Audio tape specifications

This article describes the length, tape thickness and playing times of some of the most common ones.

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.

Studio- and otherwise professional-quality 1⁄4 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.

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 1+7⁄8 inches per second (4.762 cm/s) tape speed.

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).

The tape in a compact audio cassette is nominally 1⁄8 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.

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.

Further, the playback signal was so low that it compromised the calibration of Dolby noise reduction systems.

In sound recording, magnetic tape speed is often quoted in inches per second (abbreviated ips) for historical reasons.

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.