LAME -q switch

When using LAME from the command line, the switch -q (quality) can be added in either VBR or CBR mode to alter various parameters used when encoding, which are predicted (at least in theory) to affect the quality of the output MP3 stream.

Equivalences

CBR

Some values of -q seem to produce equivalent files, at least in terms of the audio itself.

• -q5 and -q6 output identical results. LAME's header will indicate the different quality parameter, but the files, including the actual audio, are otherwise a perfect match.
• -q8 is always mapped to -q7 and thus outputs identical files, including the header.
• -q9 outputs the same file as -q7 and -q8, although the official documentation describes it as a different quality. The LAME header will be different for CBR/ABR and identical for VBR.

VBR

VBR encoding modes take some different code paths and ignore some of the listed values. In particular, the ten values of -q are mapped to just 3 values under VBR as of 3.98 and later:

• -q7 to -q9 are mapped to -q7. This level uses a psymodel but does not calculate quantisation noise when encoding: it takes a quick guess.
• -q5 and -q6 include all features of -q7, calculate and consider actual quantisation noise, and additionally enable subblock gain.
• -q0 to -q4 include all features of higher numbers and additionally use the best search when applying Huffman coding.

Effects

This table documents the parameters that are affected by the value assigned to -q. These are most relevant to CBR for the reasons noted above.

-q n	Huffman coding search	Use full outer loop for Huffman encoding	sv_qnt.substep_shaping	Subblock gain	Noise-shaping	Amplification	Stopping	Description
0	Best	Yes	2	1	Yes	2	1	Use all the slowest parameters/algorithms that are available.
1	Best	No	2	1	Yes	2	1
2	Best	No	2	1	Yes	1	1	Same as -h. Higher quality than the default -q3. -q2 was the default in older versions of LAME.
3	Best	No	?	1	Yes	1	1	Default value. Good speed, good quality. Lower values of -q may not produce significantly higher quality.
4	Best	No	?	1	Yes	0	0	Optimal speed/quality. Usually produce less distortion at high frequencies than all other values of -q, except 5-6.
5-6	Normal	No	?	1	Yes	0	0	Almost the same as -q4.
7-8	Normal	No	?	-1	No	0	0	Same as -f. Very fast, OK quality. Psychoacoustics are used for pre-echo and mid/side stereo, but no noise-shaping is done.
9	Normal	No	?	-1	No	0	0	Disables almost all algorithms including psy-model. Extremely fast. OK quality.

External links

• Current documentation
• Research & discussion that led to this summary (2013)
• More discussion mainly re: ABR (2014)