3-D TV

3D TV

With a focus on Anaglyphic Stereoscopic display on Television.

By Alexander Lentjes

When you start to think of a serious application of the display of 3D imagery on TV, you have to start thinking in television terms first.
What does television mean? It means you have to transmit a signal that is compatible with the methods in use since 1953; namely the NTSC, PAL and SECAM interlaced image encoding through use of the Y Cb Cr algorithm. Plain and short this means even/odd scanlines (fields that make up a frame) at a rate of either 30 frames per second (coming down to 60 fields per second) - with NTSC - or 25 frames per second (50 fields)- with PAL and SECAM.
The world television audience is lazy and cheap by definition (bear in mind that most people can't and won't program their VCR or even their TV set itself), so you'll have to start thinking about a way of showing 3D images without complicated or expensive extra's and procedures - like electronic glasses.

So that's the playing field. Now this does leave quite some options open for consideration. Let's look at some of the available 3D ecoding techniques and their employability with TV technology.
Again, realism dictates that your ultimate choise depends on your means and goals; the bigger the audience, the cheaper the glasses need to be and the longer the film lasts, the better the 3D needs to be to prevent a massively headached audience vomiting over their TV dinner.

Lenticular 3D

Ideally, a 3D television presentation would be one where the viewer does not wear glasses at all.
This sort of a television exists. It uses the lenticular method and basically works the same way holographics do their 3D thing. The surface of this television is a special lens that has different angles, a lot of them next to eachother. This surface can also be a plastic sheet add-on.
The resolution of the screen will drop dramatically, because of the left and right image next to eachother (even/odd vertical lines) on one screen. For example, a 1024x768 image on a computer monitor will be reduced to 438x256.
Another version on this theme implies the use of a circular fresnel lens, dispersing the light in a way that makes the image look like it comes out of the television screen. This is etremely specialized and expensive hardware.

Talking glasses, first of all, the premise of the raytube that television is rules out any practise of the polarized system; the polarized system works only when it is projected on a silver screen with two differently polarized lenses.
Polarized 3D is best known for the big themepark films like 'Terminator 3-D', 'The Amazing Adventures of Spiderman' or 'Captain EO' (That one was replaced by 'Honey, I shrunk the Audience' in 1999).
Out goes the best option for 3D presentation.

Polarized 3D

Shutterglass 3D

The shutterglass, or VR system is a great option for 3D imagery on television.
If you're willing to spend the money on expensive VR goggles and a set-top receiver that is. And that means one for every family member.
There are versions with an LCD display for each eye, and versions that close and open left and right eye in sync with the field interlacing a television does (field sequential).
Watching the LCD screens that are as close to your eyes as possible is a big strain on the focus department of your brain as well. But this focus-headache thing is a general 3D problem.

So what about the pulfrich system then?
It works just fine, as recently performed with a broadcast of '3rd Rock from the Sun' by NBC.
Problem is, as is indigenous with Pulfrich 3D, the camera has to keep on moving for the effect to work, and it only works for a short amount of time. In short, it works on the combined premise of delayed image transfer to the brain -the eye with the darkened glass- and 3-D parallax -moving sideways, one eye will see equal imagery later than the other eye, creating depth.

Pulfrich 3D

This is where the color image systems end and the monochrome (or colorized) systems start.

ChromaDepth 3D

Chromadepth is a system that works with difference in hue - with hue running from red to blue.
Red objects appear in the front, blue ones in the back. This results in a colored image in the strictest sense, but it's really monochrome when the color is fixed to its position in depth.
When you are going to present moving images, this will result in something that resembles an LSD trip. If you can live with this color design, then ChomaDepth is probably the best system for cheap, accesible 3-D television production.

Then there's anaglyphic stereoscopics. Like ColorCode, the only thing needed for this display method of sterescopics is -besides the very cheap glasses- correct red and green (or yellow and blue in the case of ColorCode) color representation.
This is where television fails to deliver blatantly. TV just can't display colors at full strength.
Computer monitors do, though, and so do beamers with computer input.
Film works perfectly with anaglyphics, too, but the cost of film print can be an obstacle.

