[oqvist]

I have a receiver with no optical out. So I have to use hdmi from my apple tv to my TV which has an optical out. Question do the tv convert the signal back to digital and do a AD conversion or would it be the same as if the source had optical out it acts as a clean passthrough? TV in this case is a philips 9002 OLED

 

[sander99]

I think you can safely assume that the TV will digitally extract the audio from the HDMI input and send it to the spdif out without DA and AD conversion inbetween, because there is nothing difficult about that and doing DA and AD conversion would be completely ridiculous in this case.
However not all TVs will be able to pass on all possible digital audio signals that spdif supports.
PCM stereo at standard sampling rates will not be a problem. But some TVs won't pass Dolby Digital and/or DTS.
(And spdif itself won't support DTS HD, Dolby TrueHD, multi channel PCM.)
When trying an unsupported audio format you would simply get nothing (silence) out of the spdif. Or maybe your apple tv will automatically negotiate with your TV over HDMI and decode/convert - digitally - to another format that the TV does handle. Or you can manually set such things in the apple's audio settings.

 

[Davesrose]

I would say it depends on your source. The latest gen Apple TV (HD or 4K) outputs multichannel PCM or DD+ Atmos (or I believe you can force it to just output Dolby Digital if you don’t have a HDMI receiver). If your receiver is getting optical through your TV, the TV does have to do a conversion for Multichannel PCM to Dolby Digital surround (that requires the TV brand to have paid for a license). Or you could force the Apple TV to just output Dolby Digital, which the TV will pass through. Otherwise, optical supports PCM stereo (and I suspect the Apple TV will default to that if the TV isn’t accepting multichannel PCM, and you haven’t set Dolby Digital as your audio output).