Understanding Channels
Hi Wes:
I was reading through your website on surround sound
and I have a question about receivers. I read that some have two channels x 100 watts,
four channels x 85 watts, five channels x 110 watts, and so on. I don't really understand
the concept of having channels clearly. Does it mean that two channels have two speaker
outputs rated at 100 watts each -- and so on?
Please let me know any additional information that
might be useful to me.
Thank you for your time and help.
castillo3
I've gotten more than a few letters like this lately, so
I'm starting to think that the audio/video industry (and people like me) have been
less than successful in explaining this brave new world of multichannel music and video.
The fact is, the whole subject is pretty confusing especially if you're not one of
us mushroom people who spend our lives in darkened rooms. So over the next few months I'll
be writing a series of articles covering the basics of home theater, starting this time
out with an exploration of -- you guessed it -- channels, stereo, and multichannel
playback. Future topics will include explanations of the different surround options, some
quick'n'easy tweaks, and a look at how the new high-resolution audio formats shape up for
music listening in a surround environment.
And, of course, if I've skipped a topic you're curious
about, just email me at wes@onhometheater.com
and I'll probably deal with it as well.
Okay, this week's topic: What are channels and why do I
need 'em?
Back at the dawn of audio, there was no such thing as
channels: A single microphone converted sound into electrical energy and that energy was
used to etch grooves into a wax cylinder or onto a lacquer coating on a disc. When it came
to playback, the process was reversed: A needle "read" the groove and the
resultant mechanical energy was converted to electrical energy, which made a loudspeaker
reproduce the original signal.
Mono, therefore, was a single-channel playback system --
only nobody thought of it that way because the concept of using more than one channel
hadn't been created. That took time. And it didn't happen because somebody thought
two channels would be better than one -- two-channel audio came about as the result of the
search for a way to reproduce sound in a more realistic fashion.
We call two-channel sound stereophonic sound. Stereophonic
does not mean two channel, it means solid sound, and it grew out of
consumers' dissatisfaction with the "flat" sound of monophonic recordings --
they had no depth or space, just the ghostly image of the original sound.
There were some other formats proposed, including one that
used three speakers in front of the listener, but two channels won out, in no small part
because it was cheaper and simpler to implement. Plus, it could be very good.
People kept searching for better and better re-creations of
the original sound anyway. Stereo, as good as it was, still only reproduced sounds coming
from in front of the listener. Music, when reproduced in the concert hall, also
comes primarily from in front of the listener. However, psychoacoustical research revealed
that the minute amounts of sound reflecting off the side and rear walls of concert venues
played an important part in the listener's perception of the quality of the sound, so
audio companies and record manufacturers began looking for ways to simulate that blend of
direct and reflected sound.
Surround evolves
In the early 1970s, two competing methods of reproducing
multichannel (more than two channels) sound emerged -- both were referred to as quadraphonic,
since they both used four speakers reproducing four channels. Neither approach caught the
public's fancy. The existence of two standards confused consumers, many of the
recordings sounded horrid, and the LP, which was the prevailing music-carrying medium of
the day, wasn't really suited for carrying all that extra information.
But over in the world of film, surround sound was starting
to come into its own. Actually, movies had been experimenting with multichannel sound for
years. Fantasia, released in 1941, had four channels of audio information -- but
that system was awkward to employ, requiring a second projector just to play the
soundtrack. In the late 1950s movie studios experimented with a gaggle of surround formats
(Cinerama and Cinemascope being the most widely known). Although these different
techniques all involved more than two channels, they were known as stereophonic sound
(or theater stereo sometimes), harking back to the use of stereophonic to
mean solid sound.
The real purpose of these systems was to make the sound
image as huge as the projected image, and most of the studios' attention was on getting
the dialogue right. There might be as many as five speakers spaced from one side of the
screen to the other and the idea was to get the voices to come from the same side of the
screen as the actors who were speaking. There were "surround channels" as well,
but the purpose of the speakers along the sides of the theater and at the rear was to
spread the sound throughout the room -- while there could be many speakers located
throughout the hall, there might only be one or two channels of separate information
feeding all of them. Big was the point, not real.
