/*Javascript script mtv_parts.js, copyright 2005 by James T. Hawes. All rights reserved.
REVISION HISTORY
5-22-2005 Rev. 1
5-29-2005 Added clear feature
5-30-2005 Revised text. Most is now present tense. Renamed indexP2.js as mtv_parts.js.
*/

function displayText(textNum)
{
  if (textNum == 1)
    document.scannerDescForm.elements['textDisp'].value = "NIPKOW SCANNING DISC. The " +
    "large, round plate is the Nipkow scanning disc. The manufacturer or builder " +
    "perforated this disc with a spiral of tiny holes. These holes distribute varying " +
    "grays from the lamp across the image field. In fact, each hole paints one line of " +
    "the TV picture. Some discs use round holes. Other discs employ square or diamond-" +
    "shaped holes. You might notice that our disc has two such spirals of holes. The " +
    "second spiral allows for picture phasing. By moving a lever, the viewer can slide " +
    "the neon lamp. The top and bottom of the TV picture frame properly at only one " +
    "lever position. At other positions, some bottom lines might appear at the top of " +
    "the picture.";
  if (textNum == 2)
    document.scannerDescForm.elements['textDisp'].value = "MAIN, UNIVERSAL MOTOR. " +
    "The main motor provides the necessary torque to spin the scanning disc and " +
    "distribute grays across the screen. Standard scanning speeds in the 1920s and "+
    "1930s were 750, 900 and 1200 rpm. Countries with 50-Hz power supported 750-rpm TV " + 
    "sets. US stations with 60-Hz power employed the other motor speeds. These " +
    "speeds yield 12.5, 15 and 20 TV frames per second.";
  if (textNum == 3)
    document.scannerDescForm.elements['textDisp'].value = "SYNCHRONIZER MOTOR. " +
    "The synchronizer motor keeps the main motor in sync with transmitted " +
    "pictures. Homemade sets often didn't include a synchronizer. In that case, " +
    "the viewer might control the sync manually. Alternately, he could substitute a " +
    "synchronous motor for both the main, universal motor and the synchronizer. " +
    "Most synchronizer sets use either cogged wheel or phonic motor synchronizers. " +
    "Both types are variations on the synchronous motor. The phonic motor synchronizes " +
    "to the picture horizontal frequency. The cogged wheel synchronizes to the power " +
    "line frequency.";
  if (textNum == 4)
    document.scannerDescForm.elements['textDisp'].value = "MOORE NEON GLOW LAMP. The " +
    "Moore glow lamp provides the necessary gray tones of the picture. The scanner " +
    "amplifier reproduces the picture signal as a varying current. This varying " +
    "current drives the lamp, producing greater or lesser brightness values. " +
    "The viewer perceives these brightness values as gray tones. The gray tones can " +
    "change as frequently as 50,000 times per second. Even so, the entire picture " +
    "never appears on the lamp. Instead, human persistence of vision reassembles " +
    "the picture. It's an illusion, but the trick works!";
  if (textNum == 5)
    document.scannerDescForm.elements['textDisp'].value = "OPTIONAL MAGNIFIER. " +
    "The optional magnifier is a typical magnifying glass. It increases picture size, " +
    "but at the expense of exaggerating picture grain and reducing brightness. Such a " +
    "magnifier also reduces the picture viewing angle. Typically, only one viewer can " +
    "enjoy telecasts on such a set. Later sets substitute a neon crater lamp " +
    "for the glow lamp. Also, a lens disc replaces the aperture disc and magnifier. " +
    "A family can comfortably watch such projection sets. Another type of mechanical " +
    "set for multiple viewers is the scanning drum set. Some drums contain apertures, " +
    "while others contain mirrored facets.";
  //
  return 0;
}

function clearTextAreas()
{
  document.scannerDescForm.elements['textDisp'].value = 
    "";
}