| Video Noise May 2003
Connections and Interfaces: Part Two
In March
I covered video connections from lowly coax cable to the latest digital video. This month
we switch to the ins and outs of audio connections.
First, some history…
Back in the dawn of television, there were no external
audio connections. The broadcast TV signal received by rabbit-ear antennas on top of the
TV or from an outdoor antenna carried both the video and audio in a single signal on a
single cable connection to the TV. There was no stereo TV. You had one speaker in the TV
unless you were a member of a TV-crazy family indulging in a large console with a speaker
on each side of the screen -- still mono sound mind you, but two speakers.
The arrival of VCRs in the late 1970s, and a little later,
the introduction of video games, started the move to separate video and audio inputs on
newer TVs. Stereo sound was coming along at the same time, so the appearance of a yellow
RCA video jack along with red and white RCA jacks for stereo inputs reflected the
beginning of a revolution in how the world used video displays in the home. Before long
people needed two and three audio/video inputs to keep up with a growing array of
"boxes" that they were connecting to their TVs. During the early days, audio
remained analog and stereo, but in the early 1980s the home-theater revolution really
began to take shape.
TVs became multi-purpose video displays with two, three, or
even four sets of audio/video inputs. The traditional stereo receiver first became
"home-theater aware" with new inputs. Within a few years, surround-sound
decoding and more amplifiers for the new surround-sound channels began appearing, and
eventually became standard equipment in almost every receiver. Today you can still buy a
stereo receiver, but they are often hard to find on the shelf at stores. The home-theater
or A/V receiver has all but replaced the stereo receiver.
Audio in the early laserdisc era
In the early 1980s, laserdisc appeared and offered the
promise of the highest-quality video and audio available in a home product for movie
enthusiasts. This drove the embryonic home-theater market. Initial releases were stereo,
but it wasn’t too long before laserdiscs were encoded with Dolby Surround: an analog
matrix system that encoded four channels of audio in the analog two-channel stereo
soundtracks. An early laserdisc-based home-theater system would have stereo analog
interconnects from the laserdisc player to a home-theater receiver or surround processor.
Dolby Pro Logic followed Dolby Surround and improved the
front-to-back presentation considerably. Pro Logic was still analog, but it was quite a
bit better with a lower noise floor than the original Dolby Surround soundtracks. You
could actually hear less "hiss" in each channel when using Pro Logic versus
Dolby Surround. Pro Logic soundtracks, by Dolby’s specification, were not supposed to
contain frequencies lower than 100Hz in the center or surround channels. This held true
for a while, but blockbuster movies with better soundtracks began breaking the rule and
you could encounter soundtracks that went deeper than 100Hz in the center and surround
channels. This started making it advantageous to have larger center and surround speakers
to reproduce the newfound bass. It also meant there was some opportunity for better audio
cables for those channels. But, audio connections from the laserdisc player to the
surround processor or home-theater receiver were still stereo analog.
The first digital surround sound for home theater
In the late 1980s when the video side of laserdiscs was
getting about as good as it would ever get, digital sound was added as a new laserdisc
feature. This gave laserdisc manufacturers and studios the excuse to re-issue older titles
as remasters with improved video and Dolby Digital 5.1 audio. This was the first time
subwoofers were singled out for their very own dedicated audio signal in a consumer
format.
Laserdisc’s digital audio was an odd duck. It used
coaxial cable connections with screw-on F-connectors. A modulated RF (radio frequency)
signal carried the digital audio from the laserdisc player to the surround decoder or
home-theater receiver. All 5.1 digital channels traveled on this single coaxial cable. To
make use of the new digital soundtrack with discrete 5.1 sound, you had to get a new
laserdisc player with an RF digital output and a new receiver or surround processor that
could decode the RF digital bitstream. Modifiers promised to protect your investment by
adding an RF modulator to your existing laserdisc player and an RF demodulator to your
existing home-theater receiver or processor. But the mods could cost almost as much as new
equipment.
Today, some top-of-the-line home-theater receivers and
surround processors include an RF digital audio input for digital-capable laserdisc
players, but this connection has disappeared from most normal receivers.
The DVD era
DVD, with its all-digital, all-discrete (some exceptions of
course) 5.1 sound is what really drove home theater to wide acceptance. With a single
digital audio connection to a home-theater receiver, movie enthusiasts could get 5.1
channels of clean, clear, discrete sound. Home-theater receivers appeared in huge numbers
and the race to even better surround sound was on.
DVD players popularized the digital coax connection and the
TosLink digital optical connection. Both interfaces deliver 5.1 channels of sound in a
single digital bitstream. TosLink is an unimpressive digital optical interface that was
designed to be "cheap" from the outset; quality was not a consideration as long
as you got sound. The result is somewhat comparable to FM-radio-quality sound -- not
exactly bad, but it could be better. There is another optical interface found in limited
numbers of high-end digital audio components that is usually referred to as "AT&T
glass." This optical interface is a lot more expensive to add to a component than
TosLink, but it can produce remarkably good digital sound. But AT&T glass interfaces
never made it into most consumer DVD players, home-theater receivers, or surround
processors. It was just too expensive to use.
