The amount of activity in an A/V stream can vary. For instance, if the data being streamed is from an empty, silent room, there is much less going on than if the data is something like a loud and explosive video game.
What I am wondering is whether the actual amount of data going up and down differs depending on this subjective interpretation of “activity”. In other words, am I downloading less data when watching a stream of the empty room versus the active video game? My hunch has always been a resounding “no”; after all, how would the program know the difference between the two?
I’m asking now, though, because I’ve noticed a difference when streaming video in the past. The video always seems to be fine during periods of subjectively “low” activity, and it begins to lag or skip during periods of “high” activity. Is this just coincidence, or is there actually some kind of algorithm or service in place which dilutes data in periods of low activity or something like that?
Well, the thing is that audio and video streams are compressed. They can be compressed with any one of a whole range of formats. Some formats will aim for a % reduction in size, some will set a quality value, others will perform the same steps whether the data is simple or complex.
Take for example the jpg and png formats. Open up your favourite editor and create a 640x480px image, filled with pure white. Now save that file and look at it’s size. Now apply noise to the image and save it as a new file. Compare the two – see the huge difference in size..
I got 1.37kb for the white image, 331kb for the noisy one. (a single 8×8 or 16×16 tile may be repeated for the entire white image, unique 8×8 or 16×16 blocks must be used for the noisy one)
VBR (variable bit rate) and CBR (constant bit rate) are two frequently used terms when video transcoding (changing from one format to another)
Anyway – the answer is ‘it depends on the format’ – some formats do work like that, some don’t.
The video card is always sending the same quantity of data to the screen each frame, even if there is very little information in it – it’s uncompressed. Transmitted audio and video on the other hand are (almost) always compressed, so when there’s less information, it takes less data to convey it.