Analog tape, or any magnetic medium such as a computer hard drive, is made of ferromagnetic materials. Ferromagnetic materials can be permanently magnetized with the application of an external magnetic field. A tape recorder takes the electrical signal mentioned earlier and applies it to an electromagnet in the record head. As the tape passes over the head, it is magnetized to varying degrees, depending on the strength of the signal feeding the electromagnet at that moment in time. Later, in playback, this process is reversed, as the varying magnetic field on the tape produces a varying electrical signal when the tape is pulled across the playback head. This electrical signal may then amplified and sent to a transducer such as a speaker (Kuhn). Multiple tracks (the left and right of a stereo pair, for example) on the tape are handled by separate electromagnets for each track, which cover their own designated area on the width of the tape.

As mentioned earlier no recording device is perfect, and compensations must be made. In the case of analog tape there are losses when recording high frequency signals, and the difference between the magnetic field strength in the record head and the remanent magnetization of the tape is also significant. These two limitations are addressed with pre-equalization and AC bias, respectively. Understanding the details of these processes is not as important as understanding that analog recording has its compensations, as does digital, which often impart more noise or a less true sound to the recording.

Pre-equalization is a process similar to the RIAA equalization of vinyl records (Kuhn). High frequency information is emphasized on recording, and reduced upon playback.

AC bias is an ultra-high frequency signal (above 100 kilohertz - the upper range of human hearing is around 20 kHz, by contrast) applied while recording to excite the ferromagnetic oxide on the tape at all times. Remember that sound is variation above and below the prevailing atmospheric pressure. These rises and dips are represented with magnetism using field regions of alternating polarity - north, south, north, etc. Refer to Figure 1.

When the oxide of the tape is not magnetized at all, such as the moment of going from a north area to a south area, it takes a stronger amount of current to then produce any magnetization at all - think of it as an inertia that must be overcome. Without applying a continuous signal such as bias, there would be gaps in the resulting field on the tape (Elsea). Refer to Figure 2.

Although the bias signal itself is well above audible range, it excites the tape in such a way that produces audible hiss - something you don't want any more of in analog tape, but is well worth trading for a more linear tape response (McIan and Wichman 43-44).

It may be important to clarify how limited and noisy analog tape is at this point. Many musicians will have experience with stereo cassette decks and four-track cassette 'portastudios,' and may assume the limitations of these machines apply to all analog tape formats. The amount of noise inherent in a professional tape deck is almost completely unnoticeable in comparison to the consumer cassette tape, and the overall fidelity is increased exponentially. The fact that the cassette tape format has been massaged to produce as high fidelity music as it does is a modern miracle. The cassette tape was originally intended for the monophonic recording of voice - not the stereo, full frequency response reproduction of music. The cassette packs 4 tracks (remember you have left and right for sides one and two) onto a 1/8" wide tape, running at a measly speed of 1.875 inches per second (ips) (Newton). How do track count, tape width, and running speed affect noise and fidelity?

The increased width and running speed of the professional formats allow for greater dynamic range, signal-to-noise ratio, and overall fidelity. A professional tape machine puts 2-24 tracks over a ½"-2" width of tape, running at 15 or 30 ips. As explained earlier, multiple tracks are handled with separate electromagnets, each assigned to a designated width area of the tape. As the tracks get spread over a greater width of tape, the width designated for each track can increase. As the tape speed increases, more length of tape moves over the head in a given amount of time. Think of the amount of tape for one track in one second of time as a rectangular area. The height of this rectangular area is determined by the track width of the format. The width of this rectangular area is determined by the running speed of the tape. In general, the greater this rectangular area, the greater the fidelity and signal-to-noise ratio. Why? Analog tape has a limit to the strength of the signal it can accept before saturation. Saturation is like an overexposed photograph - because of excessive volume, the recorded signal is distorted and high end definition is lost. As we shall see, analog tape saturation is a much-desired effect. But eventually beyond the limit of saturation is an undesirable 'brick wall' the signal will hit - the tape will produce crackly, unlistenable high-frequency distortion. The more tape area you have in a given period of time, the harder it is to saturate that entire area, and the louder the signal you can send to the tape deck (McIan and Wichman 44). A hotter signal before distortion increases the dynamic range of the system, or the maximum change in audible program levels. Dynamic range is generally measured by subtracting the noise floor (in this case the ambient noise of the environment and the gear, including tape hiss) from the loudest possible representation of the system (Davis and Jones 33). In our example of increasing tape area, the noise floor may have increased ever-so-slightly due to possible increase in bias noise, but the loudest possible representation has increased greatly - resulting in a larger dynamic range. In addition, each second in time is represented with more tape, so there is more detail in the resultant recording - especially for quieter sounds and higher frequency sounds (McIan and Wichman 44). The professional format tape machine is truly a different beast than the cassette tape.

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