Magnetic tape speed measured in inches per second (ips) — 15 ips broadcast standard, 7.5 ips archive. Directly affects frequency response and noise floor.
The choice of tape speed, both on set and in editing, determines how clean your audio comes from the recorder—and how much material you can fit per reel. 15 inches per second (ips) is the broadcast standard because the frequency response remains linear and the background noise is minimal. Working at 15 ips provides head accuracy that is essential for synchronization and post-production. The disadvantage: a 10.5-inch reel only holds about 15 minutes of material. For longer shoots without changes, this can become tight.
7.5 ips is the archive and economy speed—double the running time per reel, but with noticeably higher noise and a slight attenuation of higher frequencies. This is sufficient for ambient recordings that will be overlaid with effects in post-production anyway. I often used 7.5 ips on documentary shoots when the audio needed to provide more spatial information than brilliance. The psychoacoustic difference: 15 ips sounds "present," 7.5 ips sounds "warm" and slightly "dull."
Practical on Set: Tape speed also determines head wear speed and thus the maintenance intervals of your recorder. Faster tapes require more regular calibration. Some editors do not note the speed used in the sound log—leading to an unpleasant surprise later in editing when the pitch reference is suddenly incorrect. With analog multitrack recordings, it was crucial that all tracks ran at identical tape speeds; the slightest deviations led to phasing problems during mixing.
Modern digital recorders have solved this problem—here, tape speeds only play a role when archiving, such as when digitizing old magnetic tapes. However, for those working with vintage equipment or seeking analog authenticity, a conscious decision is unavoidable. 15 ips for everything that should be audible later; 7.5 ips for raw material and long documentaries. This also saves money on magnetic tape.