What is real-time?

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?

F.

On 22 Mar, 08:38, minfitl...@yahoo.co.uk 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.

Rune

F. wrote:
> What is considered real-time? How much latency would be necessary for
> it to be no longer real-time?

The amount of latency doesn't matter. What is important is that the
latency does not exceed a given upper bound. For technical systems,
the latency usually is constant.

There is some info on wikipedia:

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

Regards,
Andor

minfitlike@yahoo.co.uk wrote:

> What is considered real-time? How much latency would be necessary for 
> it to be no longer real-time?

Real time is when the scheduler has a specified maximum context switch
time. This could be a month and still be real time.

bye
Andreas
-- 
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Rune Allnor wrote:
> On 22 Mar, 08:38, minfitl...@yahoo.co.uk 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.

Steve

Andreas Huennebeck wrote:
> minfitl...@yahoo.co.uk wrote:
> > What is considered real-time? How much latency would be necessary for
> > it to be no longer real-time?
>
> Real time is when the scheduler has a specified maximum context switch
> time.

Not every real-time system has a "scheduler" (in fact, a real-time
system doesn't even have to be a program). This definition is
unnecessarily restrictive.

On 22 Mar, 10:15, "Andor" <andor.bari...@gmail.com> wrote:
> Andreas Huennebeck wrote:
> > minfitl...@yahoo.co.uk wrote:
> > > What is considered real-time? How much latency would be necessary for
> > > it to be no longer real-time?
>
> > Real time is when the scheduler has a specified maximum context switch
> > time.
>
> Not every real-time system has a "scheduler" (in fact, a real-time
> system doesn't even have to be a program).

Ours is not. There is a crew of people who take part
in the analysis, using various programs and systems.
The 24hr limit still stands.

Rune

Rune wrote:
> Andor wrote:
> > Andreas Huennebeck wrote:
...
> > > Real time is when the scheduler has a specified maximum context switch
> > > time.
>
> > Not every real-time system has a "scheduler" (in fact, a real-time
> > system doesn't even have to be a program).
>
> Ours is not. There is a crew of people who take part
> in the analysis, using various programs and systems.
> The 24hr limit still stands.

What is the in- and output of your system?

Steve Underwood <steveu@dis.org> writes:

> Rune Allnor wrote:
>> On 22 Mar, 08:38, minfitl...@yahoo.co.uk 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. 
-- 
%  Randy Yates                  % "The dreamer, the unwoken fool - 
%% Fuquay-Varina, NC            %  in dreams, no pain will kiss the brow..."
%%% 919-577-9882                %  
%%%% <yates@ieee.org>           % 'Eldorado Overture', *Eldorado*, ELO
http://home.earthlink.net/~yatescr

On 22 Mar, 14:10, "Andor" <andor.bari...@gmail.com> wrote:
> Rune wrote:
> > Andor wrote:
> > > Andreas Huennebeck wrote:
> ...
> > > > Real time is when the scheduler has a specified maximum context switch
> > > > time.
>
> > > Not every real-time system has a "scheduler" (in fact, a real-time
> > > system doesn't even have to be a program).
>
> > Ours is not. There is a crew of people who take part
> > in the analysis, using various programs and systems.
> > The 24hr limit still stands.
>
> What is the in- and output of your system?

These are sub-sea survey systems. The sonars are mounted
on ROVs, and data are logged top-side. From one section
of sea floor hase been finished surveyed, we have 24 hrs
to process the data and hand over first-stage products,
like shaded relief images of the sea floor, and certain
other specifics, to the client's representatives.

For all practical purposes, we do a full processing
within those 24 hrs. The stage 2 products are merely
re-plotting of the same data, and merging several
sections of data into one presentation.

Rune