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Since the widespread adoption ofreel-to-reel audio tape recording in the 1950s,audio tapes and tapecassettes have been available in many formats. This article describes the length, tape thickness and playing times of some of the most common ones.
All tape thicknesses here refer to the total tape thickness unless otherwise specified, including the base, the oxide coating and any back coating. In the United States, tape thickness is often expressed as the thickness of the base alone. However, this varies from manufacturer to manufacturer and also between tape formulations from the same manufacturer. Outside of the US, the overall thickness is more often quoted, and is the more relevant measurement when relating the thickness to the length that can be fit onto a reel or into a cassette.
The tape decks of the 1950s were mainly designed to use tape1⁄4 inch (0.64 cm) wide and to accept one of two reel formats:
In each case the shaft or hub had three splines. In machines designed to allow for vertical mounting, the upper part of the shaft or hub could commonly be rotated by 60° so the upper splines locked the reel in place (or, more recently, used a rubber stopper placed on the spindle). Some tape decks could accommodate either format by using removable hubs for the larger reel size. When in use these hubs were locked onto the cine spindles by the same mechanism used to secure the smaller reels.
Reel capacity is affected by both the reel diameter and the reel hub diameter. The standard ten and a half inch reel has approximately twice the capacity of the seven inch reel, which in turn has twice the capacity of the five inch. Some (not all) reels described as three inches are in fact three and a quarter inches (8.3 cm) in diameter, in order to have half the capacity of a five-inch reel.
The first commonly available increase in tape length resulted from a reduction in backing thickness from 1.5 to 1.0 mil (38 to 25 μm) resulting in a total thickness reduction from 42 to 35 μm (1.7 to 1.4 mils), which allowed 3,600 ft (1,100 m), 1,800 ft (550 m), and 900 ft (270 m) tapes to fit on ten-and-a-half-, seven-, and five-inch reels respectively. These were known aslong-play tapes.[1] Manufacturers also referred to3+3⁄4 inches per second (9.525 cm/s) tape speed aslong-play.
A further reduction of backing to 0.5 or 0.75 mils (13 or 19 μm) and total thickness to 26 micrometres (1.0 mil) resulted indouble-play tapes of 2,400 feet (730 m) on a seven-inch reel.[1] This and thinner tapes were not commonly used on ten-and-a-half-inch reels, as the tape was too fragile for theangular momentum of the larger reels, particularly when rewinding.
Thinner tapes with thicknesses of 18 μm, fitting 3,600 feet (1,100 m) on a seven-inch reel and 1,800 feet (550 m) on a five-inch reel were known astriple-play tapes.[1] Triple-play tape was too fragile for many tape decks to safely rewind even on a seven-inch reel, and was more commonly used on five-inch- and smaller reels. However, 3600 ft tapes on seven-inch reels were commercially available for those who wanted them.
Studio- and otherwise professional-quality1⁄4 in (6.4 mm)-width tapes thinner than long-play were not commercially available in either reel format. However, some specialised applications, such as call logging, used ten-and-a-half or larger reels of double-play or thinner tape for extended recording times. These machines were extremely restricted in the reel sizes for which they were designed, and often had no rewind or fast forward facility at all, or even playback. These functions were instead performed on a dedicated machine in the event of playback being required.
In the days when long-distance telephone calls were expensive and often very low quality, three-inch or smaller reels of triple-play or even thinner tape were used for sending long recorded messages by post, most often using1+7⁄8 inches per second (4.762 cm/s) tape speed. These were known as message tapes.
Although smaller reels could be easily mounted on any machine designed for seven inch reels, in practice there were three limitations on using varying sizes of reel:
Mixing NAB and smaller cine spindle reels was rarely if ever supported, although many machines could physically mount the combination by using one hub adaptor.
As well as1⁄4 in (6.4 mm) tape, studio andmultitrack machines use tape widths of1⁄2, 1, and 2 inches (12.7, 25.4, and 50.8 mm), and at least one 3 inches (76.2 mm)[citation needed] machine was available for a time.
There is also a 35mm width, but this variety is more similar to the motion picturestock of the same width. It is referred to by the recording and motion picture industries as "magfilm". It is used for recording analog sound for a motion picture, and for interlocked playback of sound with a picture reel when editing motion picture film on aflatbed editor. It has also seen use for studioaudio recording andmastering, a few albums released by theCommand label in the 1960s were mastered using 35 mm magfilm. It has the same sprocket holes and the thicker acetate (or polyester) base of 35 mm negative or reversal cinema stock, but instead has a magnetic oxide layer, coating the full width of the film base (as opposed to a photographic emulsion). This variety is referred to as "fullcoat magfilm". Another variant, "stripe magfilm", has only three separate oxide "tracks" on the base, with the rest of the base being clear, with the same 35 mm sprocket holes as well.
Tapes of1⁄2 in (13 mm), 1 in (25 mm) and 2 in (51 mm) width are available in many professional formulations, especially but not only formulations of 35 μm thickness (the thickness known aslong play when used as1⁄4 inch (6.350 mm) tape). The wider tape also made it possible to produce professional quality tapes of about 25 μm thickness (the thickness known asdouble play in1⁄4-inch applications) in1⁄2-inch and wider formats.
In all tape widths including1⁄4-inch, some studio machines use one-sided platters instead of reels. As normal studio practice is not to rewind immediately after recording or playing but rather to store tape with the start end in to take advantage of the even tension produced by the tape transport, tapes from these machines were generally stored on the platters even if the machine was capable of mounting an NAB reel. Other machines used NAB or custom reels in the larger width.
