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Batting practice for the audio/video pro and a primer for the novice
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The AV industry has a history of co-opting, abusing, and discounting the relevance
AV Buzz Words
of fundamental audio and video terms to the level of a 'buzz word'. The next three pages attempt to restore the relevance of many victimized terms. Restoring relevance may correlate with correct application that can lead toward 'Saving the world from poor fidelity'.
Visit Ed's AV Blog page regularly to keep up with the ever changing 'buzz'
via links to current news and my WordPress blog.
Movie Aspect Ratios
Prior to 1953 the 1.37:1 aspect ratio dominated the silver screen. Due to competition from television the movie studios introduced several widescreen formats. This included
the widescreen 1.85:1 ratio, the even wider Cinemascope anamorphic 2.35:1, and its replacement Panavision's anamorphic 2.39:1 (often referred to as 2.40) aspect ratio.
Widescreen 1.85:1 pictures are created by cropping a standard 1.37:1 35mm film frame with masking on the projector lens. The passing unmasked widescreen image is enlarged to fill the entire theater screen. But the masking sacrifices resolution lost in the remaining unused surface of the 1.37:1 film.
The original 2.35:1 or 2.39:1 or image is resurrected with a reversing anamorphic projector lens that projects the widescreen image onto the theater screen. This method salvages the film resolution lost by the 1.85:1 process.
An essential feature of the masking and anamorphic methods is they save theaters
and the studios money. Each ratio is achieved with a single film size, a standard camera,
and a standard projector with interchangeable lenses: anamorphic, masked, and standard.
Digital Cinema Initative (DCI)
Digital projection has replaced film in almost all commercial theaters. Anamorphic issues are now digitally processed. That is; digital cinema does not use anamorphic lenses.
TV Aspect Issues
The aspect ratio of HDTV and UltraHDTV is 1.78:1 (16:9). Yet there is a large library
of 4:3 TV* programing and wider screen movies that must be accommodated by 16:9 TVs.
This accommodation arrives in two forms.
1. The original ratio can be achieved by reducing its size to fit within the screen.
The remaining unused screen area is filled with black or gray bars.
2. Fill the screen top to bottom and stretched the image to fill the screen laterally.
*Handbook Note: 4:3 was the aspect ratio of NTSC TV
The DVD is a NTSC 4:3 aspect ratio source. DVD widescreen films played back on an HDTV have two options.
1. Reduce the height of the image until it fits laterally. That sacrifices many of the
DVDs 480 lines of resolution.
2. Squeeze the widescreen image laterally into the 4:3 DVD disc frame while maintaining its height. Then have the DVD player electronically 'un-squeeze' the frame to its original widescreen aspect, and maintain its 480 lines of resolution. This type of DVD disc is labeled as Anamorphic Widescreen, Enhanced for Widescreen, or Enhanced for 16:9.
The 16:9 ratio Blu-ray disc does not offer an anamorphic option. But home theater enthusiasts with a projector and screen have the option of inserting a constant-height video processor in the electronic path and substituting an anamorphic lens for the original projector lens.
The constant-height processor fills the projector's 16:9 DLP/LCD chip by squeezing the 2.35 or 2.40 image laterally while maintaining its height. The anamorphic lens reverses the 'squeeze' and fills the entire 2.35 screen.
This option opens the door to a 'cool' home theater convenience feature. Given the processor/lens system fills the screen top to bottom for all 16:9, 2.35, 2.40, and even 4:3 images; an automated vertical black masking system can be added to cover the unused white screen area. An automated masking switch from 4:3 to 16:9 to 2.40 images creates an impressive presentation.
3:2 Pull Down
Movies are shot at 24 frames per second. DVDs are produced at 30 fps. This difference causes distorting artifacts. 3:2 pull down is a solution that repeats every fourth frame of the film which results in 30fps. Film sourced DVDs may include instructions that direct a DVD player to engage 3:2 pull down.
The Blu-Ray disc @ 24 fps avoids this problem with plasma TVs that offer a 24 fps playback option. This is also true of LCD TV's with 120Hz, 240Hz or 480Hz processing.
Each is evenly divisable by 24fps.
If you have observed an LCD picture with motion that crawled slightly in and out
of focus or misstepped, then you have experienced Blur and/or Judder. Blur refers to motion distortion caused by slow LCD pixel response and LCD back lighting. Judder refers exclusively to problems caused by a 24 fps cadence misstep.
120, 240, 480 Hz LCD Processing
LCD manufacturers address blur with 120Hz, 240Hz, or 480Hz processing. This feature increases the frame rate from 60 frames per second to 120fps, 240fps, or 480fps.
But this process involves much more.
120Hz processing inserts 1 extra frame between each of the original 60 frames.
The inserted frame is either a black frame or a frame of interpolated motion.
An inserted interpolated frame predicts motion which addresses blur caused by slow pixel response.
Bright back-lighting is recognized by the human eye as blur during frame transitions. An inserted black frame reduces the blur caused by the constant 'ON' back-lighting.
240Hz processing inserts 3 frames between each of the original 60 frames. This process inserts black and interpolated frames.
480Hz processing inserts 7 frames between each of the original 60 frames.
Again, black and interpolated frames are inserted.
Multiple Hz processing also reduces judder because 24fps divides nicely into 120, 240, and 480. Each method is offered in different combinations via proprietary software
in a plethora of manufacturer copyrighted names.
Back-light Scanning ---- more blur reduction
Back-light scanning synchronizes 'blinking' compact fluorescent or LED backlights
with the video. This much as the shutter gate of movie theater film projector. Both reduce blur created by constant 'ON' lighting.
No Blur, No Judder, and No More Plasma.
You can skip to the next page. The following is just for fun. Plasma TV is long gone.
But plasma delivered better performance than LCD. The plasma TV also offered some unique solutions. 600Hz sub-field-drive and 2500Hz focus-field drive offer two examples.
600Hz sfd & 2500Hz ffd were often confused with the motion blur LCD band aids of 120Hz/240Hz/480Hz processing. But plasma did not suffer from motion blur. The sfd and ffd technologies simply created clearer pixels with tighter light emission control.
600Hz sub-field drive drives plasma cell phosphor brightness with up to 10 pulses of voltage per frame. A pulsed driven pixel ascends to full lit amplitude in less than 1.67ms.
To put this in perspective; an LCD pixel's response time equals 4ms to 12ms.
Each of the 10 pulse opportunities is a subfield. As a sub-field-driven cell begins to illuminate in the first sub-field period; the sub-field-drive can pulse or not pulse the next sub-field. This is repeated for each of the 10 sub-fields.
The total brightness of a plasma cell is equal to the sum of the actually pluse-lit sub-fields. This process offers thousands of shades of gray. And the improved gray-scale results in an expanded color gamut. As for the moniker 600Hz; 10 sub-field periods at 60 frames per second equals 600 sub-fields per second: Hence 600Hz sfd.
2500Hz focus field drive
2500Hz focus-field drive also 'drives' the brightness with pulses of voltage. However 2500Hz-ffd produces all of its light in only the last three fields of 10 sub-fields. That creates a response time of only 0.4ms.
The faster window of illumination is much as faster shutter speed in photography.
In addition, given the first 7 to 8 sub-field periods are OFF, contrast is vastly improved.
The result is an even sharper clearer image. The 2500Hz moniker is derived by dividing
1 second (1000ms) by 0.4ms.
|Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288 Revised 2018