Essay on compression

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laffik
Sun Oct 23, 2016 12:34 pm

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Essay on compression

Hi Image-Line!

Here's idea of new type of limiting. Signal is processed by adding phantom DC offset to the signal instead of gaining by multiplication by compressor ratio. Plugin like this would solve problem of various artefacts of compression like pushing, pumping or pulsing of high end while subbas is appearing rhythmically in audio track. I reckon this method is realized by Waves L2 Ultramaximizer plugin, however it may be not exactly this case described here. While analysing wave display of lots of modern mp3s mastered for loudness I noticed the fact that hats mixed with subbas get through zero (-Inf) line without distortion and are not scaled together with subbas by gain, they are rather slid down or up by phantom DC bias to fit under the threshold level. It's rather subbas that is slightly reduced in this case. It is not a problem when subbas pulses as senses are tolerant for subbas pulsing unlike pulsing of high end caused by gaining. In this case high end is amplificated by compressor in places where subbas hits over the threshold and not amplificated when subbas stops, which causes pumping. In case of adjusting level by phantom DC offset high end is not amplificated, so any artefacts like pumping pushing, growling and such are not present. It's nice and produce loud and clear sounds.

Track with limiter step by step:

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Here on picture above is presented signal with subbas hit in first half of duration and following silence in second half of plot, all covered by high end noise of hats or snare.

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Limiter calculates loudness level over the threshold for the part with subbas, which drops short after subbas stops oscillating. Picture is approximate. It shows gain reduction of 50% for signal sticking out 6dB over the threshold of limiting of 6dB. Gain reduction is popping up to 0% of reduction short after subbas stops.

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Limiter is multiplicating signal by gain reduction in place where subbas hits and after it stops according to signal of gain reduction. In effect we may observe that noise component in place where subbas hits has smaller amplitude of it's oscillation comparing to place where subbas doesn't play. It is audible as pumping.

Phantom DC offset alternative:

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The same input signal as in gain compression version.

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In this case plugin calculate level of audio sticking above 6dB. Picture is approximate. Gain reduction is not calculated, just enough to have smoothed peak level of audio, like peak level used in Maximus or any other limiter for calculating gain reduction. Only difference is fact that this level is bipolar it may have positive and negative value determined by sign of side that singal is crossing the level of threshold. Threshold is mirrored on positive and negative side of zero. For calculating peak signal level modulo is used, so this is peak signal level without modulo used.

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Instead of calculating gain, signal of audio peak level sticking above 6dB is subtracted algebraically ("-" - minus operator) from audio signal itself. (It seem like contamination of types of data, but don't worry number is number and may be used this way). In effect we got high end slid down or up according to site of zero line. Signal is slid that enough to make peak level aligned on 0dB level on positive or negative side. High end is not scaled by gain, not reduced, so there's no pumping. It is only subbas reduced slightly, which is not audible, the sound is loud and not damaged.

Cheers Image-Line and forum, have a good time and if you wish to develop plugin for brickwall limiting according to this elaborate it would be my nobilitation and pleasure. :)

Thanks.

laffik
Wed Mar 29, 2017 12:11 am

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Re: Essay on compression

P.S. Instead of phantom signal, feedback loop ...


laffik
Tue Apr 18, 2017 8:13 am

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Re: Essay on compression

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