what is realtime?

[lordsmurf]

I've seen too many variations of this in the years. I prefer to use common sense math.

Examples...

REALTIME = 60 minutes of video taking 1x60 minutes to encode. Everybody seems to understand this.

2x REALTIME = 60 minutes of video requires 2x60 minutes of process time. In other words, 2x realtime is 120 minutes (2 hours).

4x REALTIME = 60 minutes of video requires 4x60 minutes of process time. In other words, 4x realtime is 240 minutes (4 hours).

½x (0.5x) REALTIME = 60 minutes of video requires 0.5x60 minutes of process time. In other words, ½x realtime is 30 minutes.

In the world of video, you are likely going to take LONGER THAN REALTIME and not less. Why on Earth would you complicate yourself with tons and tons of fractions, when the likelihood of using them is nil.

One of my video gripes.

[JohnnyMalaria]

I think the common usage/useage is a short-hand expression for a statement such as:

"Encodes at twice the rate of realtime"

Psychologically, stating 2x is more "impressive" than 0.5x, even if mathematically inaccurate....

[Marty2003]

If you consider "REALTIME" to be a speed, rather than a measure of time, 2X would indicate it's twice as fast as "REALTIME", hence take half as much time. This way just seems more natural, since CD's and DVD's are use this notation for speed.

[daamon]

If you consider "REALTIME" to be a speed, rather than a measure of time, 2X would indicate it's twice as fast as "REALTIME", hence take half as much time. This way just seems more natural, since CD's and DVD's are use this notation for speed.

I agree, it can be seen this way too. Saying "2X realtime" can imply twice as fast:

If realtime = 1 min of video in 1 minute of encoding, then

2X realtime (could be taken as) 2 mins of video in 1 minute of encoding.

The problem is that realtime is defined, but the use of 2X (or any other number) either isn't defined, or isn't well known. Either way, the confusion now exists such that it can be taken either way.

However, Marty2003 has a good point when referring to CD's and DVD's, as these have set an unofficial, though commonly accepted, definition whereby 2x = twice as fast.

Just alter your definition lordsmurf and you'll be fine...

[ZippyP.]

Real time to me has always meant a rate, like frames per second.

[lordsmurf]

since CD's and DVD's are use this notation for speed.

CD/DVD speed "x" ratings are for the revolutions, not the minutes. The disc spins "2x as fast". It does not refer to time.

[Nelson37]

The expression "realtime" has always been taken as a measure of speed, SFAIK. Processing used to take longer than the input. Realtime processing represented an increase in speed. Faster than realtime means a further increase in speed.

So processing a 60 minute movie in "4X Realtime" would finish in 15 minutes.

[AlanHK]

The problem is that TIME is measured in seconds; but SPEED is measured in ... PER seconds; the reciprocal. So twice as fast = half the time.

People often get into a mess when using superlatives referring to something smaller numerically, saying things like "200% smaller" which are gibberish or mean quite the opposite to what the speaker intends. People seem afraid to use fractional ratios: "it's one half the size", "50% smaller".

Using the term "realtime" is fine when it's 1:1, it doesn't matter whether you're talking about time or speed. But you can see most people just assume "X" relates to speed, following the conventional rating of drives. If you say "2X realtime", you're technically clear, but will still confuse many.
If you use a percentage you'll probably be understood:
"this encoded at 250% realtime".

[lordsmurf]

People often get into a mess when using superlatives referring to something smaller numerically, saying things like "it's ten times smaller", or "200% smaller" which are gibberish or mean quite the opposite to what the speaker intends. People seem afraid to use fractional ratios: "it's one tenth the size", "50% smaller".

My point exactly. 8)


I think. It's late.

[AlanHK]

My point exactly.

(I revised my post after you read it.)

The question isn't whether you're right, it's whether you're understood. When a phrase has acquired a conventional meaning, you can't change the perception, unless you want to spend your life explaining, (like Stallman saying it's "GNU/Linux"), you find a word that isn't corrupted yet.

[edDV]

I've been involved with a lot of "realtime" video hardware and software products for either live broadcast or "live to tape". In that context realtime means processing no slower than 1x and in most cases it means completing all your processing within the 1/60th sec field with safety margin to spare.

Real time processing usually means there is a mission critical clock deadline that can't be missed. In video this is the line, field or frame.

Closest thing we do here is real time software MPeg2, divx, wmv, etc. encoding where falling behind the incoming video usually means frame loss. Some buffering is possible so long as the processor can catch up before the buffer fills.

[daamon]

The question isn't whether you're right, it's whether you're understood. When a phrase has acquired a conventional meaning, you can't change the perception.

You've hit the nail right on the head. That's it exactly.

Ask many people what "petrified" means, and they'll say something along the lines of being really scared. In fact, to petrify is to turn to stone. So, people may be so scared that they froze still as if they'd turned to stone, but they didn't (obviously) actually turn to stone.

Commonly understood meaning vs actual correct meaning.