Analog Signals

Whenever you hear about an "analog" signal, it is synonymous with a purely electrical signal, where the voltage of the signal is proportional to change air pressure that produce sound waves.  The voltage is carried through copper wiring from the output of its source to the input at its destination.

Analog: Unbalanced

The simplest connection between two devices is an "unbalanced cable," consisting of 2 conductors: one carries the voltage (+), and one acts as a ground.

As discussed in the next heading, unbalanced cables aren't great for long distances, because they're susceptible to electromagnetic interference.

Most instruments (guitar/bass/keyboard-synth) and computer inputs use unbalanced cables, because they typically don't need long cable runs.

Analog: Balanced

A balanced cable adds a third conductor to the unbalanced setup to combat the effects of electromagnetic interference. The basic premise is this lies in an understanding of phase.

In these cables, one conductor carries the voltage (+), one carries the voltage out-of-phase (-), and one acts as a ground. As the signals are carried through the length of the cable, the electromagnetic interference becomes part of both of the signals (in-phase and out-of-phase).

At the input-circuit of the device that receives the signal, the polarity of the out-of-phase voltage is reversed, and the two signals are added together. The original signal is now twice as powerful, and the electromagnetic interference is now out-of-phase with itself, thereby canceling it out of the signal altogether.

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A direct box (DI / direct-injection box) will convert an balanced into a balanced signal, and vice versa.

Digital Signals

Instead of carrying proportional voltages, data cables (digital) carry multiple streams of digital information. Typical examples include Ethernet/Cat5e/Cat6, S/PDIF, and ADAT [Lightpipe]. Because these are carrying streams of digital information, they are not susceptible to the noise issues outlined above.

Converting

Where analog devices use changes in voltage to transmit and process audio, digital devices use computers that transmit numbers.  An A/D converter translates the signals.

When converting an analog signal to digital, the A/D converter will "sample" (measure) the amplitude of the voltage, creating a rough snapshot of the analog signal. 

Sample rate (Hz): the number of snapshots taken per second

Resolution (bits): the number of available measurement points for the amplitude

A D/A converter generates an analog signal from the digital snapshot.

Quality

By nature, analog signals are susceptible to electromagnetic interference (unless you're using balanced cables), and they degrade in quality over long distances because of the resistance in the copper wiring.

Once a signal is converted to digital, the content of the signal will not change unless it's manipulated by a processor. (In a digital environment, if the signal quality isn't acceptable, there will simply be no signal present.)**REWORD**

Quantity

In addition to being able to maintain quality throughout the signal chain, the mechanisms that drive digital transmission are also incredibly efficient: with the right equipment, it becomes possible to transmit multiple streams of audio through a single copper wire. Many digital systems are able to transmit 48+ simultaneous streams through a single network cable, massively reducing the cost of the wiring infrastructure.

Limitations of Digital

Even with the benefits, it's important to remember that a digital signal is a "rough" snapshot -- and will have certain limitations - particularly based on the sample rate and the resolution of the digital signal. While no digital snapshot will ever perfectly and accurately represent an analog signal, a digital audio signal with a high enough sample rate (44.1kHz+) and a high enough resolution (24-bit+) [aka CD quality] is generally accurate-enough to be useful.

Higher sample rates and resolutions will provide more accurate representations of an audio signal, simply because there are more data points available - but, they are also more taxing on the equipment that processes the signal, so there is always a trade-off between quality and reliability when working with digital.

Wireless (RF)

In some instances, the flexibility of a wireless system provides a great benefit over running a wire. Typically, wireless signals are used to provide the ability for a speaker or performer to move around during a program.

All wireless systems consist of transmitters and receivers.

A transmitter will encode (convert) an incoming signal into a radio wave, and send it through the air.  On the other end, the receiver understands how to decode the radio wave into a usable signal (analog or digital).

It's important to note that each channel in a typical wireless system will require a dedicated radio frequency, and that every wireless system is susceptible to interference from information on surrounding frequencies.

Signal Level

Mic/Line/Speaker

Typically, input transducers (converting acoustic energy to electrical energy) will generate a relatively low voltage (known as mic level), and will require amplification before any sort of signal processor will be able to have a tangible impact.  Signal processors (mixers, compressors, equalizers, reverbs, etc.) and electric signal generators (keyboards, playback devices) are usually able to provide a higher voltage signal (known as line level).  Although their voltages tend to work in different ranges, both mic-level and line-level signals are considered "low voltage."  In order to provide enough voltage to move a speaker cone, both mic and level signals need to be amplified to speaker level.

Amplifiers

Most audio equipment is optimized to work with line-level signals.

When the signal isn't strong enough to be usable, it must be amplified to the appropriate signal level.

Pre-amps (or mic preamps) boost a mic-level signal to line-level, so that mixers and interfaces can use it.

Power amplifiers convert line-level signal to speaker level, which provide enough voltage so that when the voltage is applied to the magnet, it causes the cone in a speaker to move.

Signal Flow

In the same way that street traffic is intended to travel in specific directions (correct side of the road; one-way streets), the electricity in an audio signal travels in a very specific direction: ONE WAY through the sound system, moving from one point to another in a logical, traceable manner.

Sound information ALWAYS has a source, and always ends up somewhere (even if that "somewhere" is a broken or disabled connection).

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