Multichannel Audio Hardware Guide September 13, 2002 Alexis Dang

Overview

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This is the first part of a two part series on multichannel audio. The first part will focus on the hardware and physical attributes and requirements for multichannel audio, followed by an article about all the software formats available.

A little bit about how we hear

Most of us have noticed that we only have two ears. The part that we see helps to funnel sound waves to our tympanic membrane, which is just like the membrane on a microphone. Vibrations of the tympanic membrane transmit vibrations to various signal amplifying bones and finally to the cochlea where the mechanical vibrations are integrated into electrical signals, just like a coil in a microphone. Interestingly the cochlea is organized by frequency - certain parts of it respond to certain frequencies. That’s how one ear hears, but the sonic environment we experience is a combination of the sounds from both ears. Given that the ears have a finite space between them, the sound reaching one ear is delayed compared to the other. This delay between the sounds reaching our ears gives us the stereo effect and helps us to localize sounds in our environment.

Ok, so two ears can give us 360 degree localization of sound, but why do we need more than two speakers? Actually if you had very good headphones, you could probably recreate a 360 degree environment, although your recording must be designed for headphone listening. Another issue to consider is the ideal listening spot. Often we do not listen to music or enjoy movies by ourselves, so it would be nice if more than one person could experience the surround sound. With more speakers, you can create more ideal listening spots.

History?

Our first experience with multichannel audio was probably at the movie theaters where we were in awe of all the speakers around us. I remember going into the theater and counting how many speakers there were around us. Home usage of multichannel audio has been around for a while. Back in the 1970s there were many quadraphonic recordings on either LP or 8-track that offered four listening channels. The effort was aimed at surrounding the listener with sound to more closely create the ideal listening environment.



SIDEBAR: Many people are converting their quadraphonic recordings into DTS CDs that can be enjoyed on what is today’s conventional equipment.

Foursome

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We’ll skip the evolution of stereo but stop here to discuss four speaker audio. Simply, this is having two speakers in front and two in the back. This will be referred to as a 4.0 system if the speakers all have discrete sources.

With stereo, identical sounds from both speakers will sound like they are coming from between the speakers if the sounds reach your ears at the same time. This makes sense, but we must consider that the sounds from two speakers can only reach your ears at the same time if you are more or less equidistant from the speakers.

The next evolution came with the addition of the center channel in between the front speakers. This was primarily used for movies to carry to voices of the actors. Center channels were traditionally placed near the TV screen so that actor’s voices would appear to be coming from the screen. It turns out that this new channel now carried nearly 70% of all sound during a movie. Many people loved the center channel as it allowed voices to be reproduce without the interference from ambient sounds that were fed through the other speakers, others hated it because it robbed the imaging and soundstage that they had with two speakers before. One of the reasons for this is that many center channels are not as well designed and built as a main speaker. In addition, the aesthetic horizontal placement of the center channel wasn’t ideal and the TV screen affected the sound.

Overall, the consensus was that the center channel was a good development. It allowed people across the entire sofa to properly place sounds in space when watching a movie and made it so people didn’t have to sit directly in between the main speakers.

Around this same time, people began adding a subwoofer to their sound system. At the time, most subwoofers were actually just woofers that made up for the inadequate bass response of smaller speakers. BOSE is most famous or infamous for its design, but JBL and others had sub-sat systems in the 1970s. The reason that subwoofers have more freedom of placement is because the low frequency sounds are non-directional. This is because the wavelength of the sound is much bigger proportionally to the distance between our ears and the temporal sensitivity of our ears. It’s high school physics all over again. This is on the part of the truth however, as subwoofer placement is just as important as placement of any other speakers. Room resonances begin to play an important factor in frequencies below 80hz. A subwoofer that has a perfectly flat response in an anechoic chamber may not perform as nicely in a room. The shape of the room, the volume, the material and stiffness of the walls can all determine its resonance peaks although predicting it beforehand is difficult. Subwoofers placed in a corner sound louder because they have the reinforcement of two walls and the floor. This is similar to cupping your hands when you yell.

