Posted Fri Aug 27, 2010 at 11:15 AM PDT by Joshua Zyber
Editor's Note: Each Friday, High-Def Digest's own HD Advisor will answer a new round of questions from our readers. If you have home theater questions you need answered, send an email to [email protected]
Answers by Joshua Zyber
Is 1080i on Blu-ray Really Inferior to 1080p?
Q: Since an interlaced signal is automatically turned to progressive when it reaches your LCD/Plasma, why is it frowned upon when a Blu-ray is released in 1080i? Won't the outcome be the same? If I remember correctly, 'Terminator 3' was recalled and re-issued in 1080p after an initial 1080i release.
A: You are correct that any signal you feed into a 1080p HDTV will be deinterlaced and/or scaled to match the TV's native 1080p resolution. Also, many Blu-ray players can do this processing at that stage if desired. However, there are a couple of big advantages to having a movie authored as native 1080p on a disc rather than as 1080i.
First off, when a 1080i signal is deinterlaced to 1080p, you have to rely on the deinterlacing hardware in either the Blu-ray player or the TV (whichever you choose) to correctly re-assemble the original film frames. Some processing chips are better at this than others. If performed correctly, the result should be seamless to your eye. If performed incorrectly, this can lead to jaggies in the picture and a loss of detail. This seems like a needless chance to take when the Blu-ray format is perfectly capable of transmitting the original 1080p frames without interlacing or deinterlacing. The least processing generally results in the purest signal.
In addition to this, discs authored at 1080p resolution are also (by requirement of the Blu-ray spec) stored on the disc at the original photographic capture rate of 24 frames per second. Assuming that your Blu-ray player and HDTV are compatible with 24 fps transmission and display, you'll be able to watch the movie at that rate without the judder artifacts associated with 3:2 Pulldown. On the other hand, discs authored as 1080i must be stored at 60 Hz, and have 3:2 Pulldown forcibly applied. For more information on this, see my earlier What's the Big Deal About 1080p24? article.
While it's possible to remove 3:2 Pulldown after-the-fact, this usually requires an expensive external video processor. Many modern TVs can display a 24 fps signal at that rate (or an even multiple) if fed one, but few will convert 60 Hz to 24 fps. In my experience, conversion from 60 Hz to 24 fps is very easy to trip up if there are any bad edit points in the video, and that can cause tremendously distracting tearing artifacts on screen that can only be corrected by pausing the movie momentarily and restarting. Who wants to deal with that?
Q: Like a lot of people, I've set-up my system with low frequency signals from my main speakers redirected to the LFE channel. Mine is crossed over at 80 Hz. That is great and certainly sounds good with less stress on the main speakers. However, I was wondering what happens if, for example, there is a 40 Hz signal coming from the main speaker channels and a 20 Hz signal from the LFE channel at the same time? What is produced by the subwoofer? Is it somewhere in the middle, say 30 Hz? Or can the sub produce both frequencies at the same time? You can imagine it would get tricky if there is below 80 Hz info being produced from all channels? What gets re-produced? Is it better to put a Sub on each channel through speaker wiring and then leave the LFE channel untouched?
A: When you use bass management to redirect low frequency audio to your subwoofer, all frequencies below the cutoff you've selected will be combined together with any signal encoded in the LFE channel and sent to the sub together. The subwoofer does not average out these values. It will reproduce as much of the range as it can, depending on its size and capabilities. (Small subwoofers just can't hit the lowest of low notes. You need a large vibrating speaker surface for that.)
As for whether it's better to send everything to one subwoofer or to wire separate subs out from each main speaker channel, in most cases you're more likely to achieve better results with just one subwoofer. Because bass in non-directional, you should not lose any of the effect of the bass by having it all come from one point in your room. If you set up multiple subs all around you, you wouldn't be able to tell where the bass was coming from anyway.
Even more importantly, producing low frequency bass from multiple directions can very easily have the negative side effect of causing the bass signals to cancel each other out, which will leave you with a very muddy low-end in the soundstage. This is exactly the opposite of what you want. It's possible to set up multiple subwoofers correctly without having this problem, but it's very difficult and not usually worth the effort in most home theaters. A single, high-quality subwoofer should produce cleaner, crisper, more defined low frequency audio.
For example, suppose you're listening to classical music or a movie with an orchestral score. Ideally, you should be able to hear each and every cello note crisply and distinctly. What you don't want is a muddy block of low-end boom, which is all-too-common when using multiple subwoofers that are set up incorrectly.
