Retrofitting TVs for 3D

Reader Sam emailed me with a very interesting question:

Your review of a 3D TV on Monday was very interesting. I have an LCD HD TV which handles 1080p 50Hz inputs,  as would most, if not all, HD TVs. It would be a simple matter to synchronise polarising glasses to alternate frames with a simple set top box, which should be a lot cheaper than a full HD compatible unit. Why can’t 3D TV be presented in such a manner?

Actually, I was reporting on the Panasonic 3DTV launch, rather than doing a formal review. But back to the question.

The problem is that in order to synchronise the shutters in the glasses to the display of frames on the TV, information from inside the TV is required. The device suggested would only know about the signal being fed into the TV.

The various TV technologies are quite variable in how long they take to organise and process the signals prior to displaying them. Some manage it in a few tens of milliseconds. High end Philips TVs take about 200 milliseconds, or one fifth of a second. There’s no way an external device could know this.

Both Sony and Panasonic at their respective launches made bit of a thing about how their technology reduced ‘cross talk’ between the eyes by switching more cleanly and precisely than competing technologies. Whose is better I don’t know. What I do know is that it is clearly an issue if they both felt compelled to talk about it. Given that with a 50 hertz system each eye is receiving its picture for just 20 milliseconds, even a drift of a millisecond or two would probably be fatal to the effect.

I suppose one could have some kind of delay knob on the device, but that’s a level of complexity I’m sure everyone is keen to avoid.

Oh, one more thing: there are differences between the plasma and LCD versions of the 3D shutter glasses, due to the fact that LCD displays are polarised while plasma ones aren’t.

This entry was posted in 3D, Video. Bookmark the permalink.

8 Responses to Retrofitting TVs for 3D

  1. Sam says:

    Thanks. There is, however an output from the TV (in my case, anyway), an AV connection. I can use this, with a transmitter and receiver, to “talk” to another TV, so it must contain information relating to the frame changes. If this is not true, then my guess is that if a suitable photoreceptor was focused on the screen,the resultant signal could be analysed, with simple circuitry, to produce a signal that would have the correct frequency and phase to control the glasses.Incidentally, is the 50 Hertz frame rate controlled by the mains frequency or an internally generated signal? If the first, then again I would imagine it would be quite easy to generate a suitable signal to control the glasses.
    Thanks again for taking an interest.

  2. Stephen Dawson says:

    The output is no use. It merely echoes whatever the signal is, before it gets to the video processing circuitry.

    A photo receptor may work, but it would be incredibly complicated. It would have to detect a blanking between frames. Plasma displays actually blank their screens (these days) at up to 600 times per second (12 times per frame) so you’d still need to adjust the sync to the correct blank. The aforementioned Philips would need to be adjusted to the correct frame delay, given that the picture is typically ten frames behind the actual transmission.

    The 50 hertz used by the TV isn’t generated internally, nor directly by the mains frequency. The TV takes its timing from the signal it is reproducing (same for 60 and 24 hertz material). If it used an independent clock then there would inevitably be drift, resulting in regular lost or doubled frames.

    A second later: By taking its ‘timing’, I mean that each process to create a frame starts when the frame starts to be received, in the case of an analogue broadcast signal, or when some specified ‘new frame’ marker or flag is received in a digital stream.

  3. Craig says:

    Another point on retro-fitting 3D to a 50P display:

    A 50Hz 3D refresh rate, where only 25Hz goes flashing to each eye, would be too low, and would result in unacceptable flicker. Not to mention the fact it would only be able to convey the low-ish 25 frame rate of film/movies, and not the true smoother 50Hz field update of studio-shot material.

    That’s why 3D needs, at the bare minimum, 100Hz or better for 50Hz material, be it 50P or 50i source. (Preferably 200Hz or more; ie 100Hz to each eye)


  4. Sam says:

    Thanks, Craig,
    I don’t know what the situation is now, but years ago cinema projectors used a shutter that not only hid the frame shift, but also blanked the light during the frame,(I don’t recall how often) to reduce flicker. Is there any reason why the viewing glasses should not do the same?

  5. Craig says:

    In the cinema, each frame of a film is flashed up on the screen twice, to give a 48Hz refresh rate. Some projectors even have a triple-bladed shutter, resulting in 72Hz refresh. But the source is still only 24 frames per second. It cannot convey faster motion than that. That’s why 50Hz and 60Hz studio-shot TV systems look so smooth with regards to motion when compared to film sourced material. And that’s why proper 50Hz 3D-HDTV will look so good on sport. It will actually have 50 unique pictures per second (per eye). Even James Cameron has said that 24 frames per second is too low for movies. I agree wholeheartedly. Avatar in 3D was great, except for the fact that I was very susceptible to the 24Hz judder, especially noticeable on camera pans. If the frame rate was doubled to 48Hz or more, it would have been a huge improvement for the 3D effect. But then some would argue that it has lost that ‘film’ look, and now looks like video. Oh well, you can’t please everyone. 24 frame film was chosen many years ago as a compromise between motion portrayal and film stock costs. But with digital capture now, there really is no great penalty for making something in a higher frame rate. The Digital Cinema Projectors are all capable of higher frame rates for playback.

    If the shutter glasses emulated the 50Hz refresh rate, by flicking each frame on the screen twice (assuming 25Hz source) then the brightness of the screen would be too low. You’d have to wind up the brightness a lot to compensate.


  6. Sam says:

    Thanks again Craig,
    What frame rate do the now available 3D TVs support?

  7. Craig says:

    I believe the new current 3D TVs support 24, 50 & 60 frame per second 3D TV systems (per eye). Meaning they are really 48Hz/100Hz/120Hz, or more. I think 24 frame film source (from Blu-ray) will be more like 96Hz (ie x 4 for plasma) and 120Hz (x 5 for LCD). Some LCDs are already touting 240Hz, but I think they’re really only 120Hz, but with black frame insertion between frames to reduce crosstalk.

  8. Sam says:

    Thanks Craig,

Leave a Reply

Your email address will not be published. Required fields are marked *