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1/12/17, 17'42DIY Over-Compression Recovery – Refined Audiometrics Laboratory

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Refined Audiometrics Laboratory

Keeping music enjoyable for all! (dbm at refined hyphen audiometrics dot com)

DIY Over-Compression Recovery

I’ve been looking into the Bongiovi DPS as a means to restore overly compressed

recordings. It kinda sorta works, but that’s not really what it was intended for.

But how does it manage to re-inflate some of the damage caused by excessive

compression and limiting?

Well, to find out, we build one for ourselves and then look at the e!ects of each

component. The DPS system is really composed of 3 parts, but only 2 of them

apply for headphone listening. The third part, for room speakers, is a method of

widening the stereo image. We won’t address that part here.

The first two parts are (A) a method for leveling out the sound track among

the “frequencies that matter to the ear”, and (B) an equalization to overcome the

limitations of your headphones & speakers.

That first part, about the frequencies that matter to the ear, is a bit of a broad

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stretch. But in the words of a highly experienced recording engineer, there must

be some merit to the statement.

DPS leveling is accomplished by flattening the spectrum, going through a

broadband compressor, then un-flattening the spectrum by the opposite

operation to the pre-flattening. You can do this with a bass or treble shelving

filter centered at 1 kHz, and producing a 24 dB di!erential between the

depressed bass and elevated treble frequencies. Un-flattening uses the opposite

shelving filter, centered in the same place, and providing the same 24 dB of

di!erential gain.

The compressor is a bit of a wildcard, meaning that the choice of compression

ratio needs to be decided. Not wanting to do too much harm to the musical

content, I decided to start with a ratio of 1.5:1. And I decided to use RMS

compression rather than Peak compression.

The compressor is really just a dynamic range compression, so it should be

centered around the nominal 0 dBVU level, which in my case is at -23 LUFS.

Hence the threshold needs to be set low, e.g., -50 dBFS, and the makeup gain

should be adjusted to keep 0 dBVU at -23 LUFS. You can tell if you have the right

amount of makeup gain by alternately bypassing the compressor and placing it

in service during typical 0 dBVU passages. The two volume levels should be

roughly equal.



Why would you want to do this? About the only reason, in consumer audio

applications, would be to help overcome noise in your environment. What this

does is level out the average of the music, within limits.



The spectral flattening ahead of the compressor keeps the bass drum and bass

guitar from pumping the compressor on you. The compressor sees a nearly

uniform spectral content, and holds that part level. The un-flattening adds back

in the bass that was temporarily removed.

If you run a gentle compression ratio like 1.5:1, then it will squeeze your music

into 2/3 of its original dynamic range, centered around the 0 dBVU level. That

means the soft passages will be a little louder against the surrounding noise,

and the louder parts will be not quite as loud.

Where something like this makes a lot of sense is in broadcasting where there

are limits to the usable dynamic range in the transmitters and receivers. And in

that case, you’d pre-compress before transmission, and uncompress and un-

flatten the spectrum at the receiver.

Would I ever want to mix with this contraption in line? Probably not. But if the

room is noisy, it helps when listening to recorded albums.

So far, nothing here expands overly compressed recordings. And you can prove

this to yourself, apart from the common sense that we are compressing again,

by watching Ian Shepherd’s PSR and PLR meter readings when this is inserted

or bypassed. The PSR and PLR do not expand. If anything, they shrink a bit from

the unprocessed sound readings.

So where’s the magic re-inflation of over-compressed recordings? As it

happens, the 2nd stage of the Bongiovi DPS is a 10-band Graphic Equalizer. How

that really ought to be set up depends on your personal tastes and whatever

deficiencies in your headphones / speakers you want to overcome. And when

there is treble boost, as there almost always is, then this is what causes our

over-compressed recordings to show some signs of life. The PSR and PLR



readings expand.

So, if we don’t need any dynamic range compression, then we can completely

skip part (A) above, and cut right to the chase with nothing more than a simple

10-band or 30-band Graphic Equalizer. That’s the real secret to making music

sound “better” with the DPS processing. For some people a little bit of

compression from part (A) might also help, if they have a noisy environment or

pathetically weak amplifiers.

That’s the secret! (and there’s my bit of treble boost!)

Not much to it after all. Perhaps the hard-won knowledge of a master recording

engineer would be useful as a guide to how you should be setting your Graphic

Equalizer for best results. But most people already have a pretty good handle on



that for themselves.

Oh well… not quite what I had in mind for recovering damaged recordings. But

there you have it. I find it remarkable how so little can do as much as it does.

But I have remarked before that the ears are relatively insensitive to short term

crest factors. Except for perhaps the bass region where you want the pounding

of live drums and bass to help feel the e!ects of loud.

But to prove my point about crest factors, consider the following. We all know

that a simple square wave has an odd harmonic spectrum, where each partial

declines in proportion to its harmonic number. And we all know what a square

wave looks like, eh?



But now look at this one…



This is also a square wave… or at least it has an identical spectrum. The

di!erence is that each partial has a 90 deg phase shift relative to the partials

used in the first plot above.

The ear can’t tell the di!erence between these two waveforms. And as long as

your DAC and amplifier can handle the crest factor, it sounds identical to the

usual square waveform.

Eh? So really, what good is our treble boost really doing, apart from just making

things a little brighter in our headphones? We aren’t really expanding the over-

compression after all. We’re just boosting some high frequency components in

the hyper-compressed recording. We can’t get back the bass boom that was



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squashed away, or the sharp impact of the snare drum strikes.

DM

Author: dbmcclainAstrophysicist, spook, musician, Lisp aficionado, deaf guy View all posts by dbmcclain

dbmcclain January 12, 2017 DIY, Hearing Theory/ / /

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