Building a "Reference" Home Theater

An anonymous reader writes "FiringSquad has recently written a 14-page article on building a 'reference' home theater. They go through step-by-step and define all of the issues you need to think about when going with a new home theater setup. Exceptionally detailed but also easy to read."

Comments

What?

[Jeffrey Baker]

These jokers didn't even mention the most important part of a home theater: comfy chairs.

Too much wire/cable BS

[TheMeuge]

They dedicate an ENORMOUS amount of page space to cables... when they are by far the LEAST important part of the setup.

www.partsexpress.com has excellent Dayton-brand cables for a fraction of the price of Monster-cable. (And by the way, MENTIONING moster-cable among audio pros is a faux-pas in and of itself). Expensive digital cables are a HUGE ripoff, because jitter is largely a consequence of the source, rather than the transmission... and a well-made (yes, just look at it) $10 digital cable is going to sound no different from the $1000 MIT insanity.

Analog cables need to be well-made, but again... no need to spend more than $10-15 per channel. As long as they are well-insulated and shielded, they'll work just fine.

Trust me - on my multi-thousand $ system (Aragon, B&K, MSB, etc...) I could detect no audible difference between the most expensive cables I could borrow ($1000 MIT), and the $15 set that I soldered myself.

Re:Too much wire/cable BS

[egomaniac]

Bull. Shit.

For some reason people seem to turn off their brains and start appealing to voodoo magic when dealing with audio technology. Let's put it in a more familiar context: computers.

Suppose you're downloading a digital representation of music -- an MP3, say -- from the Internet. Now, we all know that an MP3 is just a series of bits, and as long as those bits arrive unmodified the song is going to sound exactly the same. Suppose I were to claim that you needed some super-high-fidelity Ethernet cable in order for the MP3 to sound its best after being downloaded, because otherwise the inter-edge arrival time in the digital signal will be distorted, and this, in turn, would map directly into harmonic distortion in the analog reproduction.

Anybody with half a brain would simply laugh at me. The bits either arrived properly or they didn't, and single-bit errors in the MP3 are going to produce pops and static, rather than anything so clean as harmonic distortion. Harmonic distortion is an analog problem, there's just no plausible way it can occur with a digital signal. Furthermore, we all know that you can hook your computer up with pretty much any old Ethernet cable, and unless the cable is seriously crap it's going to work perfectly even at gigabit speeds (far higher than anything you encounter in audio).

The same is, for the most part, true with digital audio as well. You're either going to get a perfect signal or horrible pops and static. There really isn't an in-between, and you're certainly not going to get harmonic distortion. Admittedly digital audio does not feature error correction, so marginal connections are more likely to give you problems, but it's not going to be subtle.

For the record: I have a home theater with a 160" screen and $15K worth of speakers and audio gear. And I use the absolute cheapest generic (but still quality) digital cables I could find, just as I hook my computers up with the cheapest (but still quality) Ethernet cable I could find. I don't think I spent more than $5 on any of my digital audio cables, and the sound from my setup is still awe-inspiring.

Video, of course, is a different story -- none of the video cable standards were really intended to span the 30' run between my equipment room and the projector, and video runs at a much higher bandwidth than audio. I found that I had to buy relatively expensive video cables in order to get a good signal (but we're still talking $100, not $1000), and again with the digital hookups it's nothing subtle. With good cables, the signal is perfect. With lousy cables, it's covered with white and black snow.

And I'd like to echo the comments of a previous poster: if you are looking for high-quality cables / connectors /etc. but don't want to get ripped off, use Parts Express. They kick all kinds of ass.

Re:Too much wire/cable BS

[fyngyrz]

Someone has fed you a load of nonsense.

Yes, frequencies are high in a digital cable. Considerably higher than in an analog cable that is trying to carry the same data. However, there are only two states that have to be reliably detected in a digital signal: Is the thing ON, or is it OFF? This means (generally speaking) that at the sample time, which is in the optimal place to catch the change, if the signal simply manages to be in the correct 50% of the range it needs to be in at the right time, it'll be interpreted correctly. Once converted back into a digital one or zero, there is no, repeat no THD or anything else that is a consequence of any distortion that the cable might have introduced. And that conversion happens at the very first digital input the signal is fed to.

The analog cable, however, has no such leeway. Any change in the analog signal comes right through as a change - the black level moves, the dynamic range decreases, images get auras, shadows, high frequency components ring and create repeating echos; analog interference, such as AC signals, CB radios, your local AM station and the crud in the AC lines when you run your vacuum cleaner or air conditioner can all get into the signal and distort it, even when only a tiny bit manages to leak through the shielding. With a digital signal, in order for those same interfering signals to have an effect, the digital signal has to already be degraded to almost 50% or there will probably be no effect at all.

The home theater image itself would tell you instantly if there is effective distortion (meaning, it's changing the bits being detected) that is getting into the signal even if the cable was just carrying visible image data (it isn't!) Because if errors were getting in there, there's no particular reason for them to be bits of low significance; they would be a random mix of all significance, and so you'd see bright spots in dark areas and dark spots in bright areas, errors as high as 50% as the most significant bit errored out. Audio would be the same - there wouldn't be a "little" THD, it would be a freaking mess. Ever watch digital satellite? Notice the huge errors, complete loss of the image frame? That is what happens when you lose bits in a digital transmission, not "increased THD." If the image data is compressed, then the visibility of errors is even worse - that's why satellite images lose partial frames, key frames, and image regions, not just individual pixels.

