Buzz
Are you familiar with how they function? Nico De Pasquale Photography / Getty ImagesMagnetic recording is a core technology of the digital age, serving as a crucial method for the permanent storage of data.
- In the music industry, magnetic tapes (such as compact cassettes) are a common medium for music distribution. People often purchase pre-recorded tapes or create their own by recording from CDs.
- In television and film, videotapes are widely used in both professional broadcasting and personal use to store footage for playback on VCRs.
- In computing, magnetic recording plays a key role in data storage through floppy disks, hard drives, and magnetic tapes.
In this article, we will explore magnetic recording. Our focus will be on cassette tapes and tape recorders, though this technology applies to all forms of magnetic recording. You will discover that the widespread popularity of magnetic recording comes from its simplicity, affordability, and reliable medium-term storage capabilities (lasting between 10 to 20 years).
The Tape
Any audio magnetic recording system consists of two essential components: the recorder (which also serves as the playback device) and the magnetic tape, which acts as the storage medium.
The tape itself is quite straightforward. It is made of a thin plastic base, with a layer of ferric oxide powder bonded to it. This oxide is typically mixed with a binder to adhere it to the plastic, and it includes a dry lubricant to prevent wear on the recorder.
Iron oxide (FeO) is the red rust we commonly see. Ferric oxide (Fe2O3) is another oxide of iron, and it is often referred to as maghemite or gamma ferric oxide.
This oxide is a ferromagnetic substance, which means that when exposed to a magnetic field, it becomes permanently magnetized. This property gives magnetic tape two of its most attractive features:
- You can instantly record anything you desire, and the tape will retain the recording for playback whenever you need it.
- You have the ability to erase the tape and record something new at any time you wish.
These two features are what make tapes and disks so widely used — they offer immediacy and flexibility, being easily updated.
Over the years, audio tapes have evolved through several different formats.
- The earliest format was not tape at all, but rather a thin steel wire. The wire recorder was created in 1900 by Valdemar Poulsen.
- In the 1930s, German engineers refined the first tape recorders, utilizing oxide tapes. The tapes initially appeared in a reel-to-reel format. See this page for an image of an early reel-to-reel recorder.
- Reel-to-reel tapes remained common until the compact cassette or "cassette tape" overtook the market. Patented in 1964, the cassette ultimately surpassed 8-track tapes and reel-to-reel systems to become the leading tape format in the audio industry.
Inside a compact cassette, you'll find a rather simple setup. It consists of two spools, a long tape, two rollers, and two halves of a plastic outer shell with various cutouts and holes designed to fit the cassette into the drive. There's also a small felt pad that serves as a stopper for the record/playback head in the tape player. In a 90-minute cassette, the tape measures 443 feet (135 meters) in length.
The Tape Recorder
The simplest tape recorders are truly basic, and this simplicity is embodied in everything from a portable Walkman to a high-end audiophile deck.
The fundamental concept involves an electromagnet that generates a magnetic flux on the oxide of the tape. This flux is what causes the oxide to permanently "remember" the magnetic field it encounters. The record head of a tape recorder is a tiny circular electromagnet with a small gap in it, like this:
This electromagnet is quite small — roughly the size of a flattened pea. It's made up of an iron core wrapped in wire, as depicted in the figure. While recording, the audio signal flows through the coil, creating a magnetic field in the core. At the gap, the magnetic flux forms a fringe pattern to bridge the gap (shown in red), which is what magnetizes the oxide on the tape. During playback, the movement of the tape moves the magnetic field across the gap, which in turn generates a fluctuating magnetic field in the core and produces a signal in the coil. This signal is amplified to power the speakers.
In a typical cassette player, there are two small electromagnets that together span approximately half the width of the tape. These two heads work together to record the two channels of a stereo program, like this:
When you flip the tape over, the other half of the tape aligns with the two electromagnets.
Looking inside a tape recorder, you'll typically see something like this:
At the top of this image are the two sprockets that engage the cassette's spools. These sprockets rotate one of the spools to move the tape during recording, playback, fast forward, and rewind. Below these sprockets are two heads: the left one is a bulk erase head, designed to clear the tape of any signals before recording, and the center one is the record and playback head, which holds the two small electromagnets. To the right, you'll find the capstan and the pinch roller, as shown below:
The capstan rotates at an extremely precise speed to draw the tape across the head at exactly the correct rate. The standard speed is 1.875 inches per second (4.76 cm per second). The roller simply applies pressure, ensuring that the tape stays tightly against the capstan.
Tape Types and Bias
Most high-end tape decks include controls like the ones shown below to accommodate different tape formulations and bias settings.
High-quality tapes usually identify their formulation by specifying a type. Four common types of tape are in use today:
- Type 0 - This is the original ferric-oxide tape, now rarely seen.
- Type 1 - Known as standard ferric-oxide tape, or "normal bias."
- Type 2 - Often called "chrome" or CrO2 tape, where the ferric-oxide particles are mixed with chromium dioxide.
- Type 4 - Referred to as "metal" tape, which uses metallic particles instead of metal-oxide ones.
As you move from one tape type to another, the sound quality improves, with metal tapes offering the best sound quality. However, a standard tape deck cannot record on a metal tape unless it has a specific setting for that. Any tape player, however, can play a metal tape.
The tape deck's controls allow you to adjust the recording bias and signal strength to match the tape type, ensuring the best possible sound quality.
