View Full Version : Audio Unit Idea /challenge /request

24th June 2005, 02:00 PM
In audio mixing we have a concept called "sidechain gating." In simple terms, it's the act of using the envelope (volume) of one channel (the sidechain) to shape the envelop of another completely different audio channel. If anybody remembers the Australian band, Bang The Drum, who had an amazingly tight match between the bass guitar and the kick drum, this sound was achieved by using the kick drum to "sidechain gate" the bass guitar to create an amazingly tight and percussive bass sound.

Now I've been reading in the developer references on Apple's site, and have just enough knowledge of programming to see this is possible to do as an audio unit, but I don't quite have enough programmer IQ points to actually do it. (Don't you hate that? ;) )

I haven't found any such AU in the realm of Garage Band/Final Cut Express level stuff out there on the net, and would pay up to AU$50 for such a plug if I could get it.

How I'd see it working is this...
[list] get a volume envelope level from one channel's audio stream get a minimum volume and maximum volume parameter from its interface create a factor based on these inputs multiply the audio stream of the second channel by the calculated factor feed the new value back to the second channel's output.

In Garage Band terms this might be taking a volume envelope from a mono or stereo kick drum track and relevelling a mono or stereo bass guitar track so that the bass guitar is only loud in the mix when the kick drum is making noise.

It may need two actual plugins, one to create the envelope signal for the sidechain input, one to do the gating. Select the first plugin in the kick drum track, the second (main) plugin in the bass guitar track.

An envelope could be taken by only reading positive-going samples every 50 to 80ms. (ie: rectify then low pass filter at 20Hz)

Sorry for the long post, and if nobody here knows how to do this, no probs. But if somebody with significantly more X-Code AU skills than me wants to give this a crack, you've got a live guineapig here to beta test it to destruction :D


1st July 2005, 10:15 AM
Gee, back to the books about X-Code for me, then :(

Anybody know how to squeeze another 36 hours into the day? <sigh />

28th July 2005, 01:23 PM
Just saw this now... :)

There&#39;s HEAPS of plugs that allow sidechaining for gates, expanders and compressors. Logic has quite a few that come with it. What I&#39;m not sure about, is if there are any for free or less than &#036;50.

www.osxaudio.com (http://www.osxaudio.com), www.versiontracker.com (http://www.versiontracker.com), or www.macupdate.com (http://www.macupdate.com) would all be good places to start searching.

28th July 2005, 02:05 PM
You also might want to try KVR audo (http://www.kvraudio.com/)

They have a fairly big selection of free vst&#39;s and au&#39;s available to download. I usually find something close to or exactly what I want there.

28th July 2005, 03:03 PM
Oh yeah&#33; Thanks for the reminder the_OM. KVR is a great site&#33;