Real World Digital Audio

been added while the signal was traveling through the cable will cancel out,

because the second inversion of the cold signal will reverse the polarity of

the noise on that cable. Positive noise plus negative noise equals no noise.

Most consumer devices are unbalanced, carrying only the signal and a ground,

as on stereo minijacks and RCA plugs. Because they’re used in fairly simple

setups with little wiring, manufacturers can save money by not worrying

about creating balanced connections. But when wiring covers longer dis

tances,

connections become more prone to noise, so pros prefer balanced connectors

like XLR and TRS. Table 3.2 provides examples of balanced and unbalanced

connections and which signals they carry.

Table 3.2 Balanced/Unbalanced at a Glance

Balanced

Mono TRS

1

⁄

4

˝,

1

⁄

8

˝ signal, inverted signal, ground

XLR Analog, Digital 3 pins: signal, inverted signal, ground

Unbalanced

Mono TS

1

⁄

4

˝,

1

⁄

8

˝ signal, ground

Stereo TRS

1

⁄

4

˝,

1

⁄

8

˝ left signal, right signal, ground

RCA Analog, Digital left, right or in, out

Digital Connections

Several protocols are available for transmitting digital audio. Each of these

formats is a specification for transmitting information, and each is different

and essentially incompatible. For instance, you can’t directly plug an ADAT

Optical output into a S/PDIF input. You’ll need to know which protocols

devices are using to ensure compatibility. Unfortunately, it’s not as simple

as looking at a jack or cable, because it’s possible to transmit different for-

mats over the same physical connector.

If you’re using consumer and semi-pro equipment, the primary digital for-

mats are S/PDIF and ADAT Optical. Most computer audio interfaces use

at least one or the other; some use both. S/PDIF is used for stereo digital

audio and compressed multichannel audio such as DTS and Dolby Digital

Surround, whereas ADAT Optical is used for 8-channel uncompressed

digital audio. Since these formats appear on everything from consumer DVD

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ETTING

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97

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players to keyboards to digital mixers and recorders, typical users will be able

to connect all their digital equipment using these two formats.

Manufacturers do sometimes get lazy about labeling inputs and outputs,

but it’s usually possible to guess which connections are which. S/PDIF, for

instance, is so ubiquitous that a connector using it may simply be labeled

“optical” or “digital.”

Pros tend to use a slightly different mix of digital formats than consumers and

semi-pros. Digidesign Pro Tools|HD and other higher-end equipment use

AES/EBU. Also known as AES3, it’s transmitted over balanced XLR cables,

which perform better over longer distances and have more rugged connectors,

making them better suited to demanding applications. Although AES3 is sup-

posed to be the industry standard, Alesis’s ADAT Optical and Tascam’s TDIF

format are also popular for multichannel communication (Table 3.3, page 99).

W

Wired for [No] Sound

Are you getting hum and other noise in your system? Isolate the problem by unplugging all components. Next, starting

with the amplifier, replug each component one by one to see if you can locate the source.Then try these strategies for

avoiding hum and noise in your systems:

•

Use balanced cables whenever possible: If you are using unbalanced connections, try to keep them short (ideally

six feet or less).

•

Use a single power source for audio equipment: Plug audio equipment into a single power source, such as a

power strip connected to one wall outlet, thus ensuring a single path to ground.

•

Don’t coil wires: Audio amateurs often coil microphone cables around the stand. Bad idea: you’ve just created a very

effective magnetic field, producing additional hum! Keep wires straight; instead of coiling, bind them with gaffer’s

tape or cable ties.

•

Use trunks for organization: If you have a lot of cables, bundle them together using cable coils and other wrapping

systems, taking care not to cross cables. Bundle similar cables together into snakes.

•

Keep power and audio cables separate: Power cords generate interference that can be picked up, even by shielded

audio cables. Keep them away from each other if possible, and if not, cross the cables at a 90 degree angle.

•

Don’t wrap power and audio cables together: Bind them separately.

•

Invest in shielding: Shielded audio cords and speaker wire have a stronger immunity to interference. Some power

strips even include RFI/EMI filtering for additional protection, and many speakers intended for computer use include

special shielding.

•

Isolate sources of interference: CRT computer monitors, cell phones, power blocks, and many other devices produce

interference. Some are so loud that they’re easy enough to locate by ear. When in doubt, put some distance between

them and your audio setup and speakers.