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The Big One

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I will say that out of all the multichannel channels, the subwoofer is my favorite. It is often referred to as the .1 channel. Most subs start at below 80hz, but few can get down to 20hz, which is the average limit of human hearing. Our ears are very insensitive to these frequencies and we often feel the sound more than we can hear it.

So let’s recap: we’ve got two front speakers, two rear speakers, a center channel and a subwoofer. This kept us happy for quite a few years as we slowly added a discrete signal for the subwoofer and created a stereo rear channel. Some companies started to get fancy and add additional speakers in the back. Yamaha was one of the first, and added a pair of rear speakers calling them surround effects speakers. Since there was no format that directly supported these speakers, the built in DSP sorted it all out and to create a matrixed pair of channels. They also came out with a pair of speakers in the front called the front effects channel where the speakers were placed lateral to the main speakers. Things were calm until George Lucas came out with Star Wars Episode 1.

This is what we have so far

Then there was another

With Star Wars Episode 1 came a rear center channel. This was more than just another set of speakers playing the same signal in the back, but a new discrete speaker just like we have in the front. With this came a great amount of hype and a movie with great sound effects. How much the addition of a rear center channel added to this, we’ll never know. But with this came a big race to bring that rear center channel to the home theater. This brought the 6.1 multichannel format.

Three’s company in the back

Why stop at one

Since it was difficult to buy a single rear speaker, manufacturers came up with the 7.1 format, with, you guessed it, an additional rear speaker. Still the data only coded for a single “rear surround” channel but manufacturers DSP’d their way to a 7.1 system. Then Yamaha decided to keep their front effects channel so we have a 8.1 system too, since they only use a single rear channel

Are all speakers the same?

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So we’ve discussed where various speakers need to be for multichannel audio, next we’ll talk about what type of speakers they need to be.

The best system I think is one where all the satellite speakers are identical plus a subwoofer. This makes the tonal quality of the sound the same as the sound moves around the room. Problems to this approach would be aesthetic. It is hard to place a large speaker as a center channel or as the rear channels. Center channels are often just a speaker placed on its side. This placement is so that it blends in better with décor, but if the speaker was not designed to be put on its side, then it won’t sound as good. The placement of a tweeter above a woofer is to better define the stereo image. Most of our spatial acuity when it comes to sound is left to right and front to back, based upon the placement of our ears. With up and down sounds we aren’t as good because the sounds usually reach both ears at the same time. When a speaker is on its side, the sound from the tweeter and woofer may not reach our ears synchronously. In addition, the dispersion pattern is changed and you may have interactions with the TV screen. The lesson here is to experiment with placement if your priorities are sound and not looks.

Back it up

In the beginning the rear channel was primarily an ambience channel where you want indistinct imaging and a diffuse sound field. That is why rear speakers were made as dipoles or bipoles. With dipole speakers, the sets of drivers fire out of phase, with bipole they are in-phase. Therefore, dipole speaker drivers both move in the same direction at the same time, while bipoles have one driver moving out while the other moves in. This helps in placing the speakers in different locations, some speakers allow you to switch between this two modes. With dipole speakers, there is a null zone of sound between the drivers, so these speakers are best placed to the side of the listener with the drivers firing forward and backward to create a diffuse sound field. Bipoles are designed to be placed at the back of the room, behind the listener, firing parallel to the back wall, creating a wider sound field with localization to the back.

A diffuse soundstage is great for movies, but with multichannel music on the horizon many prefer to go with standard direct radiating speakers. Given the option of a pair of dipoles versus two pairs of direct radiating speakers in the back, I would go with the two pairs since I think it gives you a bit more placement flexibility now and definitely into the future. In addition, I think that our brains are not as good as localizing sounds behind us anyways.



SIDEBAR: Polk Audio has rear speakers with a switch to toggle between dipole and bipole operation.

