Re: What is real-time?

Steve Underwood <steveu@xxxxxxx> writes:

Rune Allnor wrote:
On 22 Mar, 08:38, minfitl...@xxxxxxxxxxx wrote:
What is considered real-time? How much latency would be necessary for
it to be no longer real-time? The application is adaptive filtering.
Somebody told me that real-time is strictly one sample in and one
sample out but most of the processors I have met don't work that way
any more - they work on buffers of data and surely all real-time
filters have some latency or is one sample all that is required?
"Real time" means that the data are processed as they come in,
and that this processing is done inside an acceptable latency.
In a phone system, this "acceptable latency" may be on the
order of milliseconds.
Where I work, the "acceptable latency" is 24 hours. It is still
a real-time system.

The last time I built DSP for the telephone network, the acceptable
latency was 3 sample times - 375us. One to deserialise, one to
serialise, and just one left for the processing delay in the
middle. :-)

A true real time application typically doesn't have an "acceptable"
latency. It has a hard and well defined latency, it absolutely
positively must meet under all circumstances. For a control loop that
hard latency is typically very small for acceptable performance. For a
satellite TV decompressor it might be quite long. Nonetheless, the
incoming data will *always* be accepted without ever losing a bit, and
the output will be provided a very much fixed delay after the input
data is received.

In my opinion any discussion of latency should be removed from the
definition of "real-time." These are two "orthogonal" concepts.
--
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