Clock jitter can introduce THD, sure enough, but that is so easy to avoid it is pitiful. And it isn't a consequence of cables unless the clock itself is carried on the cable, which it generally isn't anyway. No matter if the signal is "buffered" or "re-clocked", in the end, some input takes it as either a one, or a zero and outputs a stable one/zero result, essentially re-thresholding it. If this process works you get a fully reconstructed digital signal of ones and zeros with extremely rare errors (like, one a day.) If it doesn't work, you get an unwatchable, unlistenable signal. Remember that those errors aren't correlated to the data; they can be bits of any significance, they can be lost encryption bits, they can be lost framing or format bits... boom, wreckage.

I run a cheap HDMI cable ($60) 30 feet (the HDMI spec says 32 feet, no more) carrying image data to my 1080p projector; and similarly cheap ($6) but shorter HDMI cables carrying audio and image data from satellite, DVD and a PS3 that also serves as a Blueray player to my receiver. The only signal we ever see any errors on (and they're a huge mess, sure enough) is the satellite signal; we're not at a strong point in the footprint (NE Montana) and we get some pretty good cloud and precipitation combinations, not to mention some solar issues at certain times of the year. These errors aren't a consequence of digital cables.

One more thing: The audio THD in a system can usually be characterized by the THD of the speakers. Most even decent stereo gear has THD

Re:Too much wire/cable BS

[fyngyrz]

Yes, I read it.

First of all, closely coupled (of neccesity, because magnetic field effects fall off as the square of the distance) induced fields of 6 MHz are not in any way real-world interference for your typical home theater setup. Though I'll come back to that.

Secondly, and as the author of the paper notes, the cable impedance is 50 ohms, and so in order to induce a change in the signal carried inside, the power of the 6 MHz signal has to be fairly extreme in order to induce any change within the cable.

Third, again as the author of the paper notes, the interfering signal is a 6 MHz two volt square wave, something you aren't going to see in a home environment even if there was a 6 MHz signal source. Heck, you won't even find that in most labs unless they're trying to make it, as is the case here.

Fourth, none of this in any way implicates the cable as a source of jitter. It implicates outside signals as a source of jitter; the only related question is, does a really fine cable do a better job of keeping real-world noise (IOW, not a 2 volt, closely coupled, high power square wave) out of your home cables? And the answer, as we all know, is no, it does not. It is trivial to make a cable with a nominal 50-ish ohm impedance on a per conductor basis.

Sixth, home theater HDMI uses differential signaling [TDMS]... so induced signals like this are irrelevant anyway, as they are by their nature common mode.)

Seventh, the paper you have there does not test cables. It tests transformers. There is no question that the recovery interface, when actually different, will provide for different types of recovery. Hence his spread of results over his spread of tested transformers.

Eighth, you should note that even in the case of this horrendously strong, not real-world induced signal, chosen specifically for its unique beat frequency against the sample rate and therefore maximal capability to induce jitter (without any regard for the likelihood of such a signal to exist in the home theater environment), the jitter he detects is +/- 10 nanoseconds. The highest frequency to be recovered out of a typical 44 KHz signal is 20 KHz, which is a signal with a period of 50 microseconds. Adding (or subtracting) 10 Ns to the center position of a 50 uS waveform (and mind you, that's absolute worst case, because 20 KHz is the fastest thing to come out of that filter, period) results in time domain distortion that resolves to 4 Hz . or .00005%, which is inaudible (and again, doesn't even hold a candle to what a speaker will do to that signal when it gets hold of it. Doing the same analysis for 20 Hz, we see that the period of 20Hz is 50,000 uS, and when we add 10 uS to that, we get 50,000.01 uS, which is time domain distortion that resolves to 19.999996 Hz, or a .0000002% time domain error. Not only inaudible, just plain irrelevant.

And all of this, of course, is reflected in the pristine visuals and amazing audio we get from high end home theater systems today - even using cheap cables.

I suppose I should own up to what I'm using, as it has become germane: I've got 30 feet of HDMI cable driving a 1080p DLP projector, which in turn produces a 17 foot (205") diagonal image. Plus a bunch of cheap ($6) short cables from various sources to the receiver. I can walk up to that image and put a finger right under a specific pixel and watch the silly thing as long as I have the patience to. I can watch it from a stable source, such as a menu out of the PS3 or a still from a Blueray disk, or I can watch it change with the signal. I can see it in context with its neighbors, and if there were positional jitter (which is what this jitter equates to in video) I'd see it if it were perceptible - and it is not. That's a 30 foot, $60 cable, laid right next to four or five AC lines, some twisted pair control signals for my alarm system, and going by several large fluorescent fixtures in the basement. This is a real-wo

Wow, this is a really, really terrible article

[hudsonhawk]

There's nothing even remotely "reference" level about anything suggested here. Their suggestion for speakers, as mentioned elsewhere, is very poor (Polk? Really?) given that there's much better stuff available from great companies like PSB, Paradigm, and NHT.

That they top it off with a $400 receiver, and completely dodge the notion of getting into separates (other than to say that it's complicated and they haven't really listened to anything), makes it lose all credibility. The cable and power conditioning sections are a joke - they steadfastly refuse to entertain any alternatives to Monster Cable (hint: almost all the alternatives are better values for the money; just because you can get Monster Cable on sale doesn't make it a good value - their markup tends to be 3x-4x that of other cable manufacturers).

I get the feeling that they've listened to a handful of mass market pieces and decided to just suggest whatever crap it was they bought. As mentioned elsewhere, they don't even touch on room acoustics and setup factors, which can greatly influence the end results.

In short these guys don't know anything more about setting up a reference level home theater than your average Best Buy salesperson. Given their selection of brands, that's probably what they are in fact.