Everything I should say has probably already been said, but here goes anyway.
The 4 MB/minute limit was invented with a modified Harrison&Stetson method, i.e. an arbitrary value was chosen with intuition and verified against a few cases, to determine whether it is a good limit.
For different consoles, the average varies. Actually, why not do the math here.
mysql> select s.abbr, min((filesize/1e6)/(length/60))mi, avg((filesize/1e6)/(length/60))av, max((filesize/1e6)/(length/60))ma from movie_file mf,movie m,filetype t,system s where mf.movieid=m.id and m.systemid=s.id and t.filetype=mf.typech and filetypeclass='T' group by s.abbr;
+---------+-------------------+------------------+------------------+
| abbr | mi | av | ma |
+---------+-------------------+------------------+------------------+
| GB | 0.572175849272519 | 1.4087359767177 | 2.33612181818182 |
| GBA | 1.34804613861386 | 2.8145274068148 | 6.20754240674907 |
| GBC | 0.767979931209014 | 1.2569075060325 | 1.8114107364037 |
| Genesis | 0.975704864864865 | 2.9559998754507 | 6.30503649295637 |
| N64 | 2.35566726298707 | 2.9585978329765 | 3.87445178347063 |
| NES | 0.646402777777778 | 2.1354393788815 | 9.8144029716529 |
| SGB | 0.60576729658376 | 0.90576560167766 | 1.43155425373134 |
| SMS | 0.870126857142857 | 1.6502879425395 | 2.4848148482854 |
| SNES | 1.1508051414501 | 2.8344977178136 | 5.48289923076923 |
+---------+-------------------+------------------+------------------+
The above is for all publications, including obsoleted AVIs. Here's the same table for currently published movies.
+---------+-------------------+-----------------+------------------+
| abbr | mi | av | ma |
+---------+-------------------+-----------------+------------------+
| GB | 0.572175849272519 | 1.3884989196617 | 2.33612181818182 |
| GBA | 1.38530359840954 | 2.9467362692611 | 6.20754240674907 |
| GBC | 0.767979931209014 | 1.2139071291976 | 1.8114107364037 |
| Genesis | 0.975704864864865 | 2.8845012522829 | 6.30503649295637 |
| N64 | 2.57872355200365 | 3.0857642439087 | 3.87445178347063 |
| NES | 0.661616585365854 | 1.837647967182 | 9.8144029716529 |
| SGB | 0.84024 | 1.0699646815687 | 1.43155425373134 |
| SMS | 0.870126857142857 | 1.714521622153 | 2.4848148482854 |
| SNES | 1.1508051414501 | 2.724627874046 | 5.48289923076923 |
+---------+-------------------+-----------------+------------------+
This graph tells us, that for every console, the average encoded AVI/MKV/whatever size has been about 2 MB/minute, give or take some 1.1 MB/minute. For SNES, GBA, Genesis, and surprisingly, NES, exceptions have been made sometimes over the 4 MB/minute limit, with the highest bitrate for SNES having been 5.5 MB per minute.
These exceptions usually need to be made for movies with lots of full-screen movement that is not compensated by either motion estimation or frame history referrals, because the screen contents change entirely, and for movies that contain colour schemes that are particularly difficult because of chroma supersampling (for example, black objects on a blue or red background).
In general, the visual and aural quality of our AVIs has been very good. ShinyDoofy's most recent encode of Shinryuu's Mega Man 6 movie is an outstanding proof of this claim. Even with a trained eye, I cannot find encoding artifacts in his video with casual inspection. His movie uses 2.2 MB per minute.
There have indeed been some movies with not so good quality. In those cases the blame lies either on the too modest bitrate that the encoder used, or on too lazy settings the encoder used for the fear of consuming too much CPU time in the encoding.
We encourage our encoders to use the strongest possible encoding settings of the codec in order to produce the best quality per bit value. The MEncoder mantra that I use ifor my encodings with x264, is this:
partitions=all:me=umh:frameref=15:me_range=64:subme=7:mixed_refs:trellis=2:ratetol=60:subq=7:direct_pred=auto, together with multi-pass encoding to produce the most optimal distribution of bits within the file. This does not even include B-frames. I don't think B-frames are particularly useful with NES, but they may be useful with 3D content on N64 and PSX (and vector/scaledbitmap content on SNES and GBA).
We also encourage our encoders to visually inspect their result and to see if there are encoding artifacts that should be addressed (by changing the codec settings and re-encoding) before the AVI is published.
In the rare cases that a larger bitrate is indeed needed -- this happens usually for racing games and for some 3D games -- a developer of the TASVideos site needs to be contacted by the publisher to manually disable the limit temporarily for the duration of filling the publication form. For now, the only person that can do that is me, but I'm hoping to change that fact soon.
Regarding Johannes's claim that the ratio is too low for N64...
+---------+-----------------------------------+------------------+
| movieid | gamename | mbpermin |
+---------+-----------------------------------+------------------+
| 1029 | Tony Hawk's Pro Skater 3 | 3.87445178347063 |
| 1190 | Super Smash Bros. | 3.82257774859752 |
| 1211 | Mischief Makers | 3.62135403109398 |
| 975 | 007 - The World is Not Enough | 3.49640452173913 |
| 886 | Wetrix 64 | 3.43624058924595 |
| 1229 | Legend of Zelda - Ocarina of Time | 3.35597524190427 |
| 530 | Tetrisphere | 3.30907050706493 |
| 957 | Duke Nukem 64 | 3.2286800456013 |
| 1197 | Super Mario 64 | 3.14568689450763 |
| 1238 | Chameleon Twist | 3.10092146788991 |
| 1007 | Goldeneye 007 | 3.02810797342193 |
| 962 | Glover | 2.96598544 |
| 963 | Legend of Zelda - Majora's Mask | 2.81573428140494 |
| 808 | Gex 64 - Enter the Gecko | 2.79031657176988 |
| 946 | Diddy Kong Racing | 2.77021529065256 |
| 850 | Super Mario 64 | 2.72489028646749 |
| 759 | Mortal Kombat 4 | 2.70063864713825 |
| 576 | Legend of Zelda - Ocarina of Time | 2.69789365087056 |
| 1095 | Turok - Dinosaur Hunter | 2.68556397802107 |
| 1237 | Gex 3: Deep Cover Gecko | 2.65161661921689 |
| 1229 | Legend of Zelda - Ocarina of Time | 2.57872355200365 |
+---------+-----------------------------------+------------------+
Is it indeed the limit that has been too low, or has it been the encoders who have been too modest? I guess Tony Hawk's World Is Gray could have used an exception. Super Smash Bros is a suspect. Zeldas and Marios look quite fine, though if they needed to be encoded at a higher rate, it could be done without offending the threshold.
My verdict is: Thus is good.
