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Hey Toon,
Thanks for your email.
The 1/4" TRS connectors on the ATEM Camera Converter accepts stereo analog audio at "Microphone Level" [ATEM Converters Manual, page 6]:
http://documents.blackmagicdesign.com/Converters/20151202-9981cc/ATEM_Converters_Manual.pdf
The analog [XLR] output via HyperDeck Studio Pro is at "Line Level".
Connecting a line-level source (such as mixer output) to a mic-level input will cause the sound to be loud and distorted because the line signal is much stronger than what the mic input will accept.
The analog input for the M/C - Analog to SDI accepts balanced "Line Level" audio, which is the reason why you didn't experience any interferences.
Blackmagic Studio Cameras accepts both "Microphone Level" and "Line Level" analog audio inputs, which can be changed using the UI [Audio -> Audio Input [Mic / Line].
Please refer to this link for more information about the differences between "Line Level" and "Microphone Level":
http://shure.custhelp.com/app/answers/detail/a_id/758/~/mic-and-line-level----what-do-they-mean%3F
I hope the information I have provided is useful to you. Let me know if you have any questions. Thanks!