Audio tape length and thickness

Since the widespread adoption of reel-to-reel audio tape recording in the 1950s, audio tapes and tape cassettes 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 USA, 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 USA, the overall thickness is more often quoted, and is the more relevant measurement when relating the thickness to the length that can be fitted onto a reel or into a cassette.

Reel to reel ¼"
The tape decks of the 1950s were mainly designed to use tape ¼" wide and to accept one of two reel formats:


 * Ten and a half inch reels, almost always with metal flanges, which fitted over a hub three inches in diameter. These reels and hubs were similar to those used for wider tape formats such as ½", 1" and 2" tape widths, and were principally used for professional and studio applications. The reels were known as NAB reels and the hubs on which they were mounted as NAB hubs.


 * Reels of up to seven inches in diameter, most commonly with plastic flanges but metal was also used, which fitted over a splined ¼" shaft known as a cine spindle. These reels dominated domestic applications. The most common sizes were seven, five and three inches in diameter.

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. 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 in diameter, in order to have half the capacity of a five inch reel.

Long play, double play, triple play
The first commonly available increase in tape length resulted from a reduction in thickness to 35 µm, which allowed 3600', 1800' and 900' tapes to fit on ten and a half, seven and five inch reels respectively. These were known as long play tapes.

Confusingly, for a short time some equipment manufacturers also referred to 3¾ ips tape speed as long play, but this usage did not persist.

A further reduction to 25 µm resulted in double play tapes of 2400' on a seven inch reel. This and thinner tapes were not commonly used on ten and a half inch reels, as the tape was too fragile for the angular momentum of the larger reels, particularly when rewinding.

Even thinner tapes fitting 3600' on a seven inch reel and 1800' on a five inch reel were known as triple play tapes. Triple play tape was too fragile for many tape decks to safely rewind even on a full seven inch reel, and was more commonly used on five inch and smaller reels. However 3600' tapes on seven inch reels were commercially available for those who wanted them.

Professional or studio quality ¼" 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 that 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 using 17⁄8 ips tape speed. These were known as message tapes.

Reel size compatibility
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:


 * Stability in rewind and fast forward modes was often optimized for a particular reel size to allow faster winding speeds. Some machines provided a switch facility to accommodate different reel sizes, others provided speed controls and relied on manual intervention, still others attempted to sense the reel size by various methods.


 * Smaller machines were built for smaller reels, and could not physically mount a full size seven inch reel.


 * Mixing reel sizes between supply and takeup spool made fast winding still more complicated, and was discouraged or prohibited by almost all manufacturers. Even mixing metal and plastic flange reels of the same size was not supported by some machines.

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.

Studio tape formats
As well as ¼" tape, studio and multitrack machines used tape widths of ½", 1" and 2", and at least one 3" machine was available for a time. There was also a 35 mm width.

Tapes of ½", 1" and 2" width were available in many professional formulations, especially but not only formulations of 35 µm thickness (the thickness known as long play when used as ¼" tape). The wider tape also made it possible to produce professional quality tapes of about 25 µm thickness (the thickness known as double play in ¼" applications) in ½" and wider formats.

In all tape widths including ¼", some studio machines used 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 varied, NAB hubs being most common, and tape lengths up to 5000' not uncommon.

Compact audio cassettes
The tape in a compact audio cassette is nominally ⅛ inch but actually slightly wider (3.81 mm). 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 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 is 1⅞ ips and a C60 cassette records 30 minutes per side, a C60 cassette in theory holds 281¼' (85.73 m) of tape. In practice there is some variation, for example Maxell quote their C60s as being 90 m (295') in length.

Tape about 11.2 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. Most equipment manufacturers optimize their equipment for C90 cassettes but fully support shorter cassettes using either 11.2 µm or 16 µm tape.

A cassette longer than a C90, such as a C120, must use even thinner tape. Most 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). However C180 and even C240 cassettes have been produced both for the consumer market and for specialised applications such as call logging.

Rumours have sometimes associated C120 and longer cassettes with double or triple play reel-to-reel tape, but in fact even C60 cassettes use tape thinner than double play and little thicker than triple play. The main thing that the longer cassettes have in common with double and triple play reel-to-reel tapes is that they are pushing similar limits of technology, and as a consequence suffer similar problems.

Microcassettes
Although the microcassette 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') of tape for 30 minutes recording per side at 2.4 cm/s (about 15⁄16 ips, making it half the standard speed of a compact cassette). Most recorders also provide a slower speed of 1.2 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.

Tape speeds
In sound recording, magnetic tape speed is often quoted in inches per second (abbreviated ips) for historical reasons. Magnetic tape speeds are commonly an even fraction of 30 ips:



Tape recording first became common enough for the issue of compatibility between tape 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 a synchronous 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 even 15⁄16 ips. The most common reel to reel speed of 71⁄2 ips is approximately 19 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 the compact 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 the microcassette is specified to have a standard record speed of 2.4 cm/s and low speed of 1.2 cm/s, in the dictaphone application for which it was designed these speeds are in practice identical to 15⁄16 ips and 15⁄32 ips. Playback speed is not specified, and on many machines is continuously variable.