Anaglyphic Glasses

Charles Poynton explaines the color related limitations of television in moderately technical terms:

Film, video, and computer-generated imagery (CGI) all start with red, green, and blue (RGB ) intensity components.
In video and computer graphics, a nonlinear transfer function is applied to RGB intensities to give gamma corrected R'G'B'.
This is the native color representation of video cameras, computer monitors, video monitors, and television.

The human visual system has poor color acuity. If R'G'B' is transformed into luma and chroma, then color detail can be discarded without the viewer noticing. This enables a substantial saving in data capacity - in "bandwidth," or in storage space.
Because studio video equipment has historically operated near the limit of realtime recording, processing, and transmission capabilities, the subsampled Y'C B C R 4:2:2 format has been the workhorse of studio video for more than a decade.

The disadvantage of 4:2:2 is its lossy compression. Upon conversion from 8-bit R'G'B' to 8-bit Y'C B C R , three-quarters of the available colors are lost. Upon 4:2:2 subsampling, half the color detail is discarded.

Of the 16.7 million colors available in studio R'G'B', only about 2.75 million are available in Y'C B C R .
If R'G'B' is transcoded to Y'C B C R , then transcoded back to R'G'B', the resulting R'G'B' can't have any more than 2.75 million colors!

RGB Image

PAL Image

So the limited colorspace of the present television system makes anaglyphic representation of stereoscopic imagery impossible.
But what of initiatives like PAL-Plus and HDTV?
Well, PAL-Plus should do the trick. This is how it works

A PALplus picture has a 16:9 aspect ratio. It appears as a 16:9 letterboxed image with 430 active lines on conventional TVs, but a PALplus TV will display a 16:9 picture with 574 active lines (with extended vertical resolution to match).
The full TV system bandwidth (5.0MHz on systems B/G, 5.5MHz on system I) is available for luminance detail.
Cross colour effects are removed by use of so-called "Clean PAL" encoding and decoding. Put simply, Clean PAL encoding can be thought-of as the transmitter removing the sorts of signals (like fine patterned chequered shirts) which cause conventional PAL receivers to display stripey coloured interference bars. 4.0:

The extra information needed to restore full luminance vertical resolution is carried by a "helper" signal coded into the letterbox black bars (they aren't quite black therefore!). The helper can almost be described as a difference signal telling the TV the differences between the original 574 line 16:9 ratio picture and the picture which would be obtained by merely taking the 430 line letterboxed 16:9 image and interpolating it into 574 lines. (Such an interpolation done without a helper would yield an image with more scan lines but no more actual vertical resolution than the original of course).

Horizontal resolution of a PALplus signal is improved by the use of a so-called co-operative Clean PAL encoding and decoding process. A conventional TV should also benefit from this when it displays the signal - some of the effects of "cross colour" in which chequered shirts produce a coloured moire pattern ought to be reduced by the action of the Clean PAL encoding. The matching Clean PAL decoder in the PALplus TV will get rid of it altogether. Conventional TVs with comb-filters in their decoders will probably benefit more from this Clean PAL encoding than a TV with mere bandpass filters in the decoder - in so much as they currently do a better job of decoding conventional PAL.

Too bad television broadcasters are not broadcasting in PAL-Plus at all. The present equipment -much the same technology as used 50 years ago- is the cheapest equipment to produce television programs with. Go do the math and throw away your brand new widescreen PAL-Plus TV.

As figures, DVD is no solution. In short, the MPEG-2 encoding uses the television color system for its digital encoding of the image.
Here's the technical scoop:

The MPEG-2 concept is similar to MPEG-1, but includes extensions to cover a wider range of applications. The primary application targeted during the MPEG-2 definition process was the all-digital transmission of broadcast TV quality video at coded bitrates between 4 and 9 Mbit/sec. However, the MPEG-2 syntax has been found to be efficient for other applications such as those at higher bit rates and sample rates (e.g. HDTV). The most significant enhancement over MPEG-1 is the addition of syntax for efficient coding of interlaced video (e.g. 16x8 block size motion compensation, Dual Prime, et al).

