There are three basic stages to converting a digital file to analogue. First, the file format needs to be decoded and then sent to the DAC as a digital bitstream. Second, the DAC converts the digital file to an analogue waveform. Third, the amp applies buffers and gain stage to the headphones. The first step is the difference between DAC mode and the standalone player.
Logically, when in DAC mode, the X5iii is receiving the decoded bitstream from the computer and bypassing the SoC to decode the files. Likely, the path for the Bluetooth output is looking for the decoded bitstream from the SoC so therefore it does not output the bitstream over Bluetooth when used in DAC mode, as the computer has taken over these duties.
@Relic: Thanks for the response. I think I can make sense of your post, please correct the following if I'm mistaken.
3 logical elements of analog audio output:
1. Input is a digital audio bitstream, PCM or DSD, either received via USB from an external source, or from the SoC which decodes a source audio file on the device, or, again from the SoC, when an app such as Spotify or Tidal receives audio over WiFi and then passes it to the SoC, which decodes it to a bitstream
2. Pass bitstream to DAC which converts to analog
3. Amplify & output analog audio
But what happens when the player outputs Bluetooth audio? I presume that the SoC consumes the source (say flac or mp3 files on the device, or OGG-Vorbis for Spotify, or AAC for Apple Music) and then, instead of decoding to PCM, decodes to a format compatible with the Bluetooth target, either SBC or Apt-X, which it then transmits to the Bluetooth receiver (say my Bose speaker) which converts it to analog, amplifies and plays it.
So the essence of your message is that when the X5 is in USB-DAC mode, the only thing the SoC does is manage the USB connection, and deliver the digital audio stream, which is produced by the computer, to the DAC. If I play a file in FooBar on my computer, the X5 receives a PCM bitstream, converts it to analog, and then amplifies and outputs the audio. Same if I run Spotify on the computer -- the computer converts the OGG-Vorbis data to PCM, and then outputs it to the X5, resulting in the same story.
However, the device cannot convert a PCM (or DSD) bitstream (delivered via USB) to SBC or Apt-X to transmit over Bluetooth, either because there is no such conversion capability, or because the point where the system consumes the bitstream via USB is 'downstream' from the module responsible for SBC encoding.
Close?
