An Introduction to Imaging Science
Excerpts and Notes from the Home Theater Cruise
Presented by: Terry Paulin
On day one we started our Home Theater Cruise with An Introduction to Imaging Science (ISF) where Terry Paulin discussed the milestones of TV imaging beginning such as:
- 1939 when RCA showed a 343 line video system at the World's Fair.
- In 1940 NTSC locked down the current 525 line system.
- HDTV - which was the first consumer electronics product that was basically mandated to succeed by the government
- Over 18.5 million HD displays served from 1998 to date; $23 billion in sales.
- DVD sales were up 94% in 2004 which outsold the box office, CDs, video games, etc
The need for proper calibration of display devices was also discussed. Displays tend to ship in what is known as "showroom mode" which is how a consumer picks up a product from the store.
The common mantra people generally follow on the showroom floor is to follow the bright light - where all of the displays are in "torch mode" and the brightest one on the showroom floor is usually the one consumers tend to pick. Combine the "torch mode" default calibration issue with typical parts tolerances of +-10% from display to display and it's each to understand the necessity of properly calibrating displays.
ISF also discussed:
- The importance of matching the display to the source
- Matching displays to the room environment (variable ambient light for both day and night should have different calibration modes accordingly)
One catch phrase that really stuck with me was:
Taking pictures is an art, making pictures is a science
ISF really drilled in the concept of "Getting the Whole Picture"
To do so, you have to avoid clipping whites which eliminate active picture elements, avoid low black levels which eliminate active picture information, watch overscan, and use the sharpness control sparingly, or not at all. In fact, they even pondered the questionable placement of sharpness/detail controls on display devices.
It's all in the black and white levels which set the best contrast ratio. Getting proper calibration over 0 IRE to 100 IRE is the basics for color signal. In addition to them looking better, proper calibration can extend light of phosphor based TVs, CRT, RPTV, and plasma. Calibrating White Level allows the displays to be operated in the linear operating range. With CRTs you need to avoid blooming and for fixed pixel devices you'll want to avoid what's known as clipping. ISF thinks Black and White levels are so very important they are even working with Microsoft in ways to better calibrate contrast for Media Center PCs and set top boxes.
NTSC Decoder adjustments are also important, including Sharpness, Color/Chroma controls for saturation, and Tint/Hue.
CIE Chart theory was also discussed. Basically, if you can't measure it, you cant manage it. This is a lesson we wish exotic cable vendors would learn! If the display cannot represent all of the available ATSC colors, you are simply not going to get the vibrancy you would like to see from today's digital video displays.
Gamma was discussed, namely the linearity of luminance values on a scale of 0-100IRE. It's important to avoid land mines such as SVMs (scan velocity modulation - ultra harsh sharpening circuits), Auto Flesh Tones (which can over saturate reds), Auto tint, Auto color - Auto anything. Are you seeing a pattern here? Manufacturers are good at dumbing-down controls, but not so good at providing automatic controls for getting accurate color and detail.
Calibration Summary Steps
- Warm up the display (20-30 minutes for most displays, especially plasma or CRT-based systems)
- Document pre-calibration performance
- Remove landmines such as most automatic settings for sharpness and color
- Test and learn the color modes
- Know the native resolution of the display device so you can feed it the best possible source format and reduce scaling artifacts
- Determine proper viewing distance to lower the visibility of artifacts
Choosing a Projector Screen
Another topic covered in the Intro to ISF session was a discussion of screen dynamics with key metrics to pay attention to regarding screen design. This included white field uniformity and spectral purity.
They also touched on some of the benefits of Gray screens when used with DLP/LCD front projectors. We encourage them in rooms that cannot have complete light control or which have highly reflective or light surfaces in the room.
While they were certainly not advocates of perforated screens we still view them as a viable alternative for those who wish to see their movies, NOT their speakers. In fact, putting the main and center speakers behind the screen is almost imperative in rooms that demand a very large screen-to-seated position - lest you place the speakers too far apart for accurate sound reproduction. In either case, we appreciate the fact that you have to be careful with perforated screens as they tend to lose light because of the holes. In addition, audio at and above 10kHz will also need to have some serious attention paid to it in order to correctly reproduce a flat response.