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Old 19th May 2008, 13:37   #1  |  Link
Vivaldi
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Bitrate, Framerate and Container definition?

Hi,

Can you please point me towards a reference guide?

- Framerate is easy to understand, but what is the optimal value? I see values of 23, 25 and 30. Why are exactly these value?

- What is bitrate (of the video). Is it the result after selecting a resolution and framerate, or is it another factor and what is the influence of bitrate on the quality.

- Container: I prefer DivX or XVid because the video can be played on a DivX DVD Player. This way I can watch the video on my TV instead of computer. Some authors use MKV, MPEG, OGM. How are they compared to DivX / XVid ?

Thanks in advance for any help.
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Old 19th May 2008, 14:21   #2  |  Link
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Quote:
Originally Posted by Vivaldi View Post
- Framerate is easy to understand, but what is the optimal value? I see values of 23, 25 and 30. Why are exactly these value?
Framerate = Number of Frames per Second (FPS)

In theory a higher framerate (more frames per second) results in more fluid motion.
Nevertheless at a certain point (around 50 fps) the human eye is too slow to see the difference.
Also there are a number of "standard" framerates, we use in video broadcast today.

So it all depends on how the video was filmed and how it is delivered/broadcaster.
For example PAL TV (as used in Europe) is 25 fps, NTSC (as used in the US) is 29,97 fps and Cinema is 24 fps.
This has historical reasons...

There are various ways to convert 24 fps film material to PAL or NTSC for TV broadcast.
But usually there is no reason the change the framerate, except for converting between different TV standards.
You can simply edit the video "as-is" and that's it.

Quote:
Originally Posted by Vivaldi View Post
- What is bitrate (of the video). Is it the result after selecting a resolution and framerate, or is it another factor and what is the influence of bitrate on the quality.
Bitrate = Number of (Kilo)Bits per Second (kbps)

The bitrate usually can be selected totally independent from the framerate and resolution of the video!
Nevertheless the bitrate has a huge impact on the quality of the video - the higher the bitrate, the higher the quality.
But a higher bitrate unavoidably results in a bigger file! So your goal is: Find the lowest possible bitrate that still gives "acceptable" quality.

Unfortunately there is no general rule to calculate the "optimal" bitrate. The bitrate needed to keep "good" quality differs from video to video!
For example a video containing only a black screen won't need much bitrate, but a video of random noise will need a lot of bitrate.
Generally "real life" footage (with lots of textures) needs more bitrate than anime and cartoons.
Of course a video with higher framerate and/or higher resolution will usually need more bitrate to retain "good" visual quality.

Furthermore, the bitrate highly depends on the video format you use!
For example MPEG-4 ASP (DivX/Xvid) is more efficient than MPEG-2. And H.264/AVC (x264) is much more efficient than MPEG-4 ASP.
The more efficient the video format compresses, the less bitrate is needed to keep the same visual quality.
And it also depends on the individual encoder: a "sophisticated" encoder can squish out better quality at the same bitrate than a "dumb" one.

Last but not least the definition of "quality" differs from person to person.
So what some people may consider a "good" quality, might be considered "ugly" quality by others. And vice versa!
This makes it even harder to (objectively) choose the right bitrate...
Quote:
Originally Posted by Vivaldi View Post
- Container: I prefer DivX or XVid because the video can be played on a DivX DVD Player. This way I can watch the video on my TV instead of computer. Some authors use MKV, MPEG, OGM. How are they compared to DivX / XVid ?
DivX and Xvid are Encoders, they are not video formats and they are not containers!
Both, DivX and Xvid, implement the "MPEG-4 ASP" Video Format, as specified in the MPEG-4 standard.
So the result will be an MPEG-4 ASP bitstream, no matter which Encoder (DivX, Xvid or another one) was used to create it!
In contrast the "x264" Encoder implements the "AVC/H.264" Video Format, so it's not compatible to MEPG-4 ASP (e.g. Xvid or DivX).

