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UAPP external dac/amp query
Hi everyone,

Yesterday I had a problem which Davy very kindly resolved quickly but i still have some queries/misunderstanding and i don't want to keep bothering Davy by email so I thought i'd throw it to the forum floor here.

I have two mobile phones both of which have hi-res dacs and UAPP installed. My LG V50 has the sabre dac and my older Huawei Mate 10 has the Hi6403. I stream Qobuz studio premier.

I recently acquired an OTG cable because i realised I can get either phone to utilise the warmer dac/amp from my old Fiio E10 should i prefer that instead of the cold, analytical sabre dac.

Connecting the LG V50 to the Fiio E10 everything worked fine and as expected. Note that the volume was only controllable from the E10 and not via the phone.

When i connected the Mate 10 everything seemed fine until buffering and then "Error starting playback!". Davy replied: 

"Unfortunately, on recent Huawei devices and in combination with certaintypes of USB DACs, Android blocks access to the USB device, making it impossible for third party apps to use their own USB audio driver. Since

the problem is inside the kernel, we cannot solve this issue. As a work-around, you can deactivate 'Use USB DAC' in the app's USB audio tweaks, but you'll get the same quality as any other app."

I did as Davy said and it worked. I then asked: "It's a noob question but how do i find out what quality my E10 is outputting using this route? I have qobuz app set to "CD quality - upto 24/192" but since it's not via UAPP i know i'm not getting that." Davy replied: "It's hard to tell. My guess is that it gets first resampled to 16-bit 48kHz, then upsampled to the highest rate of your DAC (limited to 192kHz)".

So, when i listen to the mate 10 via UAPP it gives the same data stream values for a given song as either listening to the Mate 10 via the 3.5mm jack, or listening to the LG V50. The only obvious difference is that when the Fiio E10 is plugged into the mate 10 the volume is adjustable either via the Fiio E10 OR via the phone. So:

1) Does this mean the Fiio is acting only as an amp when plugged into the mate 10? but how could the mate 10 export analogue data via its USB-C? The sound profile between 3.5mm and Fiio E10 on the mate 10 is clearly different but maybe thats just the character of the Fiio op-amp.

2) The sound coming from the Mate 10 sounds great. Clearly Hi Res - spacious - great sound-stage -sparkly- could all that be coming from upsampling as Davy suggests?

Thanks in Advance! :-)

Further to the above: i just noticed bit-perfect mode is available on the mate 10 with the Fiio E10 whereas it is not available on the mate 10 via the 3.5mm i guess that confirms that it must be using the Fiio E10 dac......i just dont get how the mate 10 retains volume control on the digital out signal?
digital volume control is a thing - it works by lowering the amplitude of the samples within the song/track, then the dac converts the lowered digital signal and then its amplified.

The problem is that this is "altering the bits" of the file to lower the volume. thats why Bit Perfect mode will disable the volume in app to ensure that you dont "lose any bits" by locking the digital output volume at 100% meaning if the song/file sample point is at 0dBFS then it will be output at 0dBFS.

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