Microphone noise reduction or stuttering FaceTime

Have you tried an external microphone either hardwired or Bluetooth? I would start there as that would isolate the issue if it is or isn't the systems mic system when on a FaceTime call.

Your system does have noise canceling. It has two microphones internally. But its not as good as the newer iMac’s ability. In any case if you are recording your self and don’t hear any issue then its not the systems microphones.

Now the tricky part! Your other devices are an iPhone or iPad which likely are connected via the cellular network Vs your cable internet connection. Its also possible your iMac is not hardwired to the cable router but instead connected via WiFi (or has a bridged connection over WiFi). So while you might have a good internet connection things are a bit more complex!

So let’s start at the cable end at the Router most folks connection is Asymmetrical that is the data flow coming to you is much higher than the flow leading out of you. So watching movies or other Internet stuff is silky smooth, but posting a big file to a server on the internet takes forever! This is the limits of your connection.

If your directly wired that would be the most likely issue you are facing. If you are connected over a WiFi connection then the distance between the iMac and the Router (AP) could be an issue as well as the given band you are using. You’ll need to check to see what you are connected via a good program like WiFi Explorer Lite which allows you to visually see the bands and what is in use around you. Ideally you want to use a band that is not heavily used and offered better throughput. Do keep in mind the 5 GHz bands don’t offer the same distances as the slower 2.4 GHz bands. In addition if you are using a WiFi bridge the slowest link will we the limits of the pathway. If you can go for a Meshed router setup.