This page lists the suggested tasks to build a 175 hour (medium sized) or 350 hour (large project) for Google Summer of Code 2023. The ideas are already assigned to example projects, but you are encouraged to use them for building your own project adding your own ideas and make it suit perfectly to you, your skills and your time line.

If you are interested in applying to GSoC, read GSoC Advice before applying or getting involved. Only beginner contributors that are active members of the Mixxx community are accepted. If this is not the case yet, just say hello at https://mixxx.zulipchat.com and discuss your Ideas and discus cases with us.

Some Controller mappings have implemented a Spin-Up/Spin-Down to mimic the inertia of a turn table. This should be generalized into a "turn table inertia model" that models all moving parts like the platter and the slip pad.

Some DJ actions on the controller or keyboard should be applied to that model rather than directly to Mixxx transport unit. The transport is than controlled by the turn table model. This should make all transport effects/actions sound more natural.

It allows Spin-Up/Spin-Down effects and keyboard/mouse scratching.

Part of the project is to record the spin up of a real turn table to get to know of the real acceleration curve and model this via software.

These are related feature requests: https://github.com/mixxxdj/mixxx/issues/8867 https://github.com/mixxxdj/mixxx/issues/11250

Here you can see such a model in action:

• Expected Outcome: A flawlessly working spin up and down effect accessible without controller
• Skills: Good understanding of sound processing, C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Medium
• Size: 350 h

Our scratching algorithm suffers from jitter noise created by the latency of the midi messages. It also suffers from a so called sticker drift, an incrementing offset shifting away form the original scratch sample. Part of this project is to filter the discrete messages from the controller in a way that you can either "play" the track without much wobbling by turning the jog wheel, and keep the position during scratching.

We may also make use of the time stamps of the midi messages. https://github.com/mixxxdj/mixxx/issues/6951

The required controller can be provided as a loan.

• Expected Outcome: Wobble fee scratching using a joggwheel
• Skills: Good understanding of real time processing, C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Hard
• Size: 350 h

Currently crossfader changes are stretched on audio buffer time to avoid pop sounds. This is too long for some scratching styles. During this project you need to dive into the audio engine code find the code that is responsible for crossfading and make it independent from the audio buffer size.

https://github.com/mixxxdj/mixxx/issues/8899 https://github.com/mixxxdj/mixxx/issues/11253

• Expected Outcome: Cut type crossfader curve, suitable for scratching.
• Skills: Good understanding of sound processing, C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Medium
• Size: 175 h

Mixxx uses a linear resample when scratching. This is blazing fast, but the sound can be improved. Here Mixxx should provide more resample options. This project involves to review the already used resample libraries RubberBand and Soundtouch and compare them with other candidates. The one with the best Sound/CPU load trade of shall be selected. Make sure that it supports on the fly changing of the sample rate without artefacts. This project may also involve to contribute a missing feature to such library.

https://github.com/mixxxdj/mixxx/issues/9328

• Expected Outcome: Optional replacement of the linear resampler.
• Skills: Good understanding of sound processing, C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Easy
• Size: 175 h

If a controller is accidentally unplugged it has to be manually reconfigured, which is a party stopper. Mixxx should do it automatically. This project has two stages, fist behave like a real DJ: fix the issue but faster. So the the time of silence on the dance floor is kept short. The advance stage of this project is to implement real Hot Plug And Play. In order to that you likely need to touch the low level inside third party libraries.

The required controller can be provided on loan.

• Expected Outcome: Re-power the controller and continue to use it, on your target platform.
• Skills: Good understanding of the USB stack, C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Easy / (Hot Plug And Play = Difficult)
• Size: 175 h / (Hot Plug And Play = 350 h)

Currently Mixxx supports Pipewire via Portaudio and Jack. These two glue layers leads to a limited feature support with a weak user experience. In this project you need to consider solutions to improve the situation. Desired features are

• Better audio routing using external tools

• Route sound to/from other applications from inside Mixxx

• Access to DAC timing info of the soundcards

• Acesss to analog volume settings of the soundcards.

