Topping D90 IIS-LVDS vs USB Interface Sound Quality
I've been doing a bit of testing of the sound quality on the I2S (IIS-LVDS) interface and the USB interface. I used exactly the same music app source (UAPP running on my Cayin N6ii DAP) and I played a set of three songs alternately using each interface a total of four times (= 12 songs per interface). I know the songs well so was looking out for presentation and detail.
The bottom line, to my ears, is this: the I2S interface on the Topping D90 sounds a little better than the USB interface. However, there is a difference in soundstage. The I2S interface sounded more 'in my head' than the USB interface. The USB interface was a little more laid back and further away in its presentation of the soundstage (which is something I do prefer generally).
However, in terms of resolution, clarity and detail of vocals and instruments, the I2S interface was slightly better. The downside of the I2S interface is that you may have to do a bit of menu configuration on the D90 to get it to work with your source. The USB interface was plug and play - I used an Android OTG dongle and a short printer-style USB cable and that combo worked perfectly. In terms of cabling for the I2S interface, I used a standard mini HDMI (DAP end) to HDMI cable (D90 end).
So, in summary, if you want a more laid back presentation with a soundstage that's in front of you then the USB interface may be to your liking. If you want a more 'in your head' presentation that is more vibrant, more forward, with a little more clarity and detail, then the I2S interface may be your preferred option.