Does anyone know what's the difference between Opto and RMS in compression

[Eva Redwood]

I happened to have a compressor plugin that has both Opto and RMS options. I tried to do my research but in every article I looked at it sounds pretty much like it functions the same.

SumatraPDF-prerel-64_SN5ExexQ3s.jpg

[Halma]

Please take a closer look at the picture you have posted.

Try to search again but this time:

1) differences between PEAK & RMS

2) differences between Opto and FET (and maybe VCA & Variable MU)

[whyterabbyt]

As Halma effectively says, the difference is that they're not in the same category of things, its apples vs oranges. Well, more like its a comparison between millilitres of orange juice versus a carton with a straw. They're not an either/or with each other, not related at all.

RMS refers to how the incoming signal is measured.
Opto refers to a type of circuit used in hardware, with a particular VCA response behaviour when it adjusts gain, ie how long it takes and how linear the adjustment isnt. Presumably because the plugin provides an approximation of that behaviour.

[imrae]

"Opto" modelling does usually include an RMS detector; but should also include some "program-dependent" ballistics for the gain reduction. I would assume this plugin has separated the two.

[TMaudio]

Think of RMS compression in terms of time variables: long attack, long release times.
Think of opto as a flavor and characteristic or type of compression: in this case a varying and responsive compression response according to level and a light sensor.

[Eva Redwood]

Please take a closer look at the picture you have posted.

Try to search again but this time:

1) differences between PEAK & RMS

2) differences between Opto and FET (and maybe VCA & Variable MU)

Thanks for the patronizing advice based on the assumption that I'm extremely dumb.

I think I do know the difference between PEAK & RMS as well as Opto and FET. But what made me curious is that Opto and RMS seems to function similar in practice as smoothing out the sidechain signal and hence softening the gain reduction changes. I'm curious how exactly they affect the incoming signal differently under the hood, whereas their effect seem to be somewhat similar.

[Eva Redwood]

Some says it just sounds different without stating how, and some music magazines write that people can think Opto as same as RMS in practice. I'm confused the more I research.

[Ploki]

Opto is an emulation of physical analog compression implementation, and RMS (root mean square) is mathematical function.

Opto doesn’t have a singular mathematical function like RMS because different implementations will yield different transfer functions and detector ballistics.

So what you might ask is “how does this specific opto compressor affect topology compared to a simple RMS circuit”

If it uses a lightbulb it’s slower (but can still be peak because the light will respond to peak or RMS if there’s an RMS circuit to drive the light source) if it uses an LED it’s faster.

The answer is “ask the developer/manufacturer” or chuck in a bunch of test signals to reverse engineer the topology.

Edit: so yes, a slow lightbulb opto might superficially act RMS-ish, but it depends on how quick the light source is and what kind of photosensor it uses.
If you used a fast sensor and an optical fibre it would be as fast as peak.

[whyterabbyt]

I'm curious how exactly they affect the incoming signal differently under the hood

Saying that would have helped.

[Eva Redwood]

Opto is an emulation of physical analog compression implementation, and RMS (root mean square) is mathematical function.

Opto doesn’t have a singular mathematical function like RMS because different implementations will yield different transfer functions and detector ballistics.

So what you might ask is “how does this specific opto compressor affect topology compared to a simple RMS circuit”

If it uses a lightbulb it’s slower (but can still be peak because the light will respond to peak or RMS if there’s an RMS circuit to drive the light source) if it uses an LED it’s faster.

The answer is “ask the developer/manufacturer” or chuck in a bunch of test signals to reverse engineer the topology.

Edit: so yes, a slow lightbulb opto might superficially act RMS-ish, but it depends on how quick the light source is and what kind of photosensor it uses.
If you used a fast sensor and an optical fibre it would be as fast as peak.

Thanks so much. That cleared up things a lot! The plugin is actually an open source project called squeezer. I'm not a coder or a developer, but maybe I can ask the developer about its topology or try to look into it or even try to understand it by myself in the future.

[jlgrimes11]

I happened to have a compressor plugin that has both Opto and RMS options. I tried to do my research but in every article I looked at it sounds pretty much like it functions the same.



SumatraPDF-prerel-64_SN5ExexQ3s.jpg

They are similar in they are typically slower acting compressors and both typically work great for smoothing applications and shines on vocals and smooth bass sounds.

Opto is a bit more organic sounding and a bit more colorful. Usually a bit easier to dial in settings using your ears.

RMS tends to be cleaner and/or more precise. I think RMS has a wider amount of use applications as well, as it can sound good on a variety of things. RMS though can be a bit sterile though but when tuned right can sound spectacular. RMS GUIs tend to be a bit more complex than opto but probably depends on the compressor.


LA2A is what I think of when thinking of an opto. Very smooth, very easy to learn compressor.

I think the Ableton stock compressor is what I think of regarding an RMS Compressor. Alot of people might overlook it but it is a fairly tweakable compressor that sounds great in the right hands.

[AGIGA]

https://fortemastering.com/optical-comp ... explained/
https://varietyofsound.wordpress.com/20 ... mpression/
https://www.soundonsound.com/sound-advi ... ompression
https://gearspace.com/board/so-much-gea ... ssion.html

[sascha]

Edit: so yes, a slow lightbulb opto might superficially act RMS-ish, but it depends on how quick the light source is and what kind of photosensor it uses.
If you used a fast sensor and an optical fibre it would be as fast as peak.

I'm not aware of any commercial opto compressors that actually use light bulbs, can you elaborate on that (just curious)?

The T4 cell in an LA-2A (and its variants) use an EL foil. That one is operated at high voltage (which is there anyway because of the tube circuitry for the drive/makeup amp and the metering). EL panels are what are also used as LCD backlights. Their response is instant.
[Their fist use was probably in military equipment (aircraft displays), as they're extremely consistent and durable. They don't erratically burn out like bulbs, they just get darker over time.]

In theory you could replace an EL foil with an LED and and the overall response won't be faster. The magic is in the LDR, which acts as program-dependent voltage divider. And which includes 'memory'. Unlike pure RMS math, the LDR can actually start reducing its resistance quite fast (as functional of photoconductance/(valence-electron flux if I recall correctly)). But once you keep on applying more and more light, flux saturation kicks in, creating inertia. Short transients can make it recover quickly (increase resistance), but prolonged exposure or - speaking of audio - especially ever-changing signals make use of the memory effect.
In contrast, RMS is a well-defined window that won't enlarge or collapse with varying load.

You can check that with an opto compressor (or emulations thereof) by applying burst signals of increasing length; program-dependency will reveal itself with a change in release time but also how the attack gets handled when it's not just a short spike, and also when such a sequence of burts is repeated multiple times in series.

[Ploki]

You know i might have read that somewhere and took it for granted
Thanks for elaborating! Interesting

[bmanic]

....

Not actually going to comment other than saying spot on! I'm not aware of any actual light bulb opto compressors either. They are almost always LEDs.

And the real reason I wanted to comment and quote you is to say publicly that I'm looking forward to your new DSP ventures with Tourage DSP. Any news you can share about this?? Sorry for pulling this off topic!