So, what is 'Real-Time'?

[z05DSL3A]

A comment in in another thread:

Within 5 seconds is hardly realtime.

I have often seen the phrase 'real-time' used and I always have to try to work out what the particular meaning is. I understand 'real-time' to have at least two meanings. The first is the common meaning of 'as it happens' or at least as soon as possible to it happening. The second is where you have a temporal constrain to work within else the 'system' is at fault.

I was just wondering what others think when they read the phrase 'Real-Time'.

[oz10RXSz]

5 seconds is realtime if exceeding it causes something to crash.

Realtime is when something is to react within a given amount of time. If sometimes it is allowed to exceed it (but should not do it too often) then it is a soft-realtime. If it is not allowed to, then it is hard realtime. Audio / Video / Games are soft realtime. Avionics software or software controlling traffic lights is hard realtime.

[helios]

1. Something that happens fast enough to seem instantaneous. E.g. right-clicking and a context menu appearing (assuming normal conditions).
2. A process whose output can be examined before it finishes. This can be equated with "on the fly". E.g. Audio and video compression technology, some physics simulations, those cool molecule animations you can configure your Folding@Home client to do.

[jsmith]

http://en.wikipedia.org/wiki/Hard_real_time

My definitions:
"hard real time" : A system that is at fault if time constraints are not met (what the page calls real-time).
"soft real time" : A system that has time constraints which it should meet, but is OK if not met all the time
(what the page calls non-real time).

[tition]

I can't help pointing out that "hard real time" and "soft real time" sound quite dirty to me...

[Edit sorry for the spam: I second helios on the topic]