Inches per second

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In sound recording, magnetic tape speed is often quoted in inches per second (abbreviated ips) for historical reasons. One inch per second is equal to 0.0254 metres per second (2.54 cm/s).

Magnetic tape speeds are commonly an even fraction of 30 ips:

120 ips: Also used by some analog instrumentation recorders and loop bin duplicators.
60 ips: Used by some analog instrumentation recorders, as well as loop bin duplicators.
45 ips: Used by 3M's first digital audio recorder in 1978.
30 ips: The highest professional speed.
15 ips: The most common professional and studio speed for reel to reel including multitrack.
7¹⁄₂ ips:
* The lowest professional speed, used on some single-speed studio recorders including multitrack.
* The highest domestic speed.
* Used on older single speed domestic machines.
* The most common speed for pre-recorded reel to reel tapes.
3³⁄₄ ips:
* Used on later single speed domestic machines.
* The second most common speed for pre-recorded reel to reel tapes.
* Used by some consumer multitrack machines using compact cassettes.
1⁷⁄₈ ips:
* The standard speed for compact cassettes.
* The lowest common reel to reel speed.
* Used for reel to reel message tapes.
* Used for message logging and similar specialised applications.
¹⁵⁄₁₆ ips: The standard speed for microcassettes (although specified as 2.4 cm/s, see below). Also used for some "talking books" on compact cassette for the blind & visually impaired, issued by the Library of Congress.
¹⁵⁄₃₂ ips: The standard alternative recording speed for microcassettes.

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.

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). 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 dictating machine 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.

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