[Tig] ASC Non-Theatrical Display Subcommittee Report

[Bob Friesenhahn]

On Fri, 18 Jan 2008, Rob Lingelbach wrote:
>> TVs with a dynamic iris tend to amplify errors in black level.
>
> dynamic iris = AGC  ?  In the case of one particular client several years

A dynamic iris is an iris (similar to what is found in an SLR camera) 
which can be used to control the percentage of light (from a fixed 
light source) which hits the screen.  For a dark scene, the iris is 
closed more, and the digital signal is scaled up (brighter) in order 
to use more of the device's dynamic range.  The hardware evaluates the 
picture level in real time in order to determine how much the iris 
should be open, and the amount of scaling to apply to the input 
signal.

A problem can occur when the iris is mostly closed and the signal is 
scaled way up since with an 8-bit digital input, only a few steps are 
used for all of the image.  This results in a sort of "pooling" effect 
as seen in dark regions on older plasma displays, and the problem is 
made much worse by MPEG encoding because the block noise is amplified 
to produce quite obvious MPEG artifacts.  This was a problem with my 
Samsung TV as delivered.  It is likely that more recent models are 
less agressive with the "black amplification" factor.

LED and laser TVs provide the opportunity to obtain the dynamic range 
without using an iris because their light output can be dynamically 
modulated.  It is likely that the LED/laser light output can only be 
controlled to a limited extent so there is probably still a need to 
establish the final level via other means (e.g. DLP uses vibrating 
mirrors).

Bob
======================================
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/