Oh, What a Night. HD Advisor Back in '63

Posted Fri Jun 18, 2010 at 11:45 AM PDT by

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Answers by Joshua Zyber

3-D Pass-Through in Receivers

Q: I had been under the impression that a receiver labeled as "pass-through" was a bad thing. That it meant a receiver could not process HD audio bitstreams from its HDMI port, so that you also had to connect an optical or coaxial digital audio cable to complete your surround setup (and also miss out on hearing lossless surround audio). Now when looking for 3-D capable receivers, the term "3-D pass-through" for video appears to be a desired feature. One retailer list the Onkyo TX-SR608 receiver as having "3-D Pass Through." Another retailer lists it as "3-D ready," which makes more sense to me. Am I the only one being confused by the new use of the term "pass-through"?

A: I think this will just come down to a matter of the same term having different implications in different contexts. In all cases, the phrase "pass-through" means that an A/V receiver can receive a video signal over HDMI on one end, pass that signal through to the other end, and transmit it to the display.

You are correct that, in the early days of HDMI receivers, "pass-through" implied that the receiver's HDMI connection was only used to pass video between the disc player and the display. These receivers could not process any audio signals carried by the HDMI cable. For audio, you would need separate connections. This was not a desirable outcome. It's preferred if the receiver can both pass the video through and process audio over HDMI. For the lossless audio formats, that's required.

These days, when it comes to 3-D, the term "pass-through" has a different implication. Because the 3-D signal has a different specification than traditional standard-def or high-def video signals, most A/V receivers can't pass the 3-D video. The video will go in on one end but never come out the other. For that reason, 3-D pass-through is very desirable.

Of course, you're going to want a receiver to process audio over HDMI in addition to that 3-D pass-through. Fortunately, as far as I'm aware, all 3-D compatible receivers should be able to do that by now.

3-D Glasses and Broadcast TV

Q: ESPN has been promoting that they are broadcasting some of the World Cup games in 3-D. The companies that are making 3-D monitors don't have the shutter glasses aspect together yet. I'm referring to non-compatibility between brands, and so forth. How will broadcast media, such as these soccer games, deliver the sync signal, and with compatibility to what kind of glasses? Also, is an upgrade in cable boxes going to be needed to pull this off?

A: To answer the first part of your question, the sync signal for 3-D is something specific to each television model and its corresponding 3-D glasses. This signal is not carried in the TV broadcast or encoded on a Blu-ray disc. The specifications for broadcast 3-D and for Blu-ray 3-D are standardized across all equipment on the output side (the cable box or Blu-ray player). The output signal from a Panasonic 3-D Blu-ray player will be identical to the output signal from a Samsung 3-D Blu-ray player. Likewise, the output signal from one 3-D cable box will be the same as any other 3-D cable box.

(Note that the standards for broadcast 3-D and Blu-ray 3-D are different than one another, however. Broadcasters will use an inferior form of 3-D that will deliver only about half the resolution to each eye as compared to Blu-ray 3-D. The Blu-ray version is called "Full HD 3D TV" or "FHD3D." Regardless, the hardware for processing both of these two standards should be built into every new 3-D TV.)

That's how it works on the output side. On the input side, each television brand may have its own way of processing those 3-D signals and syncing them with the necessary glasses. The sync signal will be added by the TV itself. That's why you'll need a pair of 3-D glasses specifically compatible with the brand of television you own.

So, in other words, if you have a Panasonic 3-D TV, you can hook a Samsung 3-D Blu-ray player to it, and it will work fine. You don't need a Panasonic Blu-ray player. But you will need a pair of 3-D glasses compatible with the Panasonic TV, and Samsung glasses may not be.

As for your question about cable boxes, this may vary by hardware model and cable provider. Some existing cable boxes may be upgradeable to 3-D with a firmware update, if the cable provider offers one. Unfortunately, I suspect that most cable carriers will force you to swap out the box for a newer 3-D model. Depending on the specifics of the hardware, there may or may not be legitimate technical reasons for that. In any case, I'm sure that they'll use this as an excuse to charge you extra each month for the 3-D box.

