| Vocademy |
Please note that this chapter is in outline form and will be fleshed out later. Use it to guide you through your own research.
Recording tape is made from "prestretched" polyester film (mylar) coated with iron oxide (rust) or mixtures of iron and cobalt or chromium.
A tape head is usually made of a core of laminated soft iron[1]. An electrical current sent through a coil of wire creates a magnetic field in the iron. This magnetic field escapes the iron core at a gap opposite to the coil of wire.
|
|
| A tape head. Notice the gap opposite to the coil. |
Magnetic tape is passed by the gap and is magnetized to record information.
 |
| The magnetic field (black lines) escapes the iron core at the gap, penetrating the tape (brown). |
Heads with a circular cross-section require pressure pads to hold the tape against the head. Heads with a parabolic cross-section do not require pressure pads. Pressure pads can wear out or become misaligned. Severe loss of high-frequency response (muddy sound) can be caused by worn or misaligned pressure pads.
At slower tape speeds, the magnetic marks made by higher frequencies tend to overlap. If the tape is moved at higher speeds, this doesn't happen. Moving the tape at higher speed results in better high-frequency response.
Cassette tapes, which typically move the tape at 1-7/8 inches per second, can only reproduce frequencies as high as 15kHz using tape head gaps of a practical size. A tape speed of 7-1/2 inches per second can easily reproduce frequencies as high as 20kHz using tape heads with gaps of a practical size.
Once magnetic particles on the tape are magnetized they resist reversing their polarity. The tape head current must pass a certain threshold before the magnetic particles will change. This is hysteresis. Because of hysteresis, a magnetically-recorded sine wave will appear as shown below.
 |
| Original Signal |
 |
| Distortion caused hysteresis. |
An ultrasonic sine wave (the bias signal) is mixed with the original signal then recorded.
 |
| Bias Signal (about 100 kHz) |
When mixed together, the original signal rides the top of the bias, remaining outside the area that is lost due to hysteresis.
 |
| Original signal mixed with the bias signal as recorded |
The playback head cannot respond to the high frequency of the bias signal. It can only respond to the frequencies of the original signal.
 |
| The current from the playback head does not follow the bias signal. The playback head current follows the original signal (blue). |
The tape transport is the mechanism that moves the tape. It is usually a series of complicated rubber-coated wheels, belts, pulleys and levers that facilitate moving the tape forward and rewinding.
The tape speed is usually controlled by a capstan and pinch roller. The capstan is a precision-machined post that rotates at the right speed to pull the tape past the heads. The pinch roller holds the tape against the capstan.
 |
| An auto-reversing compact cassette tape transport and heads. It has dual capstans and pinch rollers so it can properly drive the tape in either direction. It has a four-track head so that the head doesn't need to move when the tape changes direction. The second capstan and pinch roller take the space normally taken by the erase head. Therefore, the single head assembly must erase, record and play back[2]. |
Analog tape recorders have a mechanism to erase the tape as it is being recorded. On cheap recorders, this may be no more than a permanent magnet that is held against the tape just before it reaches the record/playback head. This is a suboptimal[3] method as it does not erase the tape but leaves it heavily magnetized at one polarity. This introduces noise (hiss) to the tape.
On most recorders, there is a separate erase head. This head has a wider gap than the record/playback head and may have two gaps. This head uses the bias signal (unmixed with the audio) to remove magnetism from the tape. As the tape moves past the energized erase head, the magnetic domains are rapidly flipped back and forth. As the domains get further from the gap, the strength of the magnetic field gets weaker, magnetizing the domains less. When the domains leave the influence of the magnetic field, they are left in an unmagnetized state.
Some recorders use the same head for recording and playback. Higher quality recorders use separate record and playback heads, each optimized for their function. Such recorders are called "three-head" recorders because they have separate erase, record and playback heads. Three-head compact cassette recorders have the record and playback heads in a single unit that looks like a single head.
The heads need to be cleaned of oxide from the tape periodically. Cotton swabs and isopropyl alcohol are the usual tools of choice.
