This tripped me up recently and I really should know better.

Don’t mix wireless and cabled microphones with differing amounts of latency because if you do you may have a nasty and difficult to remove echo or phase issues in your audio. 

Digital + Analog don’t mix well.

In my particular case I was using a couple of Sony UWP-D wireless microphones to mic up two out of 3 members of a discussion panel. For the 3rd member I had planned to use another UWP-D but that microphone became unavailable at the last minute, so instead I used a lower cost digital microphone that works on the 2.5Ghz band. There is absolutely nothing fundamentally wrong with this lower cost microphone but the digital processing and transmission adds a very slight delay to the audio.

The Sony UWP-D’s are extremely low latency (delay) microphones and the audio arrives at the camera almost instantly. However most of the lower cost digital microphones have a very slight delay. That delay may be 1 frame or less, but there is still a delay. So the audio from the digital microphone arrives at the camera slightly late. If this is the only microphone you are using this isn’t an issue. But if you mix a very low latency microphone with one with a very slight delay, if both mics pick up any of the same sounds in the background there will be an echo or possibly a phase issue.

As the delay is almost never exactly 1 frame  this can be difficult to resolve in most normal video post production suites where you can only shift things in 1 frame increments.

Phase Issues:

Phase issues occur when the audio from one source arrives very slightly out of sync with the other so that the one source cancels certain frequencies of the other out when the two are mixed together. This can make the audio sound thin or have a reduced frequency response.

So… don’t mix different types of digital wireless microphones and don’t mix lower cost digital microphones with more expensive low latency microphones. And when you are checking and monitoring your audio listen to a full mix of all your audio channels. If you monitor the channels separately the echo or any phasing issues might not be heard.