Specifications

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The conglomeration of all those dots and dashes on all the scan lines equals the picture.
Since the electron beam "paints" the screen at a constant speed, the finer the horizontal
detail (smaller dots), the faster the changes must be. The faster the changes, the higher the
frequency the video signal must be able to attain. Since signal frequency is one of the
limiting factors for video information storage and transmission, horizontal resolution is
the first to be affected.
The Circle Rule
When measuring "lines of resolution" for TV sets or monitors, a reference distance needs
to be specified. Traditionally, video resolution is measured across the largest circle (not
an ellipse) that fits in the space we are talking about. All the technical books on video say
this, although using different words. On a standard TV screen such a circle would span
3/4 of the screen width. For U.S. HDTV (16:9 aspect ratio) the circle would span a little
over half the screen width. Since the circle exactly fits the screen height (assuming no
overscan), the phrase "picture height" stands for a distance equal to the diameter of the
circle and is also used when talking about horizontal resolution. The circle rule also
applies to film.
Under this rule, if a standard (4:3 Aspect Ratio) TV set can reproduce 800 dots
alternating black and white across the screen width using an input signal representing
same, it is said to have 600 lines of horizontal resolution. However if the source material
had just 330 lines of resolution you will see just 330 lines of resolution. A 16:9 aspect
ratio TV that can reproduce just 800 dots across its screen would have 450 lines of
horizontal resolution.
Technical term: Cycles per picture height -- One cycle of a simple video signal waveform
consists of an "up" which makes a dark dot or dash and a "down" which makes a light dot
or dash. At the 4.2 MHz maximum usable NTSC broadcast video frequency, at most 165
cycles can occur while the electron beam is drawing inside the circle we were talking
about above so we can say that the horizontal resolution is 165 cycles per picture height.
The term "cycles per picture width" is also used but less often. Incidentally one cycle
corresponds to one "line pair" in photographer's terminology.
For film "lines of resolution" is typically measured as per millimeter. We cannot easily
use "lines per inch" to describe video resolution because we would expect two TV sets of
the same quality but with different sized screens to have about the same horizontal
resolution, and exactly the same the vertical resolution. This has led to the phrase "per
picture height".
When pictures are transmitted or stored digitally, each scan line has to be broken up into
dots called pixels. Both the vertical and full width horizontal pixel counts are specified,
representing a rectangular grid. A common computer video resolution is called VGA and
is stated as 640 by 480 pixels. Here the aspect ratio does not have to be known or stated.
For analog video (broadcasts, VHS tape, laser disks) we can think of the horizontal pixel
count as the maximum amount of the finest details that can be put on a line and still be
distinguished. The video source may have a given resolution in pixels but if the TV