Isochronous

Isochronous is a word derived from the Greek iso (equal) and chronos (time). It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the same time. The term is used in different technical contexts, but often refers to the primary subject maintaining a certain interval, despite variations in other measurable factors in the same system.

• In horology, isochronous refers to the timekeeping principle of pendulums noted by Galileo in the late 16th century. He observed that the oscillation period of a pendulum is constant, regardless of the angle of the swing.
• In telecommunication, isochronous has the following meanings:

1. Of a periodic signal, pertaining to transmission in which the time interval separating any two corresponding transitions is equal to the unit interval or to a multiple of the unit interval; but phase is arbitrary (and in some systems may change slowly over time)
2. Pertaining to data transmission in which corresponding significant instants of two or more sequential signals have a constant phase relationship.

• In power generation, isochronous means that the frequency of the electricity generated is 'flat' or constant, and there is zero generator droop.
• In the Universal Serial Bus used in computers, isochronous is one of the four data flow types for USB devices (the others being Control, Interrupt and Bulk). It is commonly used for streaming data types such as video or audio sources. Similarly, in the IEEE 1394 interface standard commonly called Firewire, includes support for isochronous streams of audio and video at known constant rates.^[1]
• In dynamical system theory an oscillator is called isochronous if the frequency is independent of its amplitude. For instance, in horology, isochronous refers to a mechanical clock or watch which runs at the same rate regardless of changes in its drive force, so it keeps correct time as the mainspring unwinds.
• In particle accelerators an isochronous cyclotron is a cyclotron where the field strength increases with radius to compensate for relativistic increase in mass with speed

Closely related to the property of being isochronous is the term isochrone referring to contour lines of equal time, for instance, in geological layers, tree rings or wave fronts.

See also

• Isochrony

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).