The volume unit (VU) meters are used to monitor the level of the recording.
 |
| A VU meter[4]. |
To get a good signal-to-noise ratio, the level should be as high as possible. Distortion occurs if the level reaches 0 dB. Do not allow the level to exceed +3 dB (maximum) as the distortion becomes audible at this level.
Special recording heads can make multiple tracks. Most two-channel recorders cover only half of the tape when recording. This allows the tape to be reversed and recorded again going in the opposite direction. Systems with more than two channels (such as professional studio equipment) record in only one direction.
 |
| A stereo tape head. The two tracks cover half of the tape[5]. |
For stereo recording with 1/4 inch open reel recorders, the tracks are staggered. This was done to reduce crosstalk between the left and right channels.
 |
| Reel to reel stereo tracks. When the tape direction is reversed the new tracks interleave with the old tracks. |
 |
| Compact Cassette stereo tracks. They do not interleave when the tape is reversed. |
Some recorders have auto reversing mechanisms. To reverse the tape direction the reels do not have to be removed, turned over and switched. The system reverses the travel direction of the tape and moves the record/playback heads to the new position of the tracks. Some have four-track heads, so the head does not have to be moved to switch tracks.
 |
| An open reel tape deck[6]. |
7-inch diameter reels
Some use 10-inch reels
1/4-inch tape
1-7/8 inches per second (low quality, long play)
3-1/2 inches per second (good quality)
7 inches per second (excellent quality)
15 inches per second (rare on home recorders, professional quality)
1/4-inch wide to 2-inch wide tape
4 to 24 tracks
7-1/2 to 15 inches per second
Dolby-A noise reduction. Like Dolby-B (see below), but worked on four separate frequency ranges independently.
 |
| A compact cassette[7] |
Tape is contained in a plastic shell.
 |
| Inside a compact cassette[8] |
1/8 inch tape
1-7/8 inches per second
Cassettes hold enough tape for 60 minutes of recording (30 minutes per side)
90-minute tapes use a thinner base material
120-minute tapes use a very thin base material. They are prone to jam in the recorder.
Typically low-quality sound
Poor high-frequency response
Tape noise (hiss)
High quality can be achieved with special tape formulations and Dolby noise reduction.
Chromium Dioxide (CrO2) or "metal" tape
Requires modification to the tape bias, called "high bias."
CrO2 requires equalization on playback.
Dolby Noise Reduction
Tape noise is at higher frequencies
Dolby-B emphasizes higher frequencies while recording, then deemphasizes them during playback. Tapes recorded using Dolby-B sound too "bright" or "tinny" if played back normally. Deemphasizing the high frequencies reduces tape hiss while bringing high frequency sounds back to normal.
 |
| Different tape formulations have different notches in the cassette shell. This allows recorders to automatically determine the tape type. From top to bottom: standard (Type I), CrO2 (Type II), Metal (Type IV), Metal with record-enable tab removed[9]. |
A complete system with power amplifiers and speakers.
A recorder that has no power amplifiers and works from line level signals. On home entertainment systems it connects to the tape in and tape out connectors.
The pitch of the sound changes slowly. Caused by eccentricity (deviates from a perfect circle) in wheels or pulleys.
The pitch of the sound changes rapidly. Can by caused by eccentricity in small wheels and pulleys (such as a damaged or poorly made capstan) or by the tape being rapidly grabbed and released by friction at the tape head.
A momentary loss of volume due to damaged or poor quality oxide or by wrinkles in the tape.
Loss of sound quality (distorted sound) caused by allowing the recording level to exceed +3 dB.
A bulk tape eraser is a large electromagnet that plugs into the electrical mains. The AC creates a rapidly reversing magnetic field in the electromagnet. Proper erasing is done by placing the eraser against the tape (still in the reel or cassette) and moving the tape in a circular motion against the eraser. The eraser must not be turned off while close to the tape. This will cause the tape to remain magnetized at whatever strength and polarity the eraser has when the power is cut off. The tape should be moved several feet from the eraser (arm's length) before the eraser is turned off.
| Vocademy |