Decisecond (ds)

The decisecond is a unit of time equal to one tenth of a second. Its symbol is ds. Like the centisecond it is a name that is almost never used, while the quantity it names turns up constantly under other descriptions.

A tenth of a second is roughly the shortest interval a person can consciously register as a duration rather than an instant. Voluntary reaction to a sound takes about one and a half deciseconds, and to a light rather longer. The blink of an eye lasts between one and four deciseconds. These figures set the boundary between what feels immediate and what feels delayed, and interface designers work to keep responses inside it.

Sport uses the quantity where the hundredth would be false precision. Long-distance running, road cycling, cross-country skiing and marathon swimming publish results to the tenth, because over an hour or more the differences between competitors are large and the measurement uncertainties from course length and timing points make finer figures unjustifiable. Field events measured in distance rather than time follow a parallel logic with the centimetre.

Speech occupies the same scale. An average spoken syllable lasts around two deciseconds, and the pause that listeners hear as the end of a sentence is roughly five. Speech recognition systems segment audio into frames of about one tenth to one quarter of a decisecond, and the natural rhythm they are trying to capture sits an order of magnitude above that.

Engineering meets the tenth of a second in relay timing, in the response of mechanical safety interlocks, and in the specification of how quickly a residual current device must disconnect a faulty circuit. Music sits here too: a metronome at one hundred and twenty beats per minute puts five deciseconds between beats.

The deci prefix is among the least used in the SI. It survives in the decibel and the decilitre and almost nowhere else, so a tenth of a second is nearly always written and spoken as exactly that.

Photography meets the same interval from the other side. A shutter open for a tenth of a second gathers plenty of light but records any camera shake as blur, which is why the old rule was never to hand-hold a lens slower than the reciprocal of its focal length. Cinema and video sit just below: at twenty-four frames a second a tenth of a second is a little over two frames, and at fifty it is five, so a tenth is the shortest event an ordinary recording can show as motion rather than as a single frame. Instrument logging often samples at this rate for the same reason, fast enough to catch a change and slow enough to keep the file small.

One decisecond equals 0.1 seconds, 10 centiseconds, or 100 milliseconds.

Decisecond conversion table