What is optical audio?


The RCA jack, shown here, was one of the first widely used connections for linking audio components.
Photodisc/Thinkstock

The name might be an oxymoron, like the jazz waltz "Ugly Beauty," but there's nothing off-key about transposing electrical signals to light waves to deliver powerful, high-quality sound. It's simply another interval in the evolution of audio, as progressive as Thelonious Monk's compositions. From mono to stereo to surround sound, or from analog to digital to high-def, the beat goes on.

Once upon a time, TVs and stereos were just heavy pieces of wooden furniture. They were self-contained, and their inputs or outputs were limited to antenna hookups and speaker jacks. Today, we face a wall's worth of tuners, amplifiers, game consoles, DVRs, VCRs, DVDs, Blu-rays and moribund VHS machines, to say nothing of the cassette player you keep moving but probably haven't even bothered to hook up.

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What are all those jacks for, anyway?

Just as you wouldn't pipe Pellegrino through rusty municipal plumbing, there's no point in sending high-fi audio through low-fi jacks, wires and electronics. So, as companies dream up better audio and video standards and storage media, they also add new ways to send it crisply from your gear to your ear, which is why the backs of our entertainment systems resemble a 1960s Manhattan switchboard.

It's an apt comparison, considering that the first analog audio jack, the triple contact plug, or TRS connector, was adapted from the jack first used in telephone exchanges. The tip, ring and sleeve design, in which three insulated sections of the prong handled left channel, right channel and ground, respectively, was so successful that it is still widely used today [sources: International Textbook Company; Modern Home Theater].

The Radio Corporation of America debuted its eponymous RCA jacks in the late 1940s, but the format didn't climb to the top of the charts until the early '70s. Like the Supremes, these plugs come in groups of three -- typically red, white and yellow -- each consisting of a signal-carrying pin encircled by a ground ring [source: Modern Home Theater]. Other versions of RCA jacks carry composite and component video, as well as digital audio.

Component and composite cables carry video signals but no audio. Composite cables, usually designated by a yellow RCA jack, pack all of the video info into one signal, whereas component ones split it into three channels across three plugs [source: Maxim Integrated Products].

Both require a separate cable to handle audio, which brings us back to RCA jacks and other copper-wire solutions -- and an alternative, optical audio.

Copper-based wires contend with two problems to varying degrees, depending on their shielding and quality: external electromagnetic noise interfering with the signal and resistance in the wires degrading the signal over distance.

Optical audio is immune to the first and can be free of the second if the cable quality is high enough, but it also has its own share of problems.

The Laser Light Show, or Tangled Up in Blu-ray

As anyone who has lived with a bad landline connection or a weak signal from the cable box can tell you, signal degradation can really ruin your day.

Copper, which still provides the core of most wiring, can suffer significant signal degradation over distance. Coaxial cable -- used to transmit television, telephone and computer network signals -- comes packed with shielding to prevent signal bleed-off, but losses due to electrical resistance still weaken the signal over distance. Shielded audio cables include some protection from outside noise, but suffer the same resistance issues [source: National Instruments].

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Luckily, we have fiber optics.

Fiber-optic cables offer the kind of low signal loss and high data rates that telephone and Internet users demand by transmitting information as light through thin plastic or glass wires. These cables have total internal reflectance, meaning that light bounces along inside them without escaping. Glass fibers can transmit such "lossless" signals over great distances without needing a power boost [sources: Encyclopaedia Britannica; Wood].

By the same token, optical audio delivers higher-quality sound than older copper-wire connectors by converting electrical signals to light and piping them through optical fiber.

The age of optical audio dawned in the 1980s, when Toshiba created Toslink, the first optical audio cable. The Japanese electronics company had developed its own compact disc player and was looking for a way to output the improved digital audio quality to speakers and headphones. Appropriately, Toshiba chose an optical solution for CDs, an optical storage medium [sources: Modern Home Theater; Toshiba].

In an optical audio setup, the digital electrical signal from the source -- say, a DVD player -- is converted into light by a device called a transmission module. In Toslink, this module consists of an LED, or light-emitting diode, and a drive circuit, usually sent through plastic fiber, whereas ST Fiber Optic uses glass fiber and a red laser light at a 680-nanometer-wavelength [sources: Modern Home Theater; Toshiba]. The signal then speeds along the optical cable to the destination device, usually a television or audio receiver, where a light reception module converts it back into a digital electrical signal. From there, the device transmits it to your speakers or headphones.

Because it sends only sound, an optical audio cable is usually used with a video-only cable, such as DVI (Digital Visual Interface) or S-video.

So, what's the catch? Well, optical cables tend to be somewhat brittle, and the plastic ones are not as lossless as their glass counterparts. The reconversion of light into an electrical signal has been known to introduce errors. More to the point, however, some argue that HDMI (see sidebar) has rendered the format moot [source: Johnson].

Who is right? As with most arguments among audiophiles, the truth is in the ear of the listener.

Author's Note

After years of feeling lost in the formats, I tackled this article with a combination of excitement and trepidation.

On the one hand, I hoped that, at long last, I might decode the alphabet soup, untangle the mass of cables and reel in the rhetoric to discover just what all those jacks were for. On the other hand, my previous forays into the field hinted that my hopes for clarity might end up squelched.

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It's not that I'm a technophobe -- quite the opposite. I can upgrade my own computer (although, admittedly, that used to be a lot easier) and, in college, I helped my mom program her VCR so many times that I could talk her through it blindly over the phone, like a tower jockey walking a non-pilot through landing a 747.

It's just that, somewhere along the line, a few cables became many, the color codes ceased to mean what I thought they meant and the number of prongs stopped corresponding to the number of jacks.

More than that, though, it was that no one could give me a straight answer as to which solution was the best. There's a reason for that, and it's the chief stumbling block to writing an article about consumer electronics: The world of audio is steeped more in lore than in hard facts; there's as much snake oil in those wires as sound.

I did my best to steer clear of the hype and to stick to the facts. As far as they go, they say that the choice depends on your equipment and how you intend to use it -- oh, and whether you've already lost so much high and low range to loud music and age that fancy cables no longer matter.

Contentious, normative questions like "best" and "worst" are best left to the audiophile, if for no other reason than there is simply no clear-cut answer. Don't believe me? Try walking into a guitar shop and asking for the best amp.

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Sources

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