Insane 320kbps

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Rule Uresti

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Aug 3, 2024, 5:57:38 PM8/3/24

to teatigebpe

After analysing various converted mp3's I've found out that the LAME encoder uses a lowpass filter for the insane preset (CBR 320), but the filter seems to be disabled for the extreme preset (VBR V0). That doesn't make sense right? I'm wondering why that is the case and what would be the best practice regarding the filter if filesize doesn't really matter.

I first thought this was an anti-aliasing filter, but have long since realised (and been reminded of by an anonymous passer-by viewing this post), the input signal is already band-limited.

--preset insane asumes space/bandwidth doesn't matter, so disables this information limiter. The reason other presets use this LPF is because they take into account the negative effect of accurate high-frequencies in MP3, like the effect of the 'sfb21' band, which is described later under The Y Switch heading. So it's seen as a resource-drain for little gain. A trade-off which is seen as acceptable in an 'insane' preset.

Let's look at the chart you posted here. (from wiki.hydrogenaud.io)
The LPFs(low-pass filters) in this chart are not primarily anti-aliasing filters. They are there for a few different reasons, one of which being the fact that for a specific preset, there is a maximum frequency that can be reliably stored; as the bitrate increases, the maximum frequency that can be reliably stored increases and so the cutoff frequency of the LPF is increased accordingly, until the quality of compression is such that a LPF is no longer required or desired. (The LPF frequency ranges displayed in the chart are transition periods.)

I can't give an exact reason for --preset extreme not having an anti-aliasing filter, but as an educated guess, I'd say it was simply tested and found not to be necessary. These presets have been extensively tested and tweaked for optimum performance or quality, or to find a balance of the two, depending on the preset.

What's interesting is that --preset insane has the -Y switch enabled. Now the Y switch is a bit more complicated. A quote from a Hydrogenaudio Wiki page on this subject attempts to explain it in a single sentence;

In the MP3 format, audio is processed in bands of frequencies. Band 21, or 'sfb21' handles the 16kHz plus(for 44.1kHz SR) frequencies. The difference between sfb21 and the rest of the bands, is that the sfb21 'scale factor'(it's quality, basically) cannot be independently adjusted. So if the quality of the sfb21 band needs to be increased, all the bands are consequently increased. So what the Y switch does, basically, is prevent the quality of the lower frequencies from increasing, just because sfb21 needs to be increased. Bear in mind, the -Y switch can only be activated in VBR mode. This is because in effect, the Y switch is implicit in CBR and ABR modes.

I've read some threads with people asking how to get the absolute best quality from the LAME encoder. Most replies state that --preset insane has been tweaked to allow the best possible quality, but some people seem to think that disabling the Y switch is a possible way to achieve better quality. Perceived sound quality is subjective, and will always be a much debated subject.

"Using a lowpass filter helps reducing the amount of data to encode. This isimportant in MP3 due to a limitation in very high frequencies (>16Khz).The default value depends on the target bitrate/quality. It is not recommendedto change it as a general basis."