PCB design flaw: speaker outputs are coupled #42
Description
Description
The handset earpiece (speaker_receiver+/-) and the front ringer speaker (speaker_front+/-) share a coupled signal path on the phonev4 PCB. Audio intended for one speaker bleeds into the other.
Current Workaround
The ALSA configuration (/etc/asound.conf) mitigates this in software by routing left and right USB audio channels independently using out_left_solo and out_right_solo PCM devices with route plugin ttable matrices. This mutes the unwanted channel in software, but crosstalk is still present at the hardware level because the electrical paths are coupled on the PCB.
Impact
- Ringer audio faintly audible in the handset earpiece during ring
- VoIP call audio faintly audible on the front speaker
- The software workaround is effective enough for normal use but not ideal
Fix
A future PCB revision (phonev5) should fully isolate the two speaker output paths, likely requiring:
- Separate amplifier stages or a dual-channel amplifier with independent outputs
- Physically separate traces and ground returns for each speaker path
- Possibly separate 3.5mm jacks or a single jack with proper channel isolation
References
- PCB schematic:
pcb/phonev4.kicad_sch - ALSA workaround:
host/asoundrc.example(route_left_solo / route_right_solo) - PCB README known issues section:
pcb/README.md