On 27/02/2010 12:09 AM, Geoffrey S. Mendelson wrote:
Sylvia Else wrote:

CRT TVs refresh at 50Hz or 60Hz (near enough) depending on region.

FYI, Computer CRT screens refresh at 60 to 85 Hz.

The main difference between CRT's and LED's or LCD's is persistance.
The CRT's have long persistance phosphors, when they are illuminated, they
stay lit for a relatively long time. That's why the interlacing system
works, the odd lines are still lit when the even ones are illuminated.

It's not that long, which is why photographs of television pictures look
so awful. Interlacing is used to avoid flicker without having to
transmit 50 or 60 full frames per second.

LCDs don't flicker anyway, regardless of their framerate. The frame rate
issue relates to addressing the judder you get as a result of the image
consisting of a sequence of discrete images, rather than one that
continously varies.

It doesn't help that much TV material that was recorded on film is
transmitted with with odd and even interlaced frames that are scans of
the same underlying image (or some variation thereon), so that the
effective refresh rate considerably lower that the interlaced rate.

Sylvia.