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
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
˝ signal, inverted signal, ground
XLR Analog, Digital 3 pins: signal, inverted signal, ground
˝ signal, ground
˝ left signal, right signal, ground
RCA Analog, Digital left, right or in, out
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
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).
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
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.