The sizes of platters and their hubs varies, NAB hubs being most common, and tape lengths up to 5,000 feet (1,524 m) not uncommon.
The tape in acompact audio cassette is nominally1⁄8 inch but actually slightly wider (3.81 millimetres (0.150 in)). The small mass of the spools and mechanism generally allows thinner tape to be used than is practical with reel-to-reel.
The thickest tape normally used in cassettes is about 16-18 μm in thickness, and is used in C60 cassettes and in shorter lengths such as the C46. As the standard tape speed for a compact cassette is1+7⁄8 inches per second (4.762 cm/s) and a C60 cassette records 30 minutes per side, a C60 cassette in theory holds281+1⁄4 ft (85.73 m) of tape. In practice there is some variation, for example Maxell quote their C60s as being 90 m (300 ft) in length.
Tape about 11 or 12 μm in thickness is used in C90 cassettes, and also for those intermediate between a C60 and a C90, for example the C74 produced specifically for recording a standard length CD.
A cassette longer than a C90, such as a C120, must use even thinner tape at around 9 μm. Many equipment manufacturers discourage the use of these longer cassettes, partly because the tape is so fragile, and also because of the difficulty of providing optimum recording over an extended range of tape thicknesses (thinner tape is more prone to "print through" echo).
TDK marketed an even longer play cassette which played for 90 minutes per side. Using 6 μm thick tape, the D-C180 tape was so thin, it wastranslucent. The size was not very popular, because only well set up machines could record and play it without the tape jamming. Further, the playback signal was so low that it compromised the calibration of Dolby noise reduction systems. Most users considered that the size was only suitable for speech. Having said that: the size was available for many years, though only sold in relatively small numbers. A TDK D-C240 tape was proposed[2] but never made it to the manufacturing stage.
Although themicrocassette uses tape thinner than the C90 compact cassette (the width is the same), much of its reduced size comes from using shorter tapes at slower recording speeds. The original standard MC60 microcassette contained 43.2 m (142 ft) of tape for 30 minutes recording per side at2.4 cm/s (about15⁄16 ips, making it half the standard speed of a compact cassette). Most recorders also provide a slower speed of1.2 cm/s, doubling the recording time but with poor sound quality. The only other common sizes are the MC30 with half the length of slightly thicker tape, and the MC90 with 50% more tape length then the MC60, and correspondingly thinner tape.
In sound recording, magnetic tapespeed is often quoted ininches per second (abbreviated ips) for historical reasons. Magnetic tape speeds are commonly an even fraction of 30 ips:
| ips | cm/s | Typical use |
|---|---|---|
| 120 | 304.8 | Used by some analog instrumentation recorders andloop bin duplicators. |
| 60 | 152.4 | Used by some analoginstrumentation recorders, as well as loop bin duplicators. |
| 45 | 114.3 | Used by3M's first digital audio recorder, the "3M Digital Audio Mastering System" in 1978. Also used by both theDASH-format Sony PCM-3348HR and the Studer D827 24-bit audio recorders. |
| 39.4 | 100 | Used by the firstAEGMagnetophon models in 1935. |
| 30 | 76.2 | The highest standard professional speed. |
| 22.5 | 57.15 | Audio tape coated on70 mm motion picture film, necessitated by the 112.5 ft/min film speed for 24 frame/sec projection. |
| 18 | 45.72 | Three- or four-channel magnetic35 mm film, used byEverest,Mercury,Command, andReprise Records in the 1950s and 1960s. |
| 15 | 38.10 | The most common studio- and otherwise professional speed forreel-to-reel includingmultitrack recorders. |
| 71⁄2 | 19.05 |
|
| 33⁄4 | 9.53 |
|
| 2 | 5.08 | Used for theDC-International cassette launched byGrundig in 1965 and discontinued in 1967.[3] |
| 17⁄8 | 4.76 |
|
| 15⁄16 | 2.38 |
|
| 15⁄32 | 1.19 | The standard alternative recording speed formicrocassettes. |
Tape recording first became common enough for the issue of compatibility betweentape deck manufacturers to become an issue in the 1950s. At this time the most common speeds for professional recording were 30 ips and 15 ips, and some machines already supported both speeds. As the tape speed was determined by the speed of asynchronous motor driving a capstan, one way of achieving this was to switch the poles of the motor to a different configuration, halving or doubling the speed.
This system was extended to domestic tape decks, and so slower speeds as they were adopted tended to be exactly half the previous slowest speed. Pre-recorded tapes were mostly 71⁄2 ips, with a few at 33⁄4 ips. Message tapes transmitted by post and call logging tapes were commonly recorded at 17⁄8 ips or even15⁄16 ips. The most commonreel-to-reel speed of 71⁄2 ips is approximately19 cm/s.
Another cause of incompatibility between tape decks was the lack of standardisation of track widths and the use of alternate (rather than adjacent) stereo tracks by many manufacturers (which limited compatibility with mono equipment).
When Philips introduced thecompact audio cassette, they chose to specify the reel-to-reel standard of 17⁄8 ips (approximately 4.76 cm/s), although with narrower and thinner tape. Higher speed machines using compact cassettes commonly use 33⁄4 ips.
Although themicrocassette is specified to have a standard record speed of2.4 cm/s and low speed of1.2 cm/s, in thedictaphone application for which it was designed these speeds are in practice identical to15⁄16 ips and15⁄32 ips. Playback speed is not specified, and on many machines is continuously variable.