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115 lines
4.3 KiB
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115 lines
4.3 KiB
Plaintext
Psychoacoustic Models in TwoLAME
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================================
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Introduction
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------------
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In MPEG audio encoding, a psychoacoustic model (PAM) is used to determine which
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are the sonically important parts of the waveform that is being encoded. The PAM
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looks for loud sounds which may mask soft sounds, noise which may affect the level
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of sounds nearby, sounds which are too soft for us to hear and should be ignored
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and so on. The information from the PAM is used to determine which parts of the
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spectrum should get more bits and thus be encoded at greater quality - and which
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parts are inaudible/unimportant and should thus get fewer bits.
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In MPEG Audio LayerII encoding, 1152 sound samples are read in - this constitutes
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a 'frame'. For each frame the PAM outputs just *32* values
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(The values are the Signal to Masking Ratio [SMR] in that subband). This is important!
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There are only 32 values to determine how to alloctate bits for 1152 samples - this
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is a pretty coarse technique.
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The different PAMs listed below use different techniques to decide on these 32
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values. Some models are better than others - meaning that the 32 values chosen
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are pretty good at spreading the bits where they should go. Even with a really
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bad PAM (e.g. Model -1) you can still get satisfactory results a lot of the time.
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All of these models have strengths and weaknesses. The model 'you' end up using
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will be the one that produces the best sound for your ears, for your audio.
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Psychoacoustic Model -1
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-----------------------
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This PAM doesn't actually look at the samples being encoded to decide upon the
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output values. There is simply a set of 32 default values which are used,
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regardless of input.
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*Pros*: Faaaast. Low complexity. Surprisingly good.
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"Surprising" in that the other PAMs go to the effort of calculating FFTs
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and subbands and masking, and this one does absolutely *nothing*.
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Zip. Nada. Diddly Squat. This model might be the best example of why
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it is hard to make a good model - if having no computations sounds OK,
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how do you improve on it?
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*Cons*: Absolutely no attempt to consider any of the masking effects that
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would help the audio sound better.
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Psychoacoustic Model 0
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----------------------
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This PAM looks at the sizes of the 'scalefactors' for the audio and combines
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it with the Absolute Threshold of Hearing (ATH) to make the 32 SMR values.
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*Pros*: Faaast. Low complexity.
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*Cons*: This model has absolutely no mathematical basis and does not use
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any perceptual model of hearing. It simply juggles some of the numbers of
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the input sound to determine the values. Feel free to hack the daylights out
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of this PAM - add multipliers, constants, log-tables *anything*. Tweak it until
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you begin to like the sound.
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Psychoacoustic Model 1 and 2
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----------------------------
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These PAMs are from the ISO standard. Just because they are the standard,
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doesn't mean that they are any good. Look at LAME which basically threw out
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the MP3 standard psycho models and made their own (GPSYCHO).
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*Pros*: A reference for future PAMs
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*Cons*: Terrible ISO code, buggy tables, poor documentation.
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Psychoacoustic Model 3
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----------------------
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A re-implementation of psychoacoustic model 1. ISO11172 was used as the guide
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for re-writing this PAM from the ground up.
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*Pros*: No more obscure tables of values from the ISO code. Hopefully a good
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base to work upon for tweaking PAMs
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*Cons*: At the moment, doesn't really sound any better than PAM1
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Psychoacoustic Model 4
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----------------------
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A cleaned up version of PAM2.
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*Pros*: Faster than PAM2. No more obscure tables of values from the ISO
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standard. Hopefully a good base to work from for improving the PAMs
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*Cons*: Still has the same "warbling"/"Davros" problems as PAM2.
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Future psychoacoustic models
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----------------------------
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There's a heap that could be done. Unfortunately, I've got a set of tin
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ears, crappy speakers and a noisy computer room. If you've got the
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capability to do proper PAM testing then please feel free to do so.
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Otherwise, I'll just keep plodding along with new ideas as they
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arise, such as:
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- Temporal masking (there's no pre-echo or anything in TwoLAME)
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- Left Right Masking
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- A PAM that's fully tuneable from the command line?
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- Graphical output of SMR values etc. Would allow better debugging of PAMs
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- Re-sampling routines
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- Low/High pass filtering
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