The usual suspects for compression and expansion are what would be considered "downward".
For everything below we're only talking threshold, we're ignoring make-up gain, ratio, and everything else for simplicity.
Downward Compression
The parts of your signal that are above the threshold are attenuated...downwards. Reducing the dynamic range.
Downward Expansion
The parts of your signal that are below the threshold are attenuated downwards, making them quieter. Think your typical noise gate.
Upward Compression
The parts of your signal that are below the threshold are boosted upwards.
Upward Expansion
The parts of your signal that are above the threshold are boosted even further upwards.
A Nice Easy Example - Snare
Take a snare track that has loud transients on your back beats, plus ghost notes in between.
Let's cut the image in half so we're looking at just the positive side of the waveform, this makes it easier to imagine a threshold line. This is also easier to follow than the usual diagonal-line graphs you may have seen before. (The threshold in these images definitely isn't precise to where it is set on the plugin, it's an estimate.)
Dry Signal ⤴️
The threshold is set above the ghost notes but below the back beats.
Downwards Compression ⤴️
With a regular downwards compressor, the back beats are reduced in volume, and the ghost notes remain unaffected.
Upwards Compression ⤴️
With an upwards compressor, the ghost notes are increased in volume, and the back beats remain unaffected.
Downwards Expansion ⤴️
With a downward expander, the ghost notes are reduced in volume, and the back beats remain unaffected.
Upwards Expansion ⤴️
With an upward expander, the back beats are increased in volume even further, and the ghost notes remain unaffected.
So if you want to increase the level of the ghost notes, the usual choice of a downwards compressor may not be the best choice. This would be squashing all of your loud transients in order to increase the overall volume, to emphasise the ghost notes.
An upwards compressor could increase your ghost note level, while leaving your back beats unaffected.
Of course, if you're working with real drums, this completely ignores the inevitability of cymbal bleed.
As upwards compression is targeting everything under the threshold, you may also be bringing up undesirable parts of the signal, and the noise floor.
Plugins That Offer Each Type
Waves MV2 - Downward and Upward Compression
This has both "directions" of compressor in one, with very simplified controls.
"Low level" is your upwards compressor, drag this up to increase your quiet parts of the signal.
"High level" is your regular downward compressor, drag this down to decrease the loud parts of your signal.
Fabfilter Pro-MB - All Four, and it's Multiband
You can set a single wide band to target the whole frequency range, or get more in depth with multiple bands, and different types for certain frequency areas.
- In "compress" mode with a range below zero, you have a downwards compressor.
- In "compress" mode with a range above zero, you have an upwards compressor.
- In "expand" mode with a range below zero, you have a downwards expander.
- In "expand" mode with a range above zero, you have a upwards expander.
iZotope Ozone 11 Standard or Advanced
The Maximizer module added in version 11 includes an upwards compression control.
The Dynamics module (also available in previous versions), includes regular downwards compression & limiting.
In Advanced, these are also available as separate plugins.
Flux Pure DExpander
Slightly confusingly named, the "De-Expander" is an upward compressor at heart.
They also offer Solera, which features 4 modes:
- Comp - downwards compression
- DComp "De-Compressor" - AKA upward expansion
- Exp - downwards expansion
- DExp - "De-Expander" - AKA upwards compression
Is upward compression the same as parallel compression? NO.
This is an opinion I've seen multiple times on the internet for some reason, and it's wrong.
Upwards compression by default, is not a parallel process.
Could you do it in parallel? Absolutely.
Is upwards compression the same as (downward) expansion? NO.
Again, this is a common misconception.
I think this might be from considering upwards compression the opposite of regular compression, but it's still compressing.