Resolution, pixels, bitrate and bytes transferred over network

[loveaks002@gmail.com]

Hi,

When we select low resolution (say 240p) in YouTube, the streaming is faster. Does the low resolution video has less data?
When i read the below article, it says the bytes transferred (size) would depend on the bitrate not resolution.
https://forum.videohelp.com/threads/9671-Relationship-between-size-(pixels)-bitrate-and-file-size

Can anyone tell me why does the streaming gets faster on a slow network when we choose low resolution. In YouTube, we just change the resolution not the bitrate, then how come the streaming is faster.
A general thought which used to come into my mind was - If resolution is decreased, the bytes transferred would be less, that's why the streaming is faster. Let me know what is the correct explanation for this.

[jagabo]

file size = bitrate * running time

So, for a given running time, only the bitrate determines the file size. But larger frames sizes need more bitrate to keep the video from losing quality. So Youtube uses more bitrate with larger frames.

Youtube reencodes everything you upload to it. So even if you upload a small frame video and large frame video with the same bitrate, the videos that everyone sees will be reencoded and the resulting large frame video will have a higher bitrate.

[Cornucopia]

And to add to the good things jagabo just said, when you get streaming files from the internet, on your end they are never received in unbroken, evenly-spaced segments. Instead they are received in packets & bursts of multiple packets, then your PC has a buffer/cache where it gathers them together, so that when you are actually viewing the title it will look smooth and seamless.
The problem with doing it that way is that it relies on a higher QoS (Quality of Service). So, if your connection has a low bandwidth (compared to a higher bitrate file), you can't get those packets fast enough to receive them and maintain your buffer. That means you'll get stops and starts while the buffer takes time to re-fill. With a lower bitrate requirement, you might never run out of your buffer, and you'll experience a nice smooth playback.

Scott

[loveaks002@gmail.com]

Thanks jagabo, Cornucopia..!!
So, do you mean that when we select 240p (which is the lowest resolution in YouTube), there is some effect on the bitrate? Does it increase or decrease?

[jagabo]

240p videos at youtube are encodec with much less bitrate than 1080p videos, for example.

[poisondeathray]

you can see chart of resolution and bitrates
http://en.wikipedia.org/wiki/YouTube#Quality_and_codecs

[loveaks002@gmail.com]

Hi jagabo.. I'm sorry for asking too many questions...
I understood that the 240p will have low bitrate.
1) So, as said by you file size = bitrate * time , the file size would be less right...????
2) So, the resoltion in YouTube has less size right..?? I mean less data is being transferred.??

[jagabo]

1) So, as said by you file size = bitrate * time , the file size would be less right...????

Yes.

2) So, the resoltion in YouTube has less size right..?? I mean less data is being transferred.??

Yes.

[loveaks002@gmail.com]

Thanks jagabo.. Even now i compared the file size in normal speed...and the bytes transferred over network when the bandwidth was less. there was too much difference....
Thank you so much for your detailed replies..

[jagabo]

For example, I have a 1080p video from Youtube that's 520 MB, with an average video bitrate around 5100 kbps. The same video at 240p is 31 MB with an average video bitrate of 250 kbps. Keep in mind there is also the audio and it follows the same rule. But audio is usually much smaller than video and can be ignored for rough calculations. And there's a little container overhead, possibly subtitles, etc. But that's usually less than 1 percent.

[loveaks002@gmail.com]

Hmm..got it..
Is the below thing possible..?? -
can a application has 360p as the resolution and has different bitrates say 1000, 700, 400..??
What i want to ask is the resolution remains the same only the bitrate differs..??

[Cornucopia]

One could do that on one's own website, yes. BUT,

That's NOT what Youtube does.

Scott

[loveaks002@gmail.com]

Thanks Scott.. ya, just gone through one application which uses 360p as resolution...and has 3 diff. bit rates.....but i don't why they call type of video at 1000 kbps as high quality, video at 700 kbps as medium and video at 400 as low quality video...
A video is called High quality,medium quality, low quality depending on the resolution..but in that case everything is 360p....do u have any idea..???

[Cornucopia]

Yes - "Quality" is not just a measure of resolution/sharpness. It often/usually is also a measure of lack of artifacts, trueness of color, etc...

With that in mind, a 360p file (which might be, say 480x360 @ 30fps) has 172,800 pixels in each frame, running at 30fps. That means a pixel rate of 5,184,000 pixels/second. If you have a bitrate of 1000kbps, that give you a Bits-per-Pixel of 0.1975 average bits/pixel. At 700kbps it's 0.1382bpp. At 400kbps it's 0.0790bpp. The lower the bpp, the more the artifacts & lower the quality.

Scott

[loveaks002@gmail.com]

okk..got it...thanks all for clearing some of the doubts...