-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

[[JAZ]]

I am preparing to put this on the wiki (but my user has to be validated still. Also, if someone with more knowledge sees any error in the technical description, feel free to correct me)


The -Y switch of LAME

The short and consice:

It is not:

The -Y switch is not a lowpass filter, nor does it remove high frequencies per-se.
It does not filter, it does not prevent frequencies higher than 16Khz to exist.

It is:

The -Y switch tells the encoder to use a more coarse representation for the higher frequencies, in the parts where it would cause an over-encoding of all the other bands.
The -Y switch tells the encoder to not be so strict with the higher frequencies, *IF* they are going to cause an increase of bitrate.

The technical and know how:

Preface:

MP3 audio is stored in the frequency domain instead of time domain.
The frequencies are then subdivided in several bands.
The values for these bands are quantized to increase their compressability.
The scale factor is the amount of quantization. (higher quantization, less resolution. Resolution means the amount of possible values from minimum range to maximum range)
There is also another quantizer, the global gain, that affects all bands.

The problem, described (based on mp3-tech.org):

The last scalefactor band (sfb21 for long blocks or sfb12 for short blocks) has no own scalefactor.
This scalefactor band covers the range from 16kHz up to the higher frequency limit, when using 44.1 or 48kHz sampling frequency.
If the resolution of this part of the spectrum must be increased (determined by the psychoacoustic model), the local scalefactor, which is missing, can not be used to adjust its resolution.
To increase the resolution in this case, the only solution is to reduce the global gain value (its quantization). This impacts all other scalefactors, which are also reduced.
But once they reach a value of 0, they can not be reduced anymore, meaning that a higher than needed resolution will locally be used in those bands, leading to an inflate of the bitrate.
When encoding sfb21 content, it is common to encounter some scalefactor bands that are encoded with a too high resolution just to accomodate the coding needs of sfb21.

The -Y switch in LAME:

The -Y switch considers the last scalefactor band as different, and tries not to increase its resolution to the point of causing the sbf21 problem.