Bitrate Calculation

[roach76]

Just a quickie.

I know that the higher the resolution the more bitrate you need to have less macroblocks. I just want to know if there is a mathematical way you can figure out what would be the lowest bitrate you could use for a certain resolution to get NO macroblocks at all?

Is there a difference in quality between a DVD (with very little high motion scenes) at a bitrate of 7500kb/s and 8000kb/s, if no then that would be because the extra bitrate was not needed and was wasted, right?

[adam]

There's no way to determine at what settings a macroblock, or other artifact, will occur because it is largely determined by the source. Some scenes may perform perfectly well at 1mbit and a fairly large resolution, while others may perform badly even at 9mbits.

The closest mathmatical analysis you can make is to analyze bits per pixel at different resolution/bitrate combinations. This is just simple math...

Multiply your horizontal resolution and your vertical resolution to get the total number of pixels in each frame, so if you use 480x480 then you have 230400, as opposed to 352x240 which yields 84480.
Now multiply this by your framerate (frames per second), since we want to know the total amount of pixels in each second. So an ntsc SVCD has 6905088 pixels each second...480x480x29.97.

Now take your avg bitrate and divide it by the total number of pixels each second and you have the bits per pixel ratio. You can now compare different resolution/bitrate combinations meaningfully.

Just in general if you use 352x240/288 you probably want to aim for an avg bitrate of 1.4mbits or higher to prevent most macroblocks, though this does not conform to either vcd or svcd standard.

If you use 480x480/576 you will probably need an avg bitrate of at least 1.8mbits. With the CVD compliant resolution of 352x480/576 you can use slightly lower.

Anything higher than 480x480/576 and you will need an avg of probably more than 3mbits.

The validity of these numbers are of course highly subjective though.

Regarding your dvd question, very few dvds use bitrates this high. Most use an avg bitrate between 5mbits and 6mbits with Superbit dvds being the obvious exception. Anyway, again its all dependent on the particular scene. The extra 500kbits probably won't make any noticable difference at bitrates that high, but some scenes might benefit from it. Even dvds are highly compressed media formats so compared to a flawless souce, no bitrate is ever wasted. However, at 720x480/576 its true that throwing more bitrate at the encode probably won't make much improvement in most scenes. In this sense I suppose that often bitrate is wasted on commercial dvds but its better than simply leaving that space empty on the media.