FPGA Central

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
--
Andreas Hünnebeck | email: acmh@gmx.de
----- privat ---- | www : http://www.huennebeck-online.de
Fax/Anrufbeantworter: 0721/151-284301
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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

On 22 Mar, 14:24, Randy Yates <y...@ieee.org> wrote:

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

Interesting. Could you elaborate?

Rune

Randy Yates wrote:

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

On the contrary: Latency is exactly what determines whether a system
is real-time or not.

"Andor" <andor.bariska@gmail.com> writes:

> Randy Yates wrote:
>
>> In my opinion any discussion of latency should be removed from the
>> definition of "real-time." These are two "orthogonal" concepts.
>
> On the contrary: Latency is exactly what determines whether a system
> is real-time or not.

Not in the dozens of real-time systems I have been involved with.
--
% Randy Yates % "She tells me that she likes me very much,
%% Fuquay-Varina, NC % but when I try to touch, she makes it
%%% 919-577-9882 % all too clear."
%%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr

On 22 Mrz., 14:41, Randy Yates <y...@ieee.org> wrote:
> "Andor" <andor.bari...@gmail.com> writes:
> > Randy Yates wrote:
>
> >> In my opinion any discussion of latency should be removed from the
> >> definition of "real-time." These are two "orthogonal" concepts.
>
> > On the contrary: Latency is exactly what determines whether a system
> > is real-time or not.
>
> Not in the dozens of real-time systems I have been involved with.

Define "real-time system", then.

Randy Yates <yates@ieee.org> wrote in news:m3hcsdcs5b.fsf@ieee.org:

> Not in the dozens of real-time systems I have been involved with.


Well, you can tell us why, or you can probably string us along for another
4 or 5 posts and make us beg.


--
Scott
Reverse name to reply

Randy Yates wrote:

...

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

I sort of agree with you, but the term has gotten fuzzy. It has become a
synonym for "delay". Real-time systems always accept input and usually
provide output at the input rate. Since zero delay is rarely achieved in
systems that actually accomplish something, it follows that the delay
between input and output must be fixed in one-in-one-out out systems. I
don't choose to argue with those who call that delay "latency" as long
as the meaning is clear.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 22 Mar, 14:56, Jerry Avins <j...@ieee.org> wrote:
> Randy Yates wrote:
>
> ...
>
> > In my opinion any discussion of latency ...
> ... the term has gotten fuzzy. It has become a
> synonym for "delay".

Ah... forgive a non-native English speaker, but were the
two terms ever *not* synonyms? Where is my dictionary,
when I need it...

Rune

"Rune Allnor" <allnor@tele.ntnu.no> writes:

> On 22 Mar, 14:24, Randy Yates <y...@ieee.org> wrote:
>
>> In my opinion any discussion of latency should be removed from the
>> definition of "real-time." These are two "orthogonal" concepts.
>
> Interesting. Could you elaborate?

A real-time system is a system that has a period and a well-defined
maximum amount of work to be done in that period and that can perform
that amount of work in that period.

Latency is input to output delay.
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <yates@ieee.org> % 'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

On 22 Mar, 15:11, Randy Yates <y...@ieee.org> wrote:
> "Rune Allnor" <all...@tele.ntnu.no> writes:
> > On 22 Mar, 14:24, Randy Yates <y...@ieee.org> wrote:
>
> >> In my opinion any discussion of latency should be removed from the
> >> definition of "real-time." These are two "orthogonal" concepts.
>
> > Interesting. Could you elaborate?
>
> A real-time system is a system that has a period and a well-defined
> maximum amount of work to be done in that period and that can perform
> that amount of work in that period.
>
> Latency is input to output delay.

Ah, right. So a TV set is a real time system because of the
frame rate, and all the work that needs to be done between
frames. A satellite TV system is also restricted by the
frame rate; the two-way travel time back and forth to the
satellite doesn't affect that.

Yes, I think you are right.

Rune

Randy wrote:

> A real-time system is a system that has a period and a well-defined
> maximum amount of work to be done in that period and that can perform
> that amount of work in that period.
>
> Latency is input to output delay.

Now think about it: the work a system does is to transform an input
into an output. And the "period" limits the amount of time that the
system may spend on the work, ie. the amount of time the system has
available to transform an input into an output. So, by your
definition, a real-time system is a system that has bounded input/
output latency.

Rune Allnor wrote:
> On 22 Mar, 14:56, Jerry Avins <j...@ieee.org> wrote:
>> Randy Yates wrote:
>>
>> ...
>>
>>> In my opinion any discussion of latency ...
>> ... the term has gotten fuzzy. It has become a
>> synonym for "delay".
>
> Ah... forgive a non-native English speaker, but were the
> two terms ever *not* synonyms? Where is my dictionary,
> when I need it...

