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Topic: -q n switch (Read 4793 times) previous topic - next topic
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-q n switch

Hello, i'm new to the board.

I know that Lame's -q n switch is to determine how long the encoder spending its time processing data. When I did not specify this switch, Lame seems to choose a big number (like 4) if I use high bitrate (eg. 320kbps), and lower number (like 2) if I use medium bitrate (eg. 160kbps). My question: If I use -q 2 for 224kbps, will it yield "better" result than, let's say, -q 4 224kbps? In high bitrates, does the longer encoding time provide any benefits?

Thanks.
twitter.com/pika2000

-q n switch

Reply #2 –
I already know about the presets. My question is for my own curiosity. For example, a 320kbps CBR with -q 5 vs -q 2. Will the one with -q 2 still yield better quality then the faster -q 5?
twitter.com/pika2000

-q n switch

Reply #3 –
Yes. -q 2 used to be the best because -q0 and -q1 were "broken". As it stands now, 0 and 1 work properly as far as I know, but 2 is viewed as the most worthwhile compromise between encoding quality and speed. Settings such as -q 7 and -q 9 are meant for speed.

-q n switch

Reply #4 –
The best place to look is in the code itself:
From the 3.96 code (libmp3lame/lame.c lines 273 to 401) abbreviated:
Code: [Select]
    switch (gfp->quality) {
    case 9:            /* no psymodel, no noise shaping */
        gfc->filter_type = 0;
        gfc->psymodel = 0;
        gfc->quantization = 0;
        gfc->noise_shaping = 0;
        gfc->noise_shaping_amp = 0;
        gfc->noise_shaping_stop = 0;
        gfc->use_best_huffman = 0;
        break;
...   
    case 7:            /* use psymodel (for short block and m/s switching), but no noise shapping */
        gfc->filter_type = 0;
        gfc->psymodel = 1;
        gfc->quantization = 0;
        gfc->noise_shaping = 0;
        gfc->noise_shaping_amp = 0;
        gfc->noise_shaping_stop = 0;
        gfc->use_best_huffman = 0;
        break;
   ...
    case 2:
        gfc->filter_type = 0;
        gfc->psymodel = 1;
        gfc->quantization = 1;
        if (gfc->noise_shaping == 0)
            gfc->noise_shaping = 1;
        if (gfc->substep_shaping != 0)
         gfc->substep_shaping = 2;
        gfc->noise_shaping_amp = 1;
        gfc->noise_shaping_stop = 1;
        if (gfc->subblock_gain == -1)
            gfc->subblock_gain = 1;
        gfc->use_best_huffman = 1; /* inner loop */
        break;
  ...
    case 0:
        gfc->filter_type = 0; /* 1 not yet coded */
        gfc->psymodel = 1;
        gfc->quantization = 1;
        if (gfc->noise_shaping == 0)
            gfc->noise_shaping = 1;
        if (gfc->substep_shaping != 0)
         gfc->substep_shaping = 2;
        gfc->noise_shaping_amp = 2;
        gfc->noise_shaping_stop = 1;
        if (gfc->subblock_gain == -1)
            gfc->subblock_gain = 1;
        gfc->use_best_huffman = 2;
        break;
    }

I took out a bunch of stuff there to make it shorter, so you should go look at the whole thing yourself. This should give you a good idea of what the -q settings do. For real-world effect on quality, you will have to trust your ears.

-q n switch

Reply #5 –
I think in newer lame versions, q values below 2 just use more aggressive lossless encoding of the lossy compressed data in order to save a few percent, IIRC.  So theres not much sense in useing them if you just want the transparency.

-q n switch

Reply #6 –
Quote
I think in newer lame versions, q values below 2 just use more aggressive lossless encoding of the lossy compressed data in order to save a few percent, IIRC.

I thought so too, until I took a look at the code. As far as I can see from the LAME source, the only difference between -q1 and -q2 is -q1 sets noise_shape_amp to 2 instead of -q2's setting of 1. This is used in the (lossy) quantize step, so it appears as though -q1 over -q2 does have an effect on the lossy coding stage. I have no idea what the effect on transparency would be. From 3.96 liblame/quantize.c:
Code: [Select]
 
*  Three algorithms:
 *  noise_shaping_amp
 *        0             Amplify all bands with distort[]>1.
 *
 *        1             Amplify all bands with distort[] >= max_dist^(.5);
 *                     ( 50% in the db scale)
 *
 *        2             Amplify first band with distort[] >= max_dist;
 *
 *  For algorithms 0 and 1, if max_dist < 1, then amplify all bands 
 *  with distort[] >= .95*max_dist.  This is to make sure we always
 *  amplify at least one band.  

-q0 over -q1 changes the behaviour of the Huffman coding step, which is lossless. It doesn't seem the change offers a major increase in compression and seriously slows things down, so it's probably not that beneficial.

I might be wrong about this - I have only recently started working on understanding the LAME source, so I may be seriously misguided. Maybe Gabriel can tell me if I am right.

-q n switch

Reply #7 –
It's what you hear that counts.  (Or maybe what you ABX.)  I can hear a noticable difference between "--preset cbr 128" and --preset cbr 128 -q0" (or even between -q1 and -q0).  (No, I haven't ABX'd it).  Cymbals seem to be cleaner and there seems to be significantly fewer "garbled" noises.

Of course, this is CBR 128. 

I haven't tried -q0 it with --preset standard yet because I don't think I'll be able to tell the difference.  I'm going to try it with -V5 and --preset medium, as I have heard some garbled or wavering sounds.  I'm not sure if they are real or imagined, of if they are expected (as in: "it's medium bitrate dummy, it's going to happen").  Previous encodings I performed (before I discovered HA) with -q0 at medium bitrates (V5-V3) seemed to mostly take care of these, but I need to do more tests!

Oh, I'm using LAME 3.96.

-q n switch

Reply #8 –
Quote
I can hear a noticable difference between "--preset cbr 128" and --preset cbr 128 -q0" (or even between -q1 and -q0).  (No, I haven't ABX'd it).


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