MPEG-2 Video Main Level is analogous to MPEG-1's CPB, with sampling limits at CCIR-601 parameters (720 x 480 x 30 Hz).
The sample rate is near the Nyquist limit, or better bandlimit, of terrestrial bandlimited signals such as PAL (5.4 MHz luminance including blanking) and NTSC (4.2 MHz). It can also be said that 544 pixels/line captures the full glory (or at least 405 out of the claimed 425 TVL or TV lines) of analog video laserdiscs.

VHS picture quality can be achieved for source film video at about 1 million bits per second (with proprietary encoding methods). It is very difficult to objectively compare MPEG to VHS. The response curve of VHS places -3 dB at around 2 MHz of analog luminance bandwidth (equivalent to 200 samples/line). VHS chroma is considerably less dense in the horizontal direction than MPEG source video (compare 80 samples/ line to 176!). From a sampling density perspective, VHS is superior only in the vertical direction (480 lines compared to 240), but when taking into account interfield magnetic tape crosstalk and the TV monitor Kell factor, not by all that much. VHS is prone to timing errors (which can be improved with time base correctors), whereas digital video is fully discretized. Pre-recorded VHS is typically recorded at very high duplication speeds (5 to 15 times real time playback), which leads to further shortfalls for the format that has been with us since 1977.

Philips did a massive amount of research into the possibility of anaglyphic stereoscopic television broadcast. This research led to the development of encoding hardware that allowed for a broadcast of 3D television.
This is the rough schematics what the Philips system did: it uses two luma's (Y's) that only embraced red or blue - no chroma's (C' B, C' R). This modified television signal can then only be seen in correct anaglyphic colors with the live broadcast, when the full megaherzes of the available bandwith are used.
Today, the hardware that can encode this signal, is lost, as is the schematic, and the last known survivor of the Phillips development team has forgotten how to build it. Even Charles Poynton himself could not help out on rebuilding this elusive system.
Sounds like a classic fantasy story, but it actually happened.

And yet another creative approach to the situation is to use the even fields of the television signal to display the red (left) part of the movie and the odd fields to dsplay the blue (right) part.
As it turns out, this practise was already attempted almost as soon as the color television itself was invented, and pretty soon the inventors had to give up. Because the fields interlace at 50 fields per second (with NTSC 60 fps), the resulting (purple: red-blue) flickering is thrown back to 25 fps, as it were. So because the framerate was devised to be just fast enough to not see flickering with the human eye, separating the fields results in an image fit for killing the mildest epileptic.
Only a framerate of 90 or 100 fps would be fit for this sort of practice. And PAL nor NTSC is doing, or planning to do this.

So what does it all add up to?
Anaglyphics on television will be possible in the near PAL-Plus and HDTV future. Hopefully, shutterglasses will become cheap assets and with that, as regular a consumer product as the VCR, and lately, the DVD player. Perhaps the lenticular television becomes an affordable piece of equipment as well, or the circular fresnel technology takes off.
Any which way, in this world changes come slow: it's not strange that we still employ a television system that was enstated in 1953, while technology has wildly surpassed its boundaries. Television stations broadcast in the form that's cheapest and most widespread used - television manufacturers sell TV's that get bought by the public most - The public buys TV's that are the cheapest and receive the most widespread television signal. This is sort of a deadlock, most unlikely to evaporate from one day to the other. Please remember that a new signal that can only decoded on an expensive television set (like HDTV) will make sure that it won't be seen by many people - because most people don't own those televisions. Hence sponsors won't broadcast their commercials with those transmissions, because they want to reach that big audience as well, and the cost of those broadcasts will not be covered. And that's how changes in television come at a very slow pace, if they ever come at all.

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