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April 22nd, 2018

Shaving the DJ Software Yak

I'm getting married this June. (For the Debian folks, the Ghillie shirt and vest just arrived to go with the kilt. My thanks go out to the lunch table at Debconf that made that suggestion. formal Scottish dress would not have fit, but I wanted something to go with the kilt.)
Music and dance have been an important part of my spiritual journey. Dance has also been an import part of the best weddings I attended. So I wanted dance to be a special part of our celebration. I put together a play list for my 40th birthday; it was special and helped set the mood for the event. Unfortunately, as I started looking at what I wanted to play for the wedding, I realized I needed to do better. Some of the songs were too long. Some of them really felt like they needed a transition. I wanted a continuous mix not a play list.
I'm blind. I certainly could use two turn tables and a mixer--or at least I could learn how to do so. However, I'm a kid of the electronic generation, and that's not my style. So, I started looking at DJ software. With one exception, everything I found was too visual for me to use.
I've used Nama before to put together a mashup. It seemed like Nama offered almost everything I needed. Unfortunately, there were a couple of problems. Nama would not be a great fit for a live mix: you cannot add tracks or effects into the chain without restarting the engine. I didn't strictly need live production for this project, but I wanted to look at that long-term. At the time of my analysis, I thought that Nama didn't support tempo-scaling tracks. For that reason, I decided I was going to have to write my own software. Later I learned that you can adjust the sample rate on a track import, which is more or less good enough for tempo scaling. By that point I already had working code.
I wanted a command line interface. I wanted BPM and key detection; it looked like Mixxx was open-source software with good support for that. Based on my previous work, I chose Csound as the realtime sound backend.

Where I got

I'm proud of what I came up with. I managed to stay focused on my art rather than falling into the trap of focusing too much on the software. I got something that allows me to quickly explore the music I want to mix, but also managed to accomplish my goal and come up with a mix that I'm really happy with. As a result, at the current time, my software is probably only useful to me. However, it is available under the GPL V3. If anyone else would be interested in hacking on it, I'd be happy to spend some time documenting and working with them.
Here's a basic description of the approach.

  • You are editing a timeline that stores the transformations necessary to turn the input tracks into the output mix.
  • There are 10 mixer stereo channels that will be mixed down into a master output.
  • there are a unlimited number of input tracks. Each track is associated with a given mixer channel. Tracks are placed on the timeline at a given start point (starting from a given cue point in the track) and run for a given length. During this time, the track is mixed into the mixer channel. Associated with each track is a gain (volume) that controls how the track is mixed into the mixer channel. Volumes are constant per track.
  • Between the mixer channel and the master is a volume fader and an effect chain.
  • Effects are written in Csound code. Being able to easily write Csound effects is one of the things that made me more interested in writing my own than in looking at adding better tempo scaling/BPM detection to Nama.
  • Associated with each effect are three sliders that give inputs to the effect. Changing the number of mixer channels and effect sliders is an easy code change. However it'd be somewhat tricky to be able to change that dynamically during a performance. Effects also get an arbitrary number of constant parameters.
  • Sliders and volume faders can be manipulated on the timeline. You can ask for a linear change from the current value to a target over a given duration starting at some point. So I can ask for the amplitude to move from 0 to 127 at the point where I want to mix in a track say across 2 seconds. You express slider manipulation in terms of the global timeline. However it is stored relative to the start of the track. That is, if you have a track fade out at a particular musical phrase, the fade out will stay with that phrase even if you move the cue point of the track or move where the track starts on the timeline. This is not what you want all the time, but my experience with Nama (which does it using global time) suggests that I at least save a lot of time with this approach.
  • There is a global effect chain between the output of the final mixer and the master output. This allows you to apply distortion, equalization or compression to the mix as a whole. The sliders for effects on this effect chain are against global time not a specific track.
  • There's a hard-coded compressor on the final output. I'm lazy and I needed it before I had effect chains.

There's some preliminary support for a MIDI controller I was given, but I realized that coding that wasn't actually going to save me time, so I left it. This was a really fun project. I managed to tell a story for my wedding that is really important for me to tell. I learned a lot about what goes into putting together a mix. It's amazing how many decisions go into even simple things like a pan slider. It was also great that there is free software for me to build on top of. I got to focus on the part of the problem I wanted to solve. I was able to reuse components for the realtime sound work and for analysis like BPM detection.
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