Boom town

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I would argue that after the center channel, the next most important speaker in creating an interactive sonic environment is the subwoofer. Once you get a chance to listen to a quality, non-boomy subwoofer you will realize what you have been missing. It is probably the only single speaker that will make you want to watch all your old DVDs again. The key to a subwoofer is displacement. The more volume of air the sub can displace the louder it will play. Bigger drivers tend to displace more volume although a smaller driver with more excursion can move more air than a larger driver with less excursion. Sealed subs tend to sound tighter than ported subs although there is a great amount of overlap between the good, bad, and ugly subs.

The stock SVS driver in my hands

Larger enclosures usually make for louder and lower playing subs as well. Manufacturers that are able to use small enclosures often use some form of active equalization to smooth out the frequency response. Interestingly, the greatest technology in speakers is in the sub. Servo controlled technology is basically a quality control system for subwoofers. A sensor on the surface of the driver senses driver motion and sends that signal back to the brains of the sub. The sub then compares the driver movement with the incoming signal. In a perfect world, they would be the same as your driver would be a perfect electronic to mechanical energy converter. If the compared signals are different, the sub can modify the incoming signal to better make the cone move like it is supposed to. The feedback from the servo sensor on the driver doesn’t even have to be that fast since the incoming signal is only 80hz or less. With servo technology the sub should in theory be more accurate, it’s similar to the concept behind crc encoding where you double check to see if the other side got what it was supposed to. That said, technology can’t overcome a poorly designed driver and many passive subs are well designed and do great without servo technology.

Subwoofers can also handle much more power because their voice coils are often beefier than other speakers. They are also designed to take a little more abuse, compared to a frail tweeter. Since the subwoofer operates in a frequency spectrum that we aren’t very sensitive to, you can get away with using a slightly lower fidelity amplifier, but one with more power. That’s why you see high-efficiency digital amplifier technology like BASH in subs but not yet in regular power amps.



SIDEBAR: The key with subs is their stroke volume, not the diameter Are we suggesting size doesn’t matter as much as motion? –ed

To the front of the class

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Finally we come to the set of speakers that started it all, the front speakers. Many people still prefer listening to music with only two speakers, partly because that’s what they are used to, and because these speakers tend to be higher quality than the rest of the surround sound setup.

The main thing with front speakers is whether you want “large” or “small” ones. In multichannel lingo, large speakers get the full signal range, while small speakers are designed to only play above 80hz with the subwoofer taking over the lower frequencies. This crossover frequency is traditionally 80hz because that is the THX standard and because sounds under 80hz tend to be non-directional. To get front speakers that play down to 80hz you really need at least a 5 inch woofer. The small, stylish satellites that you see in stores really can’t do this and should be crossed over higher at 125hz usually. As you give the subwoofer more frequencies to take care of, the chance of localizing the sub increases. Many receivers have the option of adjusting your crossover frequencies. The really good pre-amps let you adjust the crossover for each channel.

A great way to test out a front channel speaker is to just listen to it in two channel mode, preferably with some music you are very familiar with. Speakers that sound good when you have two of them should sound even better when you have six of them.

Amp me up

So we’ve taken care of the speakers that we need for multichannel audio, next is figuring out the input sources. You’ll need either a receiver with amplified outputs for all channels or a power amp for each channel. Some receivers only come with 5 speaker outputs but give you a pre-amp out to make your system into a 6.1 or 7.1 system. This is a nice option for upgrading down the road.

Intercession

We’ve got all the hardware we need now, the speakers and the amp. Next time we’ll discuss the software behind multichannel audio. Just like in the computer world, good hardware can’t make up for poor software and vice versa. Lately, there have been many new software players who want to fight for the signal to your surround sound system. We’ll discuss what you need to know about all this in Multichannel Audio part deux. Don’t worry, there won’t be a quiz in the meantime.

SIDEBAR: Learn anything new about multichannel audio here? What kind of setup do you have on your home theater and/or PC? Talk about it in our comments