Implement True Wireless Stereo (TWS)

[TooDissing]

The current scope for STRØM 1.0 are mono playback only. For an enhanced listening experience stereo playback should be possible.

Describe the solution you'd like
It should be possible to support stereo playback using two STRØM speakers. This feature should be implemented using a wireless connection.

The main problem are how to synchronize the clock of the two speakers, as this is required in order to minimize any delay between the playback from each speaker.

The "classic" TWS implementation in Bluetooth can't be used here, as this will require a proprietary Bluetooth chip and software stack. This is not part of the µVox hardware platform.

There seems to have been done some serious research within this area, which are worth looking further into. The following research promise a clock drift across multiple devices of under 1µs. That must be more than enough for this use case.

In the below paper two interesting concepts are mentioned: X-Sync and X-Burst.
Poster: Accurate Cross-Technology Clock Synchronization Among Off-the-Shelf Wireless Devices

Describe alternatives you've considered
A wired connection is possible, but isn't really considered as a solution.

[TooDissing]

I had a short talk with the researcher behind the above study and it seems that the X-Sync / X-Burst isn't the most ideal solution for this application. Their main focus are on syncing devices using different network stacks; i.e. mixing Bluetooth and Wifi.

A few suggestions on a different approach:

• take a further look into BLE Audio, to see if we can rely on its clock sync mechanism. Not using BLE for the audio transfer, but solely for synchronization of the device clocks
  • Some work are done to utilize this feature within the open source community
• TI have created some proprietary logic for this: https://www.ti.com/lit/an/swaa162a/swaa162a.pdf?ts=1650461646781&ref_url=https%253A%252F%252Fwww.google.com%252F
  • looks like it will be hard to implement this in the µVox platform as it requires various pre-build BIN's

[TooDissing]

another great paper about this topic: https://www.usenix.org/system/files/atc21-chen.pdf

[TooDissing]

Maybe also of relevans? https://gitlab.com/painlessMesh/painlessMesh

[TooDissing]

FTM seems to be support in some chips. Should be able to utilize this somehow: https://docs.espressif.com/projects/esp-idf/en/latest/esp32s2/api-reference/network/esp_wifi.html#_CPPv424wifi_ftm_initiator_cfg_t

[TooDissing]

An ESP32 FTM example: https://github.com/espressif/esp-idf/blob/a82e6e63d9/examples/wifi/ftm/README.md.

When utilizing this (FTM) the ESP32 devices are likely required to be setup as station+AP, so they are in "direct contact". At least this should yield a better result.

The resulting timing should then be used as a base for clock-sync.