Hi, I've already seen this answer before, but I think it's not understandable for a standard user, not knowledgeable of the basics of film compression.
And the (very) basic idea is that: - a film is made of images following each others - all codecs somehow compress each image on its own - most modern codecs additionally do store one image and then the differences between the images; i.e. instead of storing images A B C..., they store A diff[A-B] diff[B-C]... As differences are (generally) smaller than the image itself, you spare quite a lot of space. - add to this the fact that a film in B&W contains less information than a film in color. This explains why a quiet film in B&W should be way smaller than an action film with psychedelic colors flying around the screen, even if it has the same length and resolution. What you have moreover to understand is that the 1830kb/s has to be understood as a maximum bitrate (possibly a maximum average bitrate, if you understand what I mean; if not, forget about it). This means that the codec will use as much of it as possible, but if there is nothing to encode, well, there will be nothing encoded. For example, if the film would be 3660kb/s uncompressed and the codec would manage to compress it down to 33% = 1220kb/s without loss of quality, it wouldn't create extra 17% of information just to reach the 1830kb/s you've set, would it. What does it mean practically? - if you set a certain target bitrate and you're happy with the quality of the result, don't bother if the film is "too small". - if a codec doesn't use the whole bitrate you allowed him, it most probably means that you got the best quality you can ever reach with this codec. If you're not happy with the quality, try another codec. Does it make it clearer? I'm happy with the result, but I tend to understand what I'm writing ;-) Cheers, Eric > Luke Sharkey wrote: >> >> For example, a dvd I was trying to rip, "12ANGRYMEN" (brilliant classic >> b&w >> movie), which I set to be 1300Mb (working out to be a bitrate of >> 1830kbits/second). I checked the maths and as it was a movie 92 minutes >> long, it should have transcoded into a 1300Mb at that bitrate (minus >> 86Mb >> for the audio file). However, the actual file ended up being only >> 930Mb! >> This is most unfortunate. > > Has it occurred to you that there is simply nothing more > to encode than that 930Mb? Twelve Angry Men is a courtroom > drama with practically no action... Where do you expect the > bits to go? > > > -- Eric de France, d'Allemagne et de Navarre
