The SRW1 is working as a digital audio embedder,enabling your Blackmagic card to receive embedded audio down the SDI video cable.If you turn it off,it will no longer be performing this process.Your Blackmagic breakout cable also has connections for balanced analogue audio inputs or AES/S-PDIF digital audio inputs.A third-party card might also substitute for the SRW1.
Regards,
Will.
(Soho Asia Co. Ltd.,Bangkok)