near 4.2 MHz. This makes it impossible to get the tiniest details that could be
broadcast to be really black or white; they end up being dark gray and light gray instead.
Technical term: Bandwidth -- The difference between the maximum and the minimum
frequencies of a band of frequencies that a circuit or system can pass with no more than
50% attenuation (weakening). If the bandwidth does not start at zero, two of the
parameters lower bound, upper bound, and width, must be specified.
Some sources state that 4.0 MHz is the maximum usable video frequency, since typical
video amplifiers in the early days couldn't get up to 4.2 MHz while still cutting off by 4.5
MHz. This permits an even 8 million dots per second or 508 dots per scan line. Also, if
the transmitting electronics weakened the 4.2 MHz portions of the video signal by 50%
and the receiving electronics weakened it another 50%, each set of electronics by itself
meeting the definition of a 4.2 MHz bandwidth, by the time the picture got to the screen,
the tiniest dots could still be too much grayed out to be distinguished.