AVI, MKV, MP4 and OGM are not Video Formats, they are Conatiners!
Containers specify how to mux Audio and Video streams together into one file, so they can play synchronously.
MPEG-4 ASP (e.g. DivX or Xvid) Video Streams can be stored in an AVI container as well as in an MP4 or MKV container.
With AVC/H.264 it's a bit more tricky, so AVI might not be the best solution for H.264 streams...

So to sum up, there are three things you have to distinguish:
* Video Format (e.g. MEPG-2, MPEG-4 ASP, AVC/H.264, etc.) - This is specification and documents, general Standards.
* Video Encoder/Decoder aka "Codec" (e.g. DivX, Xvid, x264, etc.) - This is a specific piece software and usually implements a certain Format/Standard.
* Container Format (AVI, MP4, MKV, OGM) - This is a file format, specifying how the Audio/Video streams are stored inside a file.

Dozens of combinations are possible. For Example:
* An "MPEG-4 ASP" Video can be encoded with the "DivX" Software and then the resulting stream can be stored in an "MKV" container.
* Alternatively you could encode your "MPEG-4 ASP" Video using the "Xvid" Software and then store it in an "AVI" container.

Those terms are mix-up up often, which can be pretty confusing. So you have to take care, especially when companies want to sell their stuff!
In the end, it all depends on what software you prefer (usability and quality wise) and what formats your (hardware) player supports...
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Last edited by LoRd_MuldeR; 19th May 2008 at 15:10. Reason: typo
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Old 19th May 2008, 15:43   #3  |  Link
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Hi LoRd_MuldeR,

I truely appreciate your expert answer. Thank you for the clarification about encoder / container that I was confused about. I hope you can help me to better understand the video bitrate. I have made a following conversion test:

- Source: A BBC documentary, 45 minutes, DivX5, 1280x720, 30 FPS. Filesize: 1.37 GB

- Output: 640x360, 25 FPS, XVid. Same audio settings. The point is to reduce the file size and also to reduce the hardware requirement for play back.

- I used:
1- SUPER 2008 build 30 (http://www.erightsoft.com/SUPER.html)
2- Any Video Converter 2.59 freeware version (http://www.any-video-converter.com/p...or_video_free/)

And I compared the output of these two softwares. In short Any Video Converter 2.59 produced a file of 290 MB. SUPER's output is 366 MB and the encoding time is roughly double (didn't time accurately but it seems significantly longer than AVC).

I compare the video info of the two test AVIs by using MediaInfo 0.7.7.0 (http://mediainfo.sourceforge.net/en). The main differences are:

1- SUPER opts for a video bitrate of 1007 Kbps (AVC is 768 Kbps)

2- SUPER uses a resolution slightly bigger (640x368 instead of 360)

3- SUPER opts for MP3 48K, CBR (AVC: 44K VBR)

Among these factors, I suppose that the video bitrate is the main reason of bigger filesize and longer encoding time. Supposedly, the AVI output by SUPER should be of higher quality. This could be but I can't really say that it is really much better. My eyes are not the best and I am no video expert. So from my perspective, the video quality between SUPER and AVC is nearly identical. So then I would be attempted to opt for a lower bitrate.

Question1: is the encoding time proportional to the video bitrate?

Question2: is there any typical video bitrates per quality rating? Like for MP3 encoding (Low: 128 Kbps, Medium: 192, High: 320)

Of course setting a higher value would be a safer choice at the expense of filesize and encoding time. However, I would rather prefer to avoid wasting resource. For example, for MP3 tracks, I don't "hear" any difference between 192 Kbps and 320 Kbps (of course, this is me and my average ears).

Thanks very much in advance for your time.

Last edited by Vivaldi; 19th May 2008 at 15:49. Reason: add question
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Old 19th May 2008, 15:57   #4  |  Link
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How did you make the source DivX or where did you get it?
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Old 19th May 2008, 16:02   #5  |  Link
Vivaldi
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Quote:
Originally Posted by neuron2 View Post
How did you make the source DivX or where did you get it?
I didn't make the source DivX. It was given to me on a USB memory stick by a friend.
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Old 19th May 2008, 16:52   #6  |  Link
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