HD Advisor 17 Again (OK, Really 18)

Posted Fri Jun 19, 2009 at 12:00 PM PDT by

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Answers by Joshua Zyber

Video Bit Rates

Q: What is the difference between higher and lower AVC encode rates? For example, 'Office Space' lists an AVC encode at 33mbps, while 'Quantum of Solace' has an encode of 28mbps. Either movie doesn't seem to trounce the other in video. Looking at different covers for Blu-ray movies, some have specifications listed on the back covers and some don't. So what is the difference between encode rates?

A: In simple terms, video bit rate is a measure of the amount of digital compression used to encode a movie on a Blu-ray disc. A higher average bit rate means that less compression was used. A low average bit rate means that more compression was used.

Video compression, by definition, throws away data from the original digital master in order to conserve file space. Because of this, it's only natural to assume that compression must be a bad thing, and that discs with high average bit rates (i.e. less compression) must have better video quality. However, things are just not that simple. Video compression doesn't work on a linear scale. The amount of compression needed for any given movie will vary depending on the complexity of the content, the codec chosen, the compression tools used, and the skill of the compressionist performing the work. For example, a bright, shiny, and clean CGI animated feature is typically much less of a compression challenge than a grainy 16mm indie movie, and can get away with a lower bit rate.

Because no two movies are exactly alike, you can't compare the bit rates of two separate discs and draw any meaningful conclusions about video quality. Even within a single movie, the amount of compression needed will vary from scene to scene, and even shot to shot. Further, as authoring tools mature and compressionists gain more experience working with them, high-performance codecs like AVC and VC-1 become more efficient over time. A good compressionist should be able to take a movie originally authored back in 2006, re-compress it today using the same codec at a lower bit rate, and derive equal or better results.

I wrote about this topic at some length back in my Specs vs. Reality article. The long and short of it is that, unless you see specific digital artifacts on screen, the bit rate is just a number. It's essentially meaningless to the end viewer. There are many more important factors to rate a good video transfer than just the compression ratio. What's important is how the movie looks to your eye, not how high the bit rate meter spikes. So ignore that number on the packaging, turn off the bit rate meter on your Blu-ray player, and just watch the movie.

PCM Audio over S/PDIF

Q: I have a PS3 and a Denon AVR-3801 receiver. I use HDMI to pass video to a Samsung HL61A-750 and use an Optical link to connect the PS3 to my receiver. I picked up the 'Neil Young Archives' on Blu-ray and the on-screen display from the PS3 shows that it is playing back a Linear PCM signal at 192 kHz. Some forums I have been looking at state that it is not possible over a Toslink connection. My question is, what am I actually hearing, the full 24/192 audio or is it somehow being downsampled but the PS3 doesn't see it?

A: An S/PDIF connection (either Toslink or digital coaxial) is capable of carrying up to 2 channels of PCM audio. Movies with soundtracks in PCM 5.1 format (e.g. most early Blu-rays from Sony or Disney) will be downmixed to 2.0 configuration by the Blu-ray player when using S/PDIF. Fortunately, those Neil Young discs are PCM stereo format, so no downmixing will be required.

Where you might conceivably run into trouble is the fact that the audio on those discs is encoded at 24-bit/192 kHz resolution. While an S/PDIF connection should be capable of carrying that signal, not all audio hardware will support it. You should check your A/V receiver's documentation. If it doesn't support 24/192, either the receiver will downsample the signal to the highest resolution that it does support, or (if you get no audio) you'll have to set the PS3 to do so.

720p vs. 1080p HDTVs

Q: I've been viewing HD content for over 4 years now with my Samsung HLP-6163 HDTV monitor. It's a 720p set in which I've only had to change the bulb once. I have a Sony BDP-S550 Blu-ray player, Toshiba HD-XA1 HD DVD player, JVC D-VHS deck, and a Denon 3910 DVD Player connected to a Denon AVR-4306 receiver. Each unit is connected to the receiver via HDMI at 1080i then sent to my monitor via HDMI. I believe the picture I get looks good; however, with many monitors out in the market boasting 1080p, I'm wondering if I'm missing additional detail. I'm considering the new Mitsubishi 837 series as a replacement if I am going to get a significant benefit in definition and detail. My question is do you think an upgrade is beneficial? My primary viewing position is about 15 feet away from the monitor.

A: I'll be honest, this is a decision that I've struggled with myself when looking for a small HDTV to use in a secondary room. Raw math tells us that a 1080p (1920x1080) display has 125% more resolution than a 720p (1280x720) display. That's a lot of pixels. However, in practical application, the visible difference between 720p and 1080p is much smaller than the difference between either one and standard-def 480i. Once you make the leap to high definition, successive steps in resolution offer diminishing returns.

That's not to say that there isn't a visible difference between 720p and 1080p. There certainly is, especially with larger screen sizes and optimal seating distances. But the improvements are mitigated by smaller screens and more distant seating. Generally speaking, it's going to be very difficult to tell the difference on screens less than 40" no matter where you sit. With larger displays, the rule of thumb is to stay within 1.5x the width of the screen.

In your particular circumstance, I notice that your current HDTV is 61" diagonal, and you're looking at other sets about the same size. A screen over 60" should be more than capable of showing improvements at 1080p. On the other hand, your seating distance of 15' is more than 4 times the screen width away, which means that you'll have a hard time seeing that extra resolution.

In the end, only you can decide what to do with your money. If you decide that now is the right time to buy a new TV, I'd recommend future-proofing yourself by purchasing a 1080p model. Even if you can't discern much difference at your current seating distance, you may adjust that distance in the future. When you do, you'll want those extra pixels. But if you're otherwise happy with your current TV, you can probably hold out with what you have for a while longer.

2k vs. 4k Resolution Revisited

Q: This is somewhat of a further reply to the earlier question about digital video having less resolution than 35mm film. Wouldn't any more resolution than 1080p be a bit superfluous for the home video market? My understanding is that one already has to buy a TV on the larger end of the size spectrum to even see much difference between 720 and 1080 resolution. So I would think while one might see a difference between 2k and 4k resolution on an IMAX screen they would not on a screen small enough to fit in the house.

A: Basically, yes. For home theater purposes, I think 1080p is the sweet spot where video displays will remain for quite some time. As mentioned above, higher resolutions offer diminishing returns at the screen sizes most of us can fit in our homes (even projector owners like myself). I'm sure we'll see some manufacturers experiment with higher resolutions, but by and large I expect 1080p to be the standard for the foreseeable future.

Of course, I'm no psychic. So don't hold me to that.

Homework Assignment: You Be the Advisor

Some questions that the HD Advisor receives 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!

Surround Speaker Placement

Q: Due to the nature of my cinema room, my rear speakers have to sit up high to clear the door on one side, which pretty much means they are ceiling height. They provide good ambient sound but really lack any sort of detail akin to sitting around a friend's house and having the rears just above ear height. They are a pair of Eltax Monitor IIIs which where given to me so I cannot moan about the use I have had out of them. However, I would like to improve my audio experience, are there any tricks I should know about high speaker placement? Failing that, are their speakers designed for such a task?

Check back next week 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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