Pair your OPNs to the TV Adapter as normal.
Insert a USB/SPDIF Adapter (I used a Delock USB/Sound Adapter from Amazon £10 – plug and play) into one of the iMac USB ports.
Connect the Toslink cable from the Oticon TV Adapter to USB/SPDIF (you also need a square Toslink to 3.5mm mini optical jack converter – I used a Kenable TOS optical (Toslink) plug to Mini 3.5mm Optical Jack Plug Adapter from Amazon, £2). Open Audio Midi Setup (a utility already in MacOSX), select windows from top menu and click on Show Audio Devices.On left hand side of Audio Devices there will be a list of audio input and output devices both internal and connected.Click on + sign at bottom left, this will bring up a drop down menu either Create Aggregate Device or Create Multi-Output Device, click on Multi Output Device then right click and select Use this device for sound output.
A list of available output devices will appear on right hand side. Under Use, tick built in Output and USB Audio Device.
You can also adjust sampling rate and drift connection – I haven’t played with these yet.
Sound should now output to the OPNs and the internal speakers. The volume of the internal speakers no longer responds to the keyboard or from Sound Preferences so I use the volume control on the audio application, e.g. iPlayer, iTunes.
When streaming films / TV, there is no discernible delay between the video and audio content and the in-built speakers seem to be in sync with the hearing aids. Stereo separation is good (too good sometimes!), occasionally background film noise (e.g. crowd scenes ) can be intrusive – only apparent on some films, possibly due to audio mixing on those particular films. (If Oticon Techies can come up with a workaround for this, it would be good!)
After using Audio Midi Setup the Sound option in Mac System Preferences will include the options for Inbuilt Speakers only, Multi-Output Device (inbuilt and OPNs) and External SPDIF Interface (OPNs only) so you do not need to use Audio Midi to change outputs.
The output from the USB/SPDIF adapter doubles as an analogue stereo jack which will connect to the TV Adapter using the supplied mini-jack adapter and phono leads. I have not tried this yet.
Other options worth exploring include TOSlink splitter which will give two outputs from USB/SPDIF Adapter or direct from the optical minijack built into the Mac.
I have been told that it is simpler with PCs as they allow inbuilt speakers to be used at same time as output from headphone jack!
c. Fred Hunt & Robert Donnan 2017