The bit rate is the main factor affecting the quality. The higher the bit rate - the higher the quality. But in addition to the choice of bitrate, we have a choice of algorithms for best scale factors and Huffman coding.
-q 0: use the slowest and the highest quality algorithms. -q -q 0 and 1 are slow and can not provide a significant increase of quality.
-q 2: recommended. Similarly -h key.
-q 5: good speed, reasonable quality.
-q 7: Same key -f. Very fast encoding of acceptable quality. Psychoacoustics is used to pre-echoes and M / S, the noise masking is not performed.
9 -q disables almost all algorithms including psychoacoustic model. Low quality.