Presuming it's the same as their PEQ on other Fiio Connect devices, it's very shoddy. It's limited, peak filters only, but also on at least some devices it has problems saving, it seems to be mapping the bands onto the GEQ bands and it gets mixed up if you put them too close together. On top of this, there's no indication if it saved, and you can only go out and back and see is the change there... sometimes it will be, sometimes, no. I have several Fiio devices with their PEQ, it's terrible. While Qudelix is far more powerful with shelves, etc. but more than all of that, it just works.
The BTR15 (I don’t have it) digital part is quite complex with two parallel, but separate, paths to handle the inputs. The app needs to convert and send the user DSP settings, through BT, to the Qualcomm QCC5125 chip (Kalimba DSP core) for BT input, and to the XMOS XU316 USB bridge, via the QCC5125, for USB input—that’s how FiiO describes the BTR15 DSP implementation.
AFAIK the XU316 does not have HW DSP cores, but it’s a quite powerful chip… FiiO must leverage some software DSP libraries, developed by XMOS and/or FiiO. The issue is that these libraries may not expose the exact same capabilities as the Kalimba HW DSP. For example, it’s possible that the XU316 can only accept say 30 GEQ-type filters (PK only, fixed Fr & Q, variable Gain). The Fiio app would have to “approximate” the PEQ settings with these GEQs, and send them to the XU316 via the QCC5125, whereas it could send the actual PEQs “as-is” to the QCC5125 HW DSP. The app also needs to manage all the corresponding BT com., write & save acknowledgments, possible interrupts of the music (clicks and pops…), etc. I’m making that up to illustrate the difficulties !!!
FiiO is shouting themselves in the foot with this architecture…
[Edit] Just found that the BTR15 has two firmware: one for the QCC chip, OTA update through BT and the Fiio app, and another one for the XMOS, update through USB only, with a different tool. It makes sense… but I bet users are going to be confused!
The Q5K, on the other hand, uses the same QCC5124 for BT & USB input, mic input… and DSP. Oh and it does not have a screen and menus to manage. That’s a lot easier to implement, and Qudelix has absolutely nailed it!
The tradeoff is lower USB SINAD (see Amir’s review) but I think it’s more than acceptable for this type of device.
Incidentally, for the T71 (I have it), Qudelix went for a very powerful iMXRT600 ARM + HW DSP… and simply gave up on BT. Same choice of a simpler architecture that just works!
