SyncRecord - An Android application for creating ad-hoc microphone arrays from smartphones.

SyncRecord is an open-source Android application that enables the synchronisation of audio recordings across multiple, spatially distributed smartphones, transforming the grouped devices into an ad hoc, synchronised microphone array. The system employs an audible pseudo-random binary sequence (PRBS) signal emitted from the devices' speakers and cross-correlates the captured audio to extract propagation-delay information embedded in the audio streams. This approach reduces temporal misalignment across recordings to within 42 µs (2 samples at 48 kHz), and enables the estimation of the distance between any pair of devices to a mean accuracy of 14 mm. A key part of the system architecture is its server-side framework, which facilitates inter-device communication, real-time audio streaming, and signal processing. The server-side component can be deployed locally to minimise latency, enhance data privacy, and provide additional control over data handling. Alternatively, this component can operate via a cloud-based server when scalability or computational offloading is required. SyncRecord prioritises transient data handling, with cloud deployment having no audio stored beyond the recording session, and all existing streams removed, enhancing data privacy. The application and source code are publicly available via GitHub and CodeBerg, providing a foundation that can be adapted, extended, or integrated into existing audio capture workflows where source localisation, source separation, or other multi-channel audio processing could be beneficial.