10-Jan-2016, 21:36
The streaming device, whether Sonos or Bluesound, plays back the original lossless stream, in PCM, say 192/24.
If output through TOSLINK optical, the PCM stream will arrive (possibly with some added jitter) as is.
Variable volume control _before_ sending output through TOSLINK optical connection means that the stream is going to be processed in the Sonos/Bluesound device to apply the digital volume control. As the Phantom still expects 24bit signal, there is no other way to reduce volume than to simply send zeroes in some of the bits. So every time you reduce volume by 3dB you lose 1bit of accuracy. At -30dB you will have a signal of 14bit from source 24. If original was 16bit, you will have 6bits of resolution left.
There is no way around this without controlling the volume on the final DAC (inside Phantom).
TLDR: Digital volume control over optical output (on Sonos/Bluesound) is just applying negative gain and reducing bit-resolution, by 1bit per each -3dB.
If output through TOSLINK optical, the PCM stream will arrive (possibly with some added jitter) as is.
Variable volume control _before_ sending output through TOSLINK optical connection means that the stream is going to be processed in the Sonos/Bluesound device to apply the digital volume control. As the Phantom still expects 24bit signal, there is no other way to reduce volume than to simply send zeroes in some of the bits. So every time you reduce volume by 3dB you lose 1bit of accuracy. At -30dB you will have a signal of 14bit from source 24. If original was 16bit, you will have 6bits of resolution left.
There is no way around this without controlling the volume on the final DAC (inside Phantom).
TLDR: Digital volume control over optical output (on Sonos/Bluesound) is just applying negative gain and reducing bit-resolution, by 1bit per each -3dB.
