What has become known to most people today as a 4-track (in the Guided By Voices sense of the word) is a not-entirely-obsolete piece of equipment actually comprising two separate units housed inside the same casing: a tape recorder (in the case of a cassette-based machine) and a mixer.

The reason behind this particular combination is that it (theoretically) allows its user to produce recordings without the need for any further accessories, save for sound sources and a mixdown recorder and/or speaker system.

Let’s tackle the two comprising sections separately in order to understand how they work and interact.

The Tape Recorder:

The tape recorder included in most of these machines is actually very similar to a regular cassette deck. The key difference is in the way the tape is used.

A typical cassette deck divides the tape in two sides, each one of which is in turn comprised of two tracks. This configuration is what makes possible the recording and playback of stereophonic material.

On a 4-track, on the other side, the whole width of the tape is used at once; all the tracks on the tape are used simultaneously.

Update: Another most important difference between a 4-track deck and a typical consumer-grade stereo deck is the fact that the 4-track deck allows the user to select which track(s) he or she wants to record into at a given time.

This is what allows for many essential multitrack recording techniques, such as overdubbing and punch-in recording.

Most home stereo decks, on the other hand, switch both tracks (left and right) into record mode when the record button is pressed; it's therefore not possible to monitor the recorded signal off the tape without the addition of an additional playback head, most often found on higher-grade equipment.

There are other minor differences as well, such as the fact that most 4-tracks run at twice the speed of a regular cassette deck (in order to obtain a slightly better sound quality). 4-tracks are also usually biased for chrome (type II) tape since this makes for slightly better frequency response and s/n ratio.

Typically some form of noise reduction (such as Dolby B or dbx) and a tape counter are also included.

The Mixer Section:

The mixer section is included in these units to make possible the mixing down of the four tracks on the tape into the two output channels (left and right) which form a typical stereo programme (see mixdown). The position of each element inside the listening field is controlled by the pan control.

The other reason why a mixer is bundled with the recorder is to allow the input signal(s) to be combined and/or routed in a convenient way so as to make the most out of the relatively small number of tracks the user has at his or her disposal.

Higher-priced units often have twice or more mixer channels than the number of tracks on the tape, allowing for more flexible signal routing.

A typical channel strip in a mid-priced Portastudio includes a preamplifier, an eq section and a number of effect sends (usually two).

The preamplifier is most often used to bring up the level of a given input signal to a suitable one for recording into the tape; the amount of gain it provides is adjustable to allow for the utilization of the tape to the fullest of its dynamic range without distortion.

The equalizers (usually two of the shelving variety and sometimes a semi-parametric one for the middle frequencies) provide the user with a certain amount of control over the tonal components of a signal, mostly in order to compensate for or correct distortions introduced by the limited frequency response and relatively poor signal-to-noise ratio of cassette tape.

Equalizers can also be used to deliberately distort signals in several ways so as to provide a musician with an amount of creative control over recordings.

On some units, effect sends (sometimes known as aux sends) are also provided. These are individual outputs which are used for feeding an outboard fx unit, thereby allowing for the processing of certain elements in a composition (an example of this would be a vocal sent to a reverb unit).

Aux sends can also be used for several other things, such as providing musicians with monitoring or building alternate mixes of a recording.

Often, due to the limited number of tracks available, users find themselves in the situation of having to mix several parts together before being able to add any more elements to a composition. This is known as bouncing and it's also made possible by the inclusion of a mixer.

The simplest form of bouncing is known as internal bouncing. It consists of mixing down the contents of two or more tracks into an empty one, allowing for subsequent erasure and re-recording of the source tracks.

An example of this would be a tape recorded with drums on track 1, a guitar on track 2 and vocals on track 3. By mixing drums, guitar and vocals together on track 4, the user is left with three available tracks to continue recording on.

The downside to this process is that the original tracks are destroyed, as well as the fact that the copying of one track to another adds an amount of hiss. An alternative to this is known as external bouncing or premixing, a process which takes some more extensive preparation but allows for the preservation of the original tracks.

If you see any serious omissions or mistakes in this writeup please let me know.

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