Summary: Part 2 of our multichannel audio guide resumes with a look at the software involved. Alexis examines the standards and companies involved with audio. Learn how Dolby, DTS and THX have shaped the world of audiophiles!
This is simply your good old stereo sound. Two discrete channels feeding two speakers. Almost everything is in stereo, including most television programs, FM radio, some AM radio, and most of the better audio and video clips on the internet. We really take stereo for granted now, but it is a marked improvement over mono sound, that is a single channel signal.
kinda boring, huh?
Physically, this would be a pair of speakers and a subwoofer. In most cases, you can use a stereo, two-channel signal to drive this setup. When doing this, the receiver or the speaker’s internal crossovers determine what sound goes to the subwoofer. This separation of the channels was not done by the mixing engineer. The only way you can get a discrete subwoofer signal with this setup is with Multichannel Super Audio CD, DTS audio, DVD audio, or Dolby Digital signals downsampled to 2.1 audio. There are a few sources that are written specifically for 2.1 audio, but you will need to have the decoding hardware necessary for at least 5.1 audio to use 2.1 audio. Basically, if you have the equipment to support 2.1 audio with a discrete subwoofer channel, you have the equipment needed for a 5.1 system.
Gotta have my subs
The first taste of surround sound at home came with Dolby Surround Sound. This took a two channel stereo signal and created sound that went to four speakers. The pair of rear speakers, however, received a mono signal. So basically, the decoder was taking a two channel source and creating a third channel for the rear by analyzing the phase differences between the stereo signals. With this first generation surround sound, the rear speakers were limited to a frequency response of about 100hz to 7 khz, This limited frequency range helped to limit localization of the rear channels to prevent it from sounding too artificial. Many years ago, quadraphonic systems powered by LP’s and 8-tack machines were available, but the high buy-in cost of this technology did not allow it to catch on with the general population. These systems are now considered collectibles. Subwoofers were often used with smaller satellite speakers, but the signal was derived from the front channels.
Starting to get a little interesting…
SIDEBAR: Cochlear implants can restore hearing to the deaf because specific regions of the cochlea respond to specific frequencies.
Soon after Dolby Surround came out, it was replaced by Dolby Pro-Logic surround. This added probably the most important channel, the center channel. Again, the source for Dolby Pro-Logic was still a two channel stereo source, but engineers knowing how the decoders would interpret various phase patterns in the stereo signal were able to record with this in mind. This created a very successful surround sound system, with predictable distribution of sound, while using a two channel source. Limitations to this system included surround channels that were limited in frequency response in addition to the technical limitations of this narrowband format. Subwoofers were added to many Dolby Pro-Logic systems, but it was the decoder that distributed the sound to the subwoofer.
Around this time, THX home was first introduced which added a few new tweaks to Pro-Logic audio. THX added an adaptive correlation algorithm that created discrete rear channels. They use information from the extrapolated surround channel and the front channels to create a stereo rear surround channel. Cinema re-equalization was also added to roll off the high frequencies in the front channels. The reason for this was that in the movie theaters, the speakers need to fire through the perforated movie screen, tracks are therefore mixed with additional treble to compensate for the loss going through the screen. When these movies were brought directly to laserdisc, they used the original sound mixes intended for the theaters. In your home, since the speakers are for the most part unobstructed, THX removes this added equalization. With the advent of DVD, it is not clear how the tracks were mixed, especially with non THX discs. With THX discs, it is assumed that they benefit from going through a THX decoder. Recently, the specifics of what THX does has become less clear, THX simply says that they make the experience better. With many DVDs this is true, having the option to use THX or not is a great feature to have.
In addition, THX advocated the use of multiple subwoofers to best replicate the theater experience. Their standards called for subwoofers to play all frequencies below 80hz. THX also promoted controlled dispersion of the speakers. Front and center channel speakers were designed to have a limited vertical dispersion with wide horizontal dispersion to limit interactions from sound bouncing of the floors and ceilings. Rear speakers were dipole to create a wider, more diffuse rear sound stage. THX amplifiers were sending 5.1 signals to the speakers, discrete channels for all speakers including the subwoofer all from an analog stereo signal.
SIDEBAR: NASA research uses a pair of VMPS larger subwoofers for their research. Hmm, why can’t I be doing that research here at Firingsquad…
I would call 5.1 audio as being the tidemark in the evolution of multichannel audio. It was during this time that the source signal became digital. The source signal now contained separate channels for all the speakers including the subwoofer. The frequency response of the rear channels were upgraded to full frequency without any limitations. Subwoofers received their own LFE or Low Frequency Effects channel that was designed to help you feel the music, literally. Pioneering this signal source was Dolby Digital.