"Latent" means not visible or apparent; not yet manifest. Exposure in a
film camera creates a latent image, which becomes manifest when the film
is developed. "Latency" has a more restricted use in computer lingo, but
sot so restricted as to mean a fixed delay. The time from the assertion
of an interrupt to the interrupt routine's beginning is called
"interrupt latency". It is always a range, rather than a fixed time.
"Latency" is a fancy word with a fancy meaning. To refer to a simple
delay, "delay" is better.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

"Andor" <andor.bariska@gmail.com> wrote in news:1174574771.790706.183280
@p15g2000hsd.googlegroups.com:

> Randy wrote:
>
>> A real-time system is a system that has a period and a well-defined
>> maximum amount of work to be done in that period and that can perform
>> that amount of work in that period.
>>
>> Latency is input to output delay.
>
> Now think about it: the work a system does is to transform an input
> into an output. And the "period" limits the amount of time that the
> system may spend on the work, ie. the amount of time the system has
> available to transform an input into an output. So, by your
> definition, a real-time system is a system that has bounded input/
> output latency.
>

I think the deadline definition is better than the latency definition.

Now let's talk "hard" vs "soft" real time!!

--
Scott

On 22 Mrz., 15:50, Scott Seidman <namdiestt...@mindspring.com> wrote:
> "Andor" <andor.bari...@gmail.com> wrote in news:1174574771.790706.183280
> @p15g2000hsd.googlegroups.com:
>
> > Randy wrote:
>
> >> A real-time system is a system that has a period and a well-defined
> >> maximum amount of work to be done in that period and that can perform
> >> that amount of work in that period.
>
> >> Latency is input to output delay.
>
> > Now think about it: the work a system does is to transform an input
> > into an output. And the "period" limits the amount of time that the
> > system may spend on the work, ie. the amount of time the system has
> > available to transform an input into an output. So, by your
> > definition, a real-time system is a system that has bounded input/
> > output latency.
>
> I think the deadline definition is better than the latency definition.

What's the deadline definition?

"Rune Allnor" <allnor@tele.ntnu.no> writes:

> On 22 Mar, 15:11, Randy Yates <y...@ieee.org> wrote:
>> "Rune Allnor" <all...@tele.ntnu.no> writes:
>> > On 22 Mar, 14:24, Randy Yates <y...@ieee.org> wrote:
>>
>> >> In my opinion any discussion of latency should be removed from the
>> >> definition of "real-time." These are two "orthogonal" concepts.
>>
>> > Interesting. Could you elaborate?
>>
>> A real-time system is a system that has a period and a well-defined
>> maximum amount of work to be done in that period and that can perform
>> that amount of work in that period.
>>
>> Latency is input to output delay.
>
> Ah, right. So a TV set is a real time system because of the
> frame rate, and all the work that needs to be done between
> frames. A satellite TV system is also restricted by the
> frame rate; the two-way travel time back and forth to the
> satellite doesn't affect that.

Precisely.

> Yes, I think you are right.

Thank you.
--
% Randy Yates % "Rollin' and riding and slippin' and
%% Fuquay-Varina, NC % sliding, it's magic."
%%% 919-577-9882 %
%%%% <yates@ieee.org> % 'Living' Thing', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

"Andor" <andor.bariska@gmail.com> wrote in news:1174575643.840060.198170
@e65g2000hsc.googlegroups.com:

> On 22 Mrz., 15:50, Scott Seidman <namdiestt...@mindspring.com> wrote:
>> "Andor" <andor.bari...@gmail.com> wrote in
news:1174574771.790706.183280
>> @p15g2000hsd.googlegroups.com:
>>
>> > Randy wrote:
>>
>> >> A real-time system is a system that has a period and a well-defined
>> >> maximum amount of work to be done in that period and that can
perform
>> >> that amount of work in that period.
>>
>> >> Latency is input to output delay.
>>
>> > Now think about it: the work a system does is to transform an input
>> > into an output. And the "period" limits the amount of time that the
>> > system may spend on the work, ie. the amount of time the system has
>> > available to transform an input into an output. So, by your
>> > definition, a real-time system is a system that has bounded input/
>> > output latency.
>>
>> I think the deadline definition is better than the latency definition.
>
> What's the deadline definition?
>
>

You have to accomplish what needs to be done in a certain period of time.
Achieving a maximum value of latency has different connotations than
maintaining a deadline.


--
Scott
Reverse name to reply