TVs are always set up to generate a picture which is 10-15 percent large than the visible face of the CRT. Why?
In the early days of TV, this was probably done to make the design easier. Component tolerances and power line voltage fluctuations would be masked even if they caused changes in picture size. There certainly is almost no reason today to have any more than a couple of percent overscan. Most modern TVs have very well regulated power supplies and component values do not really drift much. Computer monitors, for example, are usually set up for no overscan at all so that the entire image is visible. We are constantly reminded of that, now that we are building TV’s with VGA inputs (PD5029C1 in the USA, US$ 2000). This mixed application has overscan in TV mode and underscan in VGA mode. Geometry adjustment is quite critical if you see border-on-border. Unfortunately, TV’s may be good but VCR’s certainly are not. If you have too little overscan and then put the VCR in any feature mode (like picture search) then one (black) picture edge may become visible. Bad form. Viewers do not like this. W
