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The blocking (artifacts) usually is a result of a slow bitrate being used for the action going on or a poor encoder was used.
The explanation given already about VBR is very good. Here are the details. When using VBR you would set a MAX rate, a MIN rate and a TARGET rate. The first pass does nothing more than analyzes the amount of data changing between each frame and makes notes to itself. (analogy of course). The faster the action, the more data will be changing between each frame. Because of this, the encoder will see that and change to a faster bitrate. It will go all the way to the MAX setting that you gave it. If the action is slow, the encoder will see that too because the data isnt changing that much or as fast so it lowers the bitrate needed. This is where you gain huge compression. Thus the term VBR.
Once the first pass is done, the encoder will go back and start to actually compress the data based on the notes it took earlier from the first pass. Thats why it takes twice as long to do a 2-pass VBR encoding job. The advantage is that you get maximum compression with maximum quality. The only disadvantage is that it takes a long time. The solution to that is to encode overnight like I always do. The amount of compression and the quality can be tweaked by just adjusting the MIN, MAX and TARGET rates within your encoder settings. (That is if you can)
CBR just takes the data and compress it to the setting you told it to do. There is no need for 2 passes because you made the encoders mind up already. The advantage is that it encodes faster, but your compression will be poor. As far as quality, if youre around 6000 to 7000 it will be about as good as you can get or notice anyway. It also depends on whats on the movie file too. If youre filming a high school play you could probably get away with 4000 maybe even 3000 and it would look fine. Doing a NASCAR race with those low settings would be a different story. There you better take it up to the ceiling which is 8000.