AstonHack 2016 and DrumHub

Published: 12 Jan, 2018
Tags: webaudio, github

More than a year ago I went along to AstonHack 2016 as part of the Majestic team. We were mainly there to support the students trying to use Majestic API, but were pretty happy to assist them with whatever tech they were having trouble with. It was reasonably busy at first, but soon I found myself thinking I should probably start working on one of my ideas since I’m there anyway and everybody seemed to be busy with their hacks.

Since listening to a talk about WebAudio API during Hackference 2016 I’ve had this idea of taking a git repository and using the commit timings to create a drum track. I wanted the rhythm to speed up when the commit frequency was high and vice versa. Instead of trying to figure out the commit frequency I decided that every period between two commits would become one 4/4 measure and I would vary the tempo according to period between two adjacent commits. The other feature I wanted to add is somehow distinguishing the notes that fall on BPM peaks and troffs (e.g. if the rhythm is slowing down and after a certain point starts picking up I want the sample at that point to be different). I ended up picking a crash cymbal to denote the peaks and a tom-tom drum for the troffs.

Luckily for me GitHub has a REST API for their repositories, so I didn’t need to do any git parsing myself. It also meant I could build the application without any server-side code and host it using GitHub Pages. It turns out I could get all the data using one request to the API (asking for a list of commits). The only thing left to do there was transforming the data into a time series. This was the first challenge as GitHub gives two timestamps - one as the “commiter” timestamp and one as the “author” one. This difference emerges from the fact that the original author of the commit might not have write access to the repository, in which case someone (the commiter) can take their commits and apply them. However, when multiple commits are applied this way the commiter timestamp of these commits is identical between the commits even if the author commits have different timestamps.

The GitHub API returns commits ordered by the commiter timestamp (as this is the order they appear in the repository). However, because of the duplication of timestamps it didn’t really fit my purpose. Of course, I could’ve removed the duplicate timestamps, but I felt that wouldn’t have been the best representation of the activity on the repository. Instead, I decided I’d use the author timestamps and simply sort them, which worked really well.

I knew that I wanted to map the period between commits into reasonable BPM as otherwise playing one repository could take months if not years. The mapping imposed a limit on the maximum BPM so that the shortest period between commits in any repository would correspond to one measure played at 480 beats per minute. When commits were spaced out more the tempo would decay exponentially. While testing I realised that many repositories are updated quite sporadically and in bursts - there were long periods of very low freaquency, so I added a limit to the lowest tempo and chose it to be slower by no more than a factor of 10. The resulting function for scaling is then

where A is the smallest time period used for one measure (4 beats at 480 BPM or 0.5 s), v is the factor limiting the lowest tempo (in this case 10), tmin is the smallest period between commits and t is the period between the current commit and the previous one.

One value that was quite difficult to get right in the above formula was the value of k - the constant that controls the exponential decay or in our case how easily the output BPM drops to the lowest value. After a good amount of experimentation and listening to what different repositories sounded like I settled to a value of k = 0.00001 s-1.

Once I had the commit data and scaled it appropriately the only thing left was to actually schedule the samples and press play. I used this intro to Web Audio API by Boris Smus to figure out how to get the browser to do what I wanted. I also threw together a simple UI using Bootstrap and a photo from Unsplash.

You can find the hosted result at gkudelis.net/drumhub and the code at the GitHub repo. Pick a GitHub repository, put the username/reponame in the text field and press play. If you’re not sure what repository to try I suggest starting with tensorflow/tensorflow.