Estimating file size -mpeg - slider Speed-Quality

[laurentje]

After the Edit phase i want to make a mpeg-file, this is done by the "Share-tab /Create video/ custon profile - mpg"; it is possible on the "Video Save Option - Compressing " to drag a slider from Speed to Quality (from 0 to 100). Normally, in the standard profiles the value is 70.
What is the exact the meaning of this value? It has an important impact on the quality and on the size of the video-file.
Because I want estimate the size of the file, I think it (the value 70) has something to do with the average bitrate. Perhaps the size of the file is also function of the maximum bitrate (vbr - 2 pass).
I read already the article about calulating bitrates of Gorf, but it helps me not if I do not know the average bitrate.

Mine question is now: how to calculate the average bitrate, base on the (peed/Quality value & the max bitrate value?

[Trevor Andrew]

Hi

The Speed / Quality slider does not alter the file size of the rendered clip.
Afaik it allows the encoder more time to complete the render therefore being more accurate in its render.

The bit rate, video and audio bit rates are the factors regarding file size. Associated with the length of the video. (Kbps)

As a guide:-

8000 kbps is top quality dvd for one hour.
6000 kbps is good quality dvd for 90 minutes
4000 kbps is good quality vhs for 120 minutes.

Each will produce a file size under 4.3 Gb.

Using a bit rate calculator will help, read here
http://dvd-hq.info/Calculator.html

Also read my guide below, Bitrate and file sizes, may give some ideas.

I use constant bit rate, just a personal preference.

Regards

Trevor

[Ken Berry]

Trevor is of course quite right about the slider. It is supposed to represent a balance between speed of processing and the final quality. The default of 70 is supposed to represent an acceptable speed of encoding and a good end quality. Set the quality slider higher, and it will take (much) longer to process, and will be of notionally higher quality (though as Trevor says, size is not affected). Set it lower, processing speed will increase but quality drops off, and becomes a drastic loss the more you lower the number.

Personally, I simply leave the quality slider where it is and I have never been disappointed in the final result.

But it is a matter of personal choice and preference. Some people (many?) claim they can notice with the naked eye a better quality if they increase the number.

Note though that depending on your computer architecture, if you get closer to 100%, some computers have been known to lock up.