I'm looking to install a roomkit plus, using a third party DSP.
I cannot find and audio specifications for the "mic" inputs on the codec plus. Some people on the forums mention that only cisco mics are supported, other people say all third party mics are supported.
Can I please be pointed to some official documentation showing which is true?
Cisco's install guide states that Cisco mics are "recommended", but nowhere do they say they are required. Cisco does support installations with third party mic's - I have dozens of Cisco systems in my company (including new Dual 70's) with external DSP's and third party microphones. One note - the Room Kit/Plus does not give you the ability to set the mic input to line level, only mic, so you have to plan for that. Easily doable once you know that information.
Similar to @PJMack, in the majority of our installations we also use external DSP for both the audio in, as well as the audio out, of the endpoints. The issue about using external mics on the unit is usually around Phantom power, but if you are powering the microphone from the DSP, then this is not an issue.
You will still need to feed the output from the DSP in to the endpoint as a mic level feed, and with a 4-pole cable.
See the "Connect a third party microphone to a Cisco SX20" article which describes some ways to achieve this (and to directly connect a mic that needs phantom power).
Third party is supported. However not all microphones would work straight away. It is a 4 conductor 3.5mm connector. The 4 conductor is necessary and using a standard 3 conductor 3.5mm like a headphone jack would not work and sounds horrible. You must leave the R2 connection open and not short it to ground or other pins.
See this thread regarding Pin outs: https://community.cisco.com/t5/telepresence-and-video/sx20-pinout-description/td-p/2768025