If you have a choice of digital interfaces on your DVD
player, go with a well-selected digital coax cable. You will get better sound than you can
with a TosLink connection -- in most cases. Digital coax cables can vary widely in cost.
You must use an actual "designed for digital" cable and not an audio cable to
get good sound using the digital audio connection. Video cables (yellow color code) can
make good digital audio cables because they carry signals having similar frequencies. But
some of the "free" yellow video cables that come with equipment are poorly
designed or constructed -- so be wary if you are trying to use those freebies. If you
upgraded your DVD video connection to component video and you have an old composite video
cable sitting around doing nothing, it might make a perfectly good digital coax cable --
it’s worth a try.
One of the most frequently asked questions I encounter is:
"What’s a good digital coaxial cable to use with my DVD player and home-theater
receiver?" My perennial answer has been the MSB Technologies digital coax cable
(about $35 retail). You won’t find it in many stores, so you may have to resort to
mail order. But the results are worth the trouble. This cable warms both movie soundtracks
and CD sound, giving them a rich, golden sound you rarely hear from systems built around
home-theater receivers. This is not a good cable choice for systems using more
sophisticated electronics than typical home-theater receivers, though. The added warmth
and body the cable brings with it are going to be highlighted in the more expensive
equipment.
Fixing TosLink
Quite a few years ago, various black boxes that went
between CD players and external DACs (digital-to-analog converters) became popular because
they noticeably improved the sound of CDs. These boxes claimed to remove
"jitter" errors within the digital bitstream. Jitter is a form of digital
distortion. With the jitter removed from the bitstream, you get more ideal (lower
distortion) decoding of the digital signal. These "jitterbugs," as they came to
be called, would work fine for CD playback, but were incompatible with the slightly higher
48kHz frequency of DVD movie soundtracks combined with the extra Dolby Digital or DTS
flags and codes. A new generation of jitterbugs has been designed to work with DVD players
and can improve the digital audio signal enough to improve the sound of CDs and DVD
movies. In fact, most of these devices can accept a TosLink optical input and improve it
so much that it sounds as good as a good digital coax interface. Of course, to maintain
that improvement you have to use a good coaxial digital cable from the jitterbug to the
surround processor or receiver.
Getting into trouble with DVD-A and SACD
With the introduction of DVD-A and SACD, pirating hysteria
has caused the content owners to restrict the connection between the player and receiver
or processor to six analog cables or a proprietary digital interface that is incompatible
with computer or home CD or DVD recorder equipment. That means if you get an itch for a
new "universal" disc player that will play your CDs, DVD movies, DVD-Audio
discs, and multichannel SACDs, you’ll need at least two audio connections to get the
best possible sound. You’ll want a digital audio connection for CD and DVD movies,
and you’ll want six analog cables for DVD-Audio and SACD playback. Six channels of
good analog audio cables can get mighty pricey, especially if you need two- or three-meter
lengths. Only a few top-of-the-line players offer the digital connection option for DVD-A
and SACD. The ones that do offer it require you to match one of the company’s
top-of-the-line (i.e., expensive) DVD players with one of their top-of-the-line
receivers. Pioneer and Denon both offer this option, but you can’t mix brands; both
of the digital interfaces are unique to each manufacturer. It would be ideal to keep all
digital audio on that single digital cable, but the content providers have decided to make
your life more complicated (and expensive) by limiting your options for DVD-A and SACD
connections.
DVD-A and SACD have another potential "oops"
factor that might prevent you from listening to your new DVD-As or multichannel SACDs the
first night. Some products compatible with multichannel analog connections don’t
provide six RCA jacks as they probably should. They provide what is called a DB-25
connector. This is like the cable you use to connect some printers to some computers. It
has 25 pins and two small screws to hold it in place so it doesn’t fall out. If you
get a new DVD-A or SACD player you may be surprised to find that your new player has six
RCA jacks while the receiver has a DB-25 connector. All is not lost: You can get special
cables with DB-25 on one end and six RCA connectors on the other end.
Future audio
There is no doubt that audio’s ultimate future is
wireless. It will take a long time to get there, but it will happen sooner or later. An
intermediate stop is likely to be a single bi-directional digital bus that allows
components to communicate with each other while audio signals are being transmitted. The
surround processor will know what size all the speakers are without your having to guess
whether they are large or small and how many channels are active. Setting up your audio
system will be easier than ever.
If you were hoping you could get a home-theater system and
use it for the next 20 years or more, it’s not very likely. The digital revolution is
far from over. The outcome will affect every aspect of our home-theater experience and
periodic obsolescence will be unavoidable as things move forward.
...Doug Blackburn
db@hometheatersound.com |