3-D Frame Rates

Q: Do you have any information about the refresh rates for 3-D material stored on Blu-ray, or the refresh rates of 3-D material displayed using 3-D players or TVs? I was curious if the 24fps rate of film was going to be matched or multiplied for home viewing, or if 3-D technology requires higher rates (to prevent flickering) that would involve 3:2 Pulldown processing at some point in the presentation. For example, the Panasonic Viera TC-P50VT25 appears to refresh each eye at 60Hz. But if a 3-D Blu-ray is going to be backward compatible with 2-D players that want to play at 24Hz, I would think that both the left and right views of the film would still be encoded at the standard 24Hz.

A: My understanding is that 3-D Blu-ray discs will be encoded with 1080p video at 24 frames per second, the same frame rate as traditional 2-D Blu-ray discs. The difference is that the 3-D version will be encoded with each frame at a resolution of 1920x2205 pixels. This equates to both the left eye and right eye views for each frame, stacked on top of one another, with some extra blanking pixels in between. (See the HD Guru web site for a graphic representation.)

These "packed" frames will be transmitted to the TV at 24 fps. At that point, the TV will decide how it wants to process them and display them on screen. Most models will unpack the frames into separate left and right frames at the traditional 1920x1080 resolution each, and then display them in sequence, much like interlacing. Those original 24 frames now become 48 frames per second. The TV will then multiply those 48 frames to its native display rate of either 120 Hz or 240 Hz.

Now, this is where things get tricky. If the TV is a 120 Hz model, that means that the left eye and right eye views must be displayed on screen at 60 frames per second respectively. Because 60 is not an even multiple of 24, the TV will apply 3:2 Pulldown to each of the original frames. Thus, you will lose the 24 fps cadence and may notice image judder.

At first, I assumed that a 240 Hz model TV would display the left and right views at 120 frames per second each, thus allowing the set to apply 5:5 Pulldown and maintain the original cadence through simple multiplication of frames. However, reader Lee pointed out that these sets actually insert black frames in between the video content frames. Therefore, the screen still only shows 60 video content frames per eye per second. Essentially, this means that there's no way to avoid 3:2 Pulldown with 3-D at the present time. Whether that might change in the future, I can't say.

[Note: Portions of this answer have been updated since original publication.]

3-D Compatibility of Older 120 Hz TVs

Q: I realize there are soon to be many different 3-D TVs on the market, but right now they all seem to be quite expensive. I realize 3-D technology is achieved by a 120 Hz signal that alternates each frame and the shutter glasses are synchronized to match these alternations. I was wondering, if I had a regular 120 Hz TV that supported a 120 Hz input signal, would it be possible to buy some kind of device that I could place between my 3-D enabled device and my TV that would interpret the 3-D signal, send the signal that the glasses use to synchronize, and then pass that signal to the TV? This way anybody with a 120 Hz TV that could read a 120 Hz signal would be able to be converted into a 3-D TV. This is a similar idea to back when HDTVs required a digital tuner set top box in order to actually receive an HDTV signal. Are there any such devices currently planned?

A: From a theoretical standpoint, what you describe might be possible if the TV could accept a 120 Hz input signal. Unfortunately, I don't know of any HDTVs that actually will accept a 120 Hz signal. All that I'm aware of are limited to 24 or 60 HZ input, and then multiply those signals internally to the 120 Hz or 240 Hz display. So, that pretty much makes this a moot point, I'm sorry to say.

Homework Assignment: You Be the Advisor

The HD Advisor knows many things, but he doesn't know everything. Some questions are best answered with a consensus of opinions from our readers. If you can help to answer the following question, please post your response in our forum thread linked at the end of this article. Your advice and opinions matter too!

Connecting Laptop to HDTV

Q: I have some playback problems when connecting my Dell laptop to my Samsung LN46B640 46" HDTV. The computer image accurately translates to the television except when I watch certain videos. Sometimes video streaming through Windows Media Player and other media players will not display on the television. The image works perfectly on the computer screen, but only a black image displays on the television.

Check back soon for another round of answers. Keep those questions coming.

Joshua Zyber's opinions are his own and do not necessarily reflect those of this site, its owners or employees.

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