That changed with Star Wars, which was one of the
first movies to utilize Dolby Stereo. That system added effective use of the side plus
rear channels to create immersive environmental effects to heighten the realism of the
theatrical experience. Then, in 1982, Dolby introduced Dolby Surround, a version of the
movie-theater surround scheme that could be used on videocassette in the home.
The modern age
Dolby Surround wasn't perfect -- it didn't really allow for
four completely independent audio channels but rather piggybacked the surround information
onto the primary channels. In 1994, Dolby released a new surround standard designated AC-3
(now known as Dolby Digital). This was called a 5.1-channel system because it incorporated
five completely separate channels of audio information, along with a single channel's
worth of deep-bass information (to be sent to a subwoofer).
This whole business of there being 5.1 channels is
very confusing to some people -- and with good reason, since it has also been referred to
as a six-channel system! However, the .1 designation has become the standard
way to refer to the subwoofer channel -- possibly because the deep-bass information is
shared by all of the other channels.
Dolby Digital (and DTS, which came later) uses five discrete
channels of sound to enhance the movie experience. That is to say, a DVD will have five
different channels of information digitally stored upon it (plus that sixth for the bass)
and, when the disc is played, the surround-sound processor (or the decoder
contained within a surround-sound receiver) has to decode all of those channels and send
them to amplifiers, which, in turn, drive the loudspeakers.
Now this is where that whole question of channels of
amplification comes in. Each of those channels requires a separate amplifier. If you own
an A/V receiver, it combines the different amplifiers (and a surround-sound
processor and a preamplifier) into a single chassis. Multichannel amplifiers do the
same thing: they put many separate channels of amplification into a single box.
The predominant forms of surround-sound encoding (Dolby
Digital and DTS) these days support 5.1 channels, which means you need five amplifiers and
an output carrying the signal to the subwoofer. That works out to three front channels
(front left, center [dialogue], and front right) and two surround channels.
Notice I said "surround," not rear -- while
frequently called the "rear" channels, those speakers belong on the side walls
behind the listener. Both Dolby and DTS now offer multichannel formats that do
incorporate rear-channel information, however. Adhering to the famous "same but
different" principle, they offer the same thing in slightly different ways.
Dolby Digital Surround EX adds two more channels (carrying
identical information) to the rear wall of the room, whereas DTS-ES adds a single speaker
in the middle of the rear wall. The differences between the two formats are subtle, if
indeed audible. Either one, however, requires additional channels of amplification (one
for DTS; two for Dolby EX).
So answer the question already!
Okay, you've got all these different surround systems --
5.1, 6.1, 7.1 -- where on earth did three-, four-, and five-channel amplifiers come from?
After all, shouldn't there just be five-, six-, and seven-channel amps out there?
Ah, but what about all the people who already have stereo
amplifiers? When 5.1 was first introduced, manufacturers rushed to make three-channel
amplifiers, figuring that consumers would simply buy a new amp to augment the equipment
they already owned. Some did, although many others took the opportunity to buy completely
new systems. Three-channel amps weren't a raging success, but they did fill a niche, and
companies offered -- and continue to offer -- variations on the theme. Some companies that
cater to the home-installation market sell modular amp racks that can take anywhere from
three to seven amplifier modules.
What you need in the way of amplifier channels depends on
how you configure your system -- for example, whether or not you use a receiver,
separates, or something in between. But a rule of thumb is that you will need an amplifier
channel for each loudspeaker you employ. Seven speakers? That's seven channels and you
need seven amplifiers (although they can be in the same box).
When reading amplifier specifications, you will see a form
of shorthand, such as 7 x 85 or 5 x 105. That does indeed mean seven channels, each
delivering 85Wpc, or five channels providing 105Wpc.
I hope that sheds some light on the channel question.
Next time we'll take a look at what a surround-sound
processor does.
...Wes Phillips
wes@onhometheater.com
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