Musings about a portable LCD TV screen

Pieter-Tjerk de Boer, PA3FWM pa3fwm@amsat.org

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, May 2018.)

Recently I needed a not-to-large TV screen, for use with a camera with zoom lens for SMD soldering. For this I bought a 7" screen at a hamfest; the screen had been part of a portable DVD player. While studying the electronics to find out how to repurpose it, a couple of things caught my attention.

It turns out the screen expects a composite video signal (PAL or NTSC). That's quite convenient for my purpose, but it's strange considering the original application. After all, the colour information on a DVD is surely not encoded as PAL or NTSC. Why not simply feed the red, green and blue signals directly to the screen, rather than first converting them to composite, and then convert them back to RGB, with loss of quality, after transporting them as composite over literally just a few centimeters? I guess somehow this was cheaper.

The screen has up/down buttons to control the brightness. It turns out that these buttons are connected to a chip (Intersil X9511), which simply emulates a potentiometer: it contains a set of resistors connected in series, one tap of which is selected by the up/down buttons. It even contains some flash memory to remember the "potentiometer's position" when power is off. Wouldn't it have been simpler to just mount a mechanical potentiometer, and in fact easier to use? Again, it's probably a matter of price, or perhaps a rotary knob was considered old-fashioned (even if easier to use).

The screen's vertical resolution is just some 250 lines. But if around 2005 (the estimated production date of this screen), this was considered enough for an acceptable picture quality, then why did 1950s engineers go to great lengths to make our TVs show some 625 lines (of which about 550 visible)?