Originally Posted by ndabunka
Good content here. Is there a possiblity that the frequencies used in your area of the world may impact what you are observing?
or perhaps connection-based factors? US household connections have a 3rd wire for ground but (if I recall correctly) some UK circuits are simply a 2-prong socket. Could the noise you happen to be hearing be related to the absent ground prong in your power stream?
The PJ is on a normal UK power socket (50Hz) so if there were any ground loop/hum issues I'd expect something at 50Hz or a multiple. Nothing like that, the peaks with 4Ke on are very precisely multiples of the exact input frequency.
For 50Hz input (Sky HD box, 1080i) there is indeed a smallish sharp peak with 4Ke on compared to off. It is at 1350Hz, which is 27x the input. However, I struggled to hear the difference. The 60Hz input has the main peak at 419.6Hz (7x) and smaller ones at 540Hz (9x) and 1378.6Hz (23x) but it is the 419.6Hz one which is particularly evident. Lots of odd but exact multiples there, you'd think they were trying to avoid resonances...
My guess is that whatever opto-mechanical arrangement is driving the 4K enhancement is bound to be operating on various multiples of the frame rate, and the 419.6Hz noise is simply the sound of some component of that operation. It does vary according to listening position, the wavelength is about 86cm so plenty of scope for interference effects with the reflected sound from the ceiling just above the PJ.
I'd still like confirmation or otherwise that a 60Hz or 59.94Hz input gives this noise for other users. In my case it is an Amazon Fire TV 4K box. It is also clear in my case that for 50Hz and 24Hz inputs, and with 24Hz UHD, there is no noticeable noise.
Edited to add: I've updated the firmware, and no noticeable difference in noise.