Dolby Digital is commonplace now with Xbox and most DVDs supporting it. In the beginning, it was only available on Laserdiscs and was often called AC-3 audio. Some argue that AC-3 Dolby Digital is superior to DVD Dolby Digital because the Laserdisc used no compression on the front channels. Dolby Digital is a lossy compressed digital audio format. Some purists would argue that any compression is detrimental to the fidelity of the signal. What I think you should be interested in is the perceptibility of this fidelity. For the most part, the compression of Dolby Digital is good enough.
Enter the DTS
Not everyone agreed about that last point and DTS came out with their own 5.1 signal carrier. The setup is the same as Dolby Digital. The major differences are that DTS uses less aggressive compression and can be carried on a standard PCM data bus. This last point enabled DTS to be used on any machine with an optical PCM output. Old laserdisc players could support DTS, DTS could be recorded on standard redbook CD audio format. On paper, DTS looks more practical and technically superior with less compression, yet it is well overshadowed by Dolby in terms of market penetration. One of the main reasons for this is that DTS in being less compressed, takes more storage space, limiting the number of extras that can be crammed on a DVD. Some would argue that DTS hasn’t taken over because most purchasers of DVDs can’t hear a difference.
More recently, standards have come out to take two channel sources into 5.1 channel outputs. Dolby Pro-Logic II and DTS Neo-6 are the most popular of these algorithms, with individual manufactures developing their own such as Circle Surround or Logic 7. These additions really give new life to your stereo signal.
Super Audio CD was Sony and Philips’ love child, and has the backing of major music labels. They also make hybrid discs that are readable by standard CD players, but at the standard CD resolution. Regular CD’s sample sound at a rate of 44.1khz at 16bits of resolution, Super Audio CD does it at 2.8224Mhz at 1 bit, nearly an increase of an order of magnitude. DVD Audio samples at 192khz at 24bits of resolution. Both these new format extend the recording frequency response to over 100khz compared to 20khz for standard CDs. While bats and dogs are sensitive to sounds above 20khz, people are not. The thought is, however, that interactions between these ultrasonic sounds can be audible. Whatever the explanation, SACD and DVD Audio do sound better than standard CDs, not just for their discrete multichannel format, but also for their improvements in resolution and imaging.
This began with George Lucas and Star Wars Episode I. The thinking was that since a center channel in the front was such a great advance, why not add a center channel in the rear. This was termed Dolby Surround EX. Creating the center rear channel wasn’t actually that difficult, the early variants only used a standard 5.1 signal, but ran the rear channels through a Dolby Pro-Logic decoder, remember those? So, this center surround speaker started out as a derived channel, not a discrete one. At this same time, DTS came out with DTS-ES Discrete and DTS-ES Matrix. Basically, DTS-ES Matrix was using a 5.1 DTS source and extrapolating the rear center using an algorithm similar to Dolby Pro-Logic. Always the underdog, DTS went above and beyond and created DTS-ES Discrete which encodes a discrete center rear channel, here no extrapolation or interpretation of what the sound engineer intended is necessary. The one caveat to DTS-ES Discrete is that it is only available on a limited number of DVDs and Music CDs
Doesn’t that rear center speaker look lonely?
Soon after 6.1 audio was release, 7.1 became popular. I think it was mainly a practical point as most speakers are sold in pairs and having four speakers in the back would make use of newly purchased speaker sets. This is primarily a hardware issue as the sources are only 6.1 at best. The center surround speaker from 6.1, has become two speakers. Good receivers will do more than just make that a mono signal, but will use the effects from the left and right channels to help the center rear speakers mesh better with the system. 7.1 speaker systems also take advantage of more ideal speaker placement if you have the space, with surround speakers both to your sides and rear.
Looks better than my local theater
Not to be left out of advances in multichannel audio, THX released THX-EX which is THX technology for 6.1 and 7.1 systems.
There are reports that deep in the Lucas ranch, there are prototypes of 10.2 systems. The extra channels include dual subwoofers, front effects channels and a height channel. I say, the more speakers the merrier. No reports yet on how close to production or mainstream this technology is, but don’t be surprised if it isn’t introduced with one of the new Star Wars films.
To summarize, if you build the speaker systems, they’ll make the technology to drive it. Next you have to ask yourself how many speakers do I actually need for my two ears? I would recommend at least a 5.1 DTS and Dolby digital system now if you tend to watch movies in small groups. With more people, the added expansion in the rear soundfield with 6.1 and 7.1 sources is beneficial. THX is nice to have and generally assures better than average performance, although many manufacturers simply do not seek certification for their systems. Unfortunately, the criteria for THX are not explicitly known to the public and makes it difficult to compare non-THX certified products with THX products. So with the information from these two multichannel audio articles, you’ll be prepared to separate the hype from the facts the next time you audition audio equipment.
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