• Expected Outcome: A rock solid interface to Pipewire.

• Skills: Experience with Audio processing and C++

• Possible Mentor: Daniel Schürmann

• Difficulty: Difficult

• Size: 350 h

Different codecs used to encode sound files introduce slight shift in the audio samples comparing the input with the output stream. This is no issue when you use always the same file and the same decoder. It becomes however an issue when the decoder is updated or replaced because than all stored metadata like waveform, beat-grids and cue points are no longer at the correct position. To protect against surprises Mixxx has unit-tests to verify that seeks inside that track do not introduce an offset and checks if the fist sound sample can be found at the expected time.

We have currently the issue that the Microsoft has introduced such a breaking change in Windows 11. https://github.com/mixxxdj/mixxx/issues/11094 In this project we need to investigate the issue and implement a fix so that the stored metadata of a user are still valid after an upgrade to Windows 11. As a project extension, this may also fix an issues that prevents us from moving to ffmpeg as an encoder wrapper where the underlying encoder library is no longer in control of us.

• Expected Outcome: No Beat-Grid offset after Windows 11 update
• Skills: Experience and C++
• Possible Mentor: Daniel Schürmann
• Difficulty: Medium
• Size: 175 h (350 h for a general solution)

Sometimes we have reports of hard to find crashes that my have to do with dangling pointer or other memory addressing issues. When hunting such a general environment that helps to find it would be great. However it is often not worth the effort for that particular issue and that's why we don't have it. This project is for contributors who have real experience with such memory sanitizer facility. We like to learn from you, what is reasonable to use and how we can integrate checks in out continuous integration. This may also something integrated in Mixxx itself to have more info users can provide in case of a crash.

• Expected Outcome: A new workflow how to deal with crasher bugs along with an improved set of tools.
• Skills: Experience with address sanitizer tools like ASAN or Valgrind and CMake
• Possible Mentor: Jörg Wartenberg
• Difficulty: Medium
• Size: 175 h

Mixxx on MacOS Intel uses an older Qt version, due to a nasty performance bug, which occurs with the OpenGL based waveform code. Furthermore the OpenGL waveform code uses the deprecated https://doc.qt.io/qt-5/qglwidget.html class, which is no longer available in Qt6. But there is already a reimplementation https://github.com/mixxxdj/mixxx/pull/10959 and https://github.com/mixxxdj/mixxx/pull/10989 which solves the performance issue for one waveform type on MacOS Intel using the https://doc.qt.io/qt-6/qopenglwindow.html class, which is available in modern Qt versions. This is a huge maintenance burden for the Mixxx developers.

The project consists of several steps:

1. Make the QOpenGLWindow waveform implementation working on all platforms (MacOS Intel, Linux and Windows) using Qt5.15 This requires extensive testing on all platforms and providing test builds to other Mixxx users
2. • Add OpenGL code to implement all remaining waveform styles using the QOpenGLWindow
3. • Delete all QGLWidget based code and clean up the waveform render code in Mixxx

• Expected Outcome: QOpenGLWindow based waveforms running on all three platforms using the same source code on Qt5.15
• Hardware: Student need to have a MacOS system with Intel processor and access to Windows or Linux systems
• Skills: Experience with C++ programming on MacOS
• Possible Mentor: Jörg Wartenberg
• Difficulty: Difficult
• Size: 350 h

As always with Summer of Code, you aren't limited to the suggestions we've made here. If you've got a great idea for a project involving Mixxx then we're looking forward to hearing about it. We recommend spending more than a few days using Mixxx and participating in the community to develop a better understanding of areas where Mixxx could use improvement. Our issue tracker is full of feature requests and other ideas scattered throughout, so if you browse through it, you may find many more ideas for GSoC projects.

IMPORTANT: You should contact us first to get feedback if you're going to submit a proposal for your own project idea! We very rarely approve ideas students propose. If you're not already experienced with DJ equipment, we recommend sticking with one of the ideas above.