-V2 gives way too high bitrate!?! Options

[masterridley]

I use dbpoweramp and I encode flac tracks to LAME 3.98r -V2. Usually this results in ~170 to ~220 kbps files (close to 192kbps CBR quality)

Well, not so for 2 As I Lay Dying albums that I have (think REALLY LOUD metal). When I encode those I get 230-270 kbps and of course the resulting files are quite big.

My question is: is this a bug or completely natural?
Furthermore (if it's not a bug), isn't it just better to encode in 192kbps and get a smaller file, since that's the quality I was going for anyway?

Cheers

[shadowking]

I suggest not to fiddle with -Y . If you want a lower bitrate and still keep reasonable quality: V3
V2 - Y is hardly better than V3 on problem samples. Also -Y will totally change the mp3 bitrate distribution and quality is affected - whether you hear it or not.

Another approach would be -V2.99 ~ -V2.5 as you still encode HF content but less aggressively.

The V3 is a more efficient method than cbr 192k encoding, it isn't restricted. It will use 200k when needed and drop to 160k when not.

This post has been edited by shadowking: Mar 30 2010, 11:21

[Aleron Ives]

I suggest not to fiddle with -Y . If you want a lower bitrate and still keep reasonable quality: V3
V2 - Y is hardly better than V3 on problem samples. Also -Y will totally change the mp3 bitrate distribution and quality is affected - whether you hear it or not.

I find this to be a bit counter-intuitive. -V 3 has the Y switch enabled by default, so would it not be true that using -V 2 -Y would still yield superior results to -V 3, since -V 3 is actually using -V 3 -Y? Also, I think I should point out that saying "quality is affected - whether you hear it or not" is rather irrelevant, as the entire point of MP3 is to reduce quality as long as you don't hear it in the name of saving space.

As probably the resident "lay" person in this topic, I will attempt to repeat [JAZ]'s technical explanation, and hopefully it will give some idea as to how effective that post would be in the wiki to answer the -Y question for people who aren't experts. I fully expect this summary to be less technically accurate than his post, but the purpose is theoretically to show how much of the explanation's content was absorbed more-or-less correctly by someone unfamiliar with the inner-workings of MP3.

In bullet form:

• MP3 groups audio data by frequency.
• Frequency range groups are known as bands.
• Bands are quantized to make them compress better.
• "Scale factor" refers to how much quantization (compression) is applied to each band, where higher quantization causes greater compression and consequently less variation between the minimum and maximum values (resolution).
• Each band has its own scale factor, so that its quantization can be adjusted independently from the others.
• Global gain is the quantizer that affects all bands simultaneously.
• The only band without a scale factor is sfb21, which stores frequencies >= 16 kHz.
• Since sfb21 doesn't have a scale factor, if LAME determines that sfb21 needs more resolution, there is no way for LAME to increase the resolution of sfb21 alone, since there is no scale factor.
• The only way to increase the resolution on sfb21 is therefore to reduce the global gain, which then increases the resolution and lowers the quantization of sfb21.
• The side effect of doing this is that since global gain applies to all bands, resolution will be increased and quantization will be lowered on every other band, too.
• The result is that unnecessary resolution is applied to every other band, so the bitrates used in all the other bands will increase, too.
• This means that LAME is forced to increase the bitrate of the entire file just so that the frequencies >= 16 kHz will be adequately quantized.
• -Y, then, does not remove frequencies above 16 kHz like a lowpass would-- it only prevents LAME from reducing the global gain value when the psy-model says it should to achieve the desired quality in the 16 kHz + range. The result is that all the 16 kHz + frequencies still get encoded, but the ones that would normally have needed higher resolution to satisfy the criteria of the psy-model don't receive that treatment, while ones that wouldn't need higher resolution are unaffected by the Y switch.
• In effect, -Y gives you the same global gain that you would have if you used a lowpass of 16 kHz, but you still get to keep the 16 kHz + frequencies. They just get encoded "as-is", without any extra resolution being given to them by lowering the global gain, which is what LAME would do without -Y.

...so, how far off am I?

The one thing that I'm still hoping someone will further enlighten me on is: does CBR mode employ a tactic like the Y switch by default? That is to say, if you encode at a CBR bitrate where the lowpass filter is above 16 kHz, does LAME focus its efforts on the scale factor bands other than sfb21, so that since the available bits are restricted, it can apply what's available to the bands that are deemed more important? If so, I would assume this means that, like with VBR -Y, CBR mode uses lower resolution in sfb21 because it can't afford to lower the global gain to accomodate sfb21's needs, as CBR bitrate limitations mean that LAME has to be frugal in the way it allocates bits.

I continue to bring this up, because I think if my analysis is correct, it might be worthwhile to add the correlation between -Y and CBR to in effect reassure people that if they are generally satisfied with CBR's frequeny range reproduction, they need not fear using -Y in VBR mode, as CBR always has a similar behaviour enabled.