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 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.
Reel-to-reel quarter-inch
The tape decks of the 1950s were mainly designed to use tape wide and to accept one of two reel formats:
reels, almost always with metal flanges, which fit over a hub three inches in diameter. These reels and hubs were similar to those used for wider tape formats such as , , and tape widths, and were principally used for studio and other professional applications. The reels were known as NAB reels and the hubs on which they were mounted as NAB hubs. Reels of up to in diameter, most commonly with plastic flanges but metal was also used, which fit over a splined shaft known as a cine spindle, because it is of the same dimensions as the spindle used for Standard Standard 8mm cine film. These reels dominated domestic applications. The most common sizes were seven, five (12.7 cm) and three inches (7.6 cm) 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 (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 (8.2 cm) 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 from 42 μm to 35 μm, which allowed , , and tapes to fit on ten-and-a-half, seven-, and five-inch reels respectively. These were known as long play tapes.
Also manufacturers also referred to tape speed as long-play.
A further reduction to 26 μm resulted in double-play tapes of on a reel. This and thinner tapes were not commonly used on reels, as the tape was too fragile for the angular momentum of the larger reels, particularly when rewinding.
Thinner tapes with thicknesses of 18 μm, fitting on a seven-inch reel and on a 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 ft tapes on seven-inch reels were commercially available for those who wanted them.
Studio- and otherwise professional-quality -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 using 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:
