I started a support ticket back in August to try and get some clarity on this issue.
Yes I only get distortion on the right hand channel, the left is clear, and it only happens when sending 1080i50 from the camera, it's not just the A7Sii but also the Sony FS and FS7, they can't all have issues.
Here's what I got back from Blackmagic support.
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Hello Robert,
Thanks for you patience on this matter.
The Devs have tested logged this as a bug and are discussing this with Sony on how to resolve this behaviour. It may be something that is fixed in future firmware versions, however I would not be able to put a timescale on how long this may take.
I hope this helps and will recontacting when I have more information on this matter.
Thanks,
Matthew Madeley
Technical Support Consultant
Blackmagic Design
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Nothing since then, I've sort of given up as I don't get the issue with other converters.