Does the SB3 SPDIF run at 16 or 24 bit? IEC-958 allows for the use of 24 bit audio in the SPDIF interface where previously, the low order 4 bits were reserved for non-audio data use.
New to SB3, but as I understand it, the 16 bit data samples are scaled by the volume control using 8 bit coefficients to create 24 bit words. This is fine for the internal BB DAC which runs to 24 bits but there are implications for the digital output, even if the output is scaled by the maximum 8 bit value of 255. If the SPDIF only runs at 16 bit, the 24 bit words will be truncated and dither should be applied in the SB3 to the low order bit; there's a slight increase in noise but a reduction in distortion. If the SPDIF runs at 24 bit, there are implications for the DAC downstream. If the DAC only supports 16 bit, the digital audio data will be truncated to 16 bit with no application of dither because the DAC will simply ignore the low order 8 bits. Therefore, if your DAC is only 16 bit, you should run the SB3 at maximum volume and rely on the DAC (if it has one) or the analogue pre-amp for volume control. If the SPDIF runs at 24 bit and your DAC supports 24 bit but without a volume control, you can safely use the SB3 volume control - the add-on DAC is effectively emulating the SB3's internal DAC. If your DAC has it's own volume control, which is best to use? The choice is influenced by how the DAC volume control is implemented. The issue with simply scaling the data is that you increase the word length by doing so. Taking the 24 bit samples and scaling by 16 bit coefficients creates 40 bit words which then have to be truncated back to the internal DAC bit length, 24 bits. The low order 16 bits contains information which is now lost and the least significant bit of the truncated data should be dithered by adding a triangular probability density function coefficient to the 16 bits of lost data. All fine, except at low listening levels where the number of active bits in the DAC are reduced. Each bit corresponds to a level change of 6dB, so at a listening level of -48dB below maximum, your expensive 24 bit DAC is suddenly back to 16 bit because the 8 high order bits are never being used. A better approach is the use both digital and analog scaling so that as the volume is changed, the gain of the DAC is changed as well as the data being scaled in the digital domain. In that way, at a given volume setting, you are continuing to use a greater number of bits of the DAC and benefit from greater resolution. One approach, for example, it to use part of the volume coefficient to set the analog gain of the DAC and to scale the the digital data by the rest of the volume control coefficient, applying dither as normal. If you have this sophisticated type of volume control, it doesn't much matter which you use though I prefer to keep the SB3 at maximum volume and use the DAC volume control. If your DAC's volume control scales only in the digital domain, the SB3 volume should definitely be left at maximum. Better yet would be an audiophile software option to disable the SB3 volume control when using SPDIF. Don't mess with the data, just give me the raw 16 bit samples (with the low order 8 bits set to 0) and my DAC will take care of them. Even scaling at a maximum coefficient value of 255 will change the data. In the meantime, and assuming the SB3 outputs 24bit SPDIF, the message is clear for anyone adding a DAC - make sure you have a 24 bit DAC, not a 16 or 20 bit one. -- DitherMan ------------------------------------------------------------------------ DitherMan's Profile: http://forums.slimdevices.com/member.php?userid=3189 View this thread: http://forums.slimdevices.com/showthread.php?t=19697 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/lists/listinfo/audiophiles
