When this build is up and running, a record player feeding the line-in jack gets streamed through Music Assistant to every Sendspin speaker in the house, all in sync, and all fully local. The demo setup has a turntable with 33 and 45 RPM buttons next to a white speaker, with the audio traveling from a larger white box to a smaller one hooked to that speaker. The maker imagines several Sendspin units scattered through a home, all locked together, and the same path also handles CDs or TV audio without touching the cloud.
On the hardware side, the CAST PRO-1 packs a PCM1808 analog-input stereo ADC for line in, rated at 24-bit and 96 kHz, plus a PCM5122 as the output DAC. Audio out is available in both analog and optical forms. There is built-in Ethernet, which in my view is a smart call for a streaming box, since wired links tend to be the most dependable path for synced audio. A screw terminal can drive an addressable LED strip, with a music mode that lights up while playback runs, a fun touch.
The software stack is ESPHome, Home Assistant, Music Assistant and Sendspin, and the maker's code is open source on GitHub, with credit given to the Open Home Foundation for building in the open. Line in and audio out can run together, but only over optical, because line in shares the analog connection.
One open item: the hardware can handle 96 kHz 24-bit, yet Music Assistant currently converts all line-in audio to 48 kHz and 16-bit, so higher quality would need support added there first. The maker reports latency working incredibly well, though no official Sendspin latency number is known, and says line in has been running well so far, with no app, account or cloud, at a price below a WiiM mini.
Project by Apollo Automation. Watch it on their YouTube channel: