Mono, Stereo, and
Multichannel Connections
In addition to choosing a connector type, you must figure out how many
channels of audio you need to transmit.
Mono signals involve only one channel. Therefore, using an analog connec-
tion, you’ll need only one electrical connection to carry audio signal.
If you’re transmitting a stereo analog signal, you’ll need electrical connections
for two channels: left and right. This can be done either with two mono
cables or with a stereo cable. A stereo cable can carry a mono signal, but a
mono cable can only transmit half of a stereo signal. Note, however, that
a single-connector cable can carry a stereo digital signal.
You can also transmit surround audio and other multichannel formats
either via multiple mono and stereo connectors or through specialized digi-
tal formats that transmit multiple channels using one cable.
Voltage Levels
As discussed in Chapter 1, “Understanding Digital Sound,” analog signal
translates sound amplitude into an analogous voltage level. If the voltage level
of this incoming signal is too weak, a signal will sound faint. If it’s too strong,
it will sound distorted. Preamplifiers, whether they are external devices or part
of
your interface (for example, a dedicated jack), increase the voltage level. Exter-
nal
preamps are often connected between devices like microphones, guitars,
and turntables to a mixer or interface input. Integrated preamplifiers are either
in the form of jacks or switchable preamplifiers. By setting a button or switch
to “pro” or “consumer,” “line” or “mic,” you’re activating or deactivating
preamplifier circuitry to adjust the voltage of the incoming signal. Whichever
level you choose, the signal for your devices must be set at the same level.
Line level and consumer vs. pro: “Line level” can refer to either con-
sumer or professional level. Consumer equipment transmits audio signal at
–10 dBV (“consumer line level” or sometimes “instrument level”), whereas
professional equipment uses a hotter signal of +4 dBu (“professional line
level”). Many interfaces and mixers have a switch between these two levels
so you can match the signal levels of your devices.
Microphone level: Microphone level uses a much lower signal level, ranging
from about –40 dB to –60 dB; this signal level must be amplified to –10 dB
or +4 dB when being connected to other gear. Many interfaces and mixers
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have a simple mic/line switch that activates an internal preamplifier. With
the switch in the mic position, the preamp is on; when it’s switched to the
line position, the preamp is off. Therefore, if you’re using a separate mic
amp in the signal chain prior to the input, you should choose line-level
input on your interface to turn off the built-in preamp.
Guitar level: Guitar level, like microphone level, must also be amplified,
though it has a larger signal range of –60 db to 0 db. Inputs may also have a
switch for this input, as in Figure 3.7.
Non-optical digital connections use voltage too, but you don’t have to worry
about the signal level unless you’re building your own digital audio circuitry!
Impedance
Impedance, which is measured in Ohms, is the resistance of direct current to
alternating current. Even if you don’t care to understand how it works, you’ll
need to make sure the impedance of an audio signal you want to use is
appropriate for the input into which you’re plugging it. According to author
Craig Anderton in Home Recording for Musicians (Amsco), “For minimum
signal loss in an audio system, in most cases an input impedance should be
approximately ten times greater than the output impedance feeding it.”
Using the correct impedance is especially relevant when you’re connecting
an output of an electric guitar. If the input isn’t correct, the full frequency
range of the instrument will be compromised, making for a dull sound.
The solution is to use an input specifically designed for the guitar. Some
computer audio interfaces are designed for this purpose, as are some mixer
inputs. Most often, a guitarist will plug into a guitar direct box, which ampli-
fies the signal and solves the impedance problem.
Figure 3.7 Typical inputs,
shown here on an M-Audio
Fast Track USB interface.The
input level button switches
between guitar-level and
line-level signal. (Photo
courtesy M-Audio)
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