Update: The Advisor is willing to acknowledge that he's not all-knowing or infallible. Some of the recommendations in this column are open to debate or disagreement. After this article went live, reader Chase – a professional acoustic and home theater contractor – wrote in with the following:
Chase: While every room and scenario are different, it is generally accepted among acousticians these days that multiple subwoofers are better than a single subwoofer for bass-managed systems. While it is true that you can configure multiple subwoofers in advanced systems with variable level and delay for each unit (requires advanced test gear to calibrate), it is also beneficial to set up simple, multi-subwoofer systems with all units receiving exactly the same signal. In fact, this sort of arrangement is sometimes the optimal solution for a room and can be easier to implement than a single subwoofer. This is especially true if you are trying to achieve smooth bass response for more than one seat. Single subwoofers are notoriously difficult to place for smooth response because they drive bass resonances asymmetrically, resulting in efficient pressurization of the resonances (a bad thing) unless the subwoofer is carefully placed in a node. Multi-subwoofer systems drive resonances symmetrically, canceling them out and restoring smooth bass with proper timing.
If the low-pass filter portion of the crossover is set according to industry standards (typically 80 Hz with a 24dB/octave slope), localization of a single subwoofer or multi-subwoofer system should be at a minimum. I personally prefer to run the low-pass for LFE at 120 Hz, which results in much greater limitations on subwoofer placement (must be relatively near the screen). Fortunately, I have no aesthetic restrictions on my systems, so subwoofers can occupy the best possible acoustic locations at the front of the room. My recommendation for most people is still to stay with the 80 Hz/24dB/octave filter for everything.
Like many things, multi-subwoofer setups can really be as simple or as complicated as you make them. For example, a straightforward four subwoofer layout for rectangular rooms might have one unit at the mid-point of each wall. You set all their volume controls to the same level and split the AVC/R's subwoofer output four ways. A similar but smaller configuration using only two subwoofers is almost as effective. These are only marginally more complicated to set up than a single subwoofer, yet vastly outperform it.
On the other end of the spectrum, professional contractors can do custom acoustic modeling to predict optimal subwoofer locations and then employ sophisticated analyzers to tweak the level and delay/phase of each unit to achieve the smoothest frequency and time response possible. I personally recommend that everyone who cares about good sound try to implement a multi-subwoofer system, regardless of how simple or complex it may be.
In my experience, the biggest obstacles that people face are expense and aesthetics. Four subwoofers is four times as expensive as one, and most people won't tolerate that many boxes out in the room. Still, multi-subwoofer setups remain one of the most effective ways to combat bass resonances, which overshadow all other bass problems in small rooms. When you consider that other solutions include bass absorption (rife with snake oil), flexible wall/floor/ceiling construction (expensive!), and electronic equalization (only marginally effective), multi-subwoofer systems don't look so bad!
JZ: This is all very good and useful information. I'm still not 100% convinced that I would recommend this for the average home theater, though. I just think that this is very complicated, and there's quite a bit of risk that an inexperienced user is likely to get something wrong, which will result in a muddy soundstage and an overall worse audio experience. Those more knowledgeable and experienced in audio matters, or those who seek professional guidance, may find benefits in a multi-subwoofer arrangement as Chase describes, however.
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!
Dark Blue Stripe on Screen
Q: I have a fat PS3 hooked up to an Onkyo TX-SR708 receiver via an HDMI 1.4 cable. The receiver is connected to a JVC 47" LCD TV via another HDMI 1.4 cable. The TV is three years-old and is 60 Hz. The problem is that sometimes I see a dark vertical stripe down the center of the picture. It appears most frequently when dark blue is the dominant color on the screen. For example, on the Blu-ray version of 'John Adams,' there is a scene where he's riding his horse near dusk or dawn in the snow. The screen is mostly dark blue. That's when I first noticed the vertical stripe. Playing 'Batman: Arkham Asylum' in "Detective Mode" has revealed another dark stripe. This one is horizontal along the top of the screen, about an inch from the top. Both stripes show up in this situation. I didn't see these stripes before I got the receiver and the HDMI 1.4 cables. How can I fix this? Is the TV's 60 Hz rate to blame?
JZ: My first thought was that this could be a simple electrical ground loop problem. However, those usually manifest as horizontal bands that scroll from top to bottom through your picture. I've never heard of a ground loop causing a vertical stripe. And I don't know why the stripe would be blue. Just to be sure, disconnect any cable TV in your system and see if the issue goes away. If so, you just need a small doodad called a Ground Loop Breaker that shouldn't cost more than a couple dollars.
Failing that (and there's a strong possibility I'm wrong here), I'll have to ask if any of our other readers have experienced anything like this.
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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