Volume Control on a SBT via an external DAC

[vinylnvalves]

Whilst at a friends house at the weekend i had a listen to his SBT system. What confused me is this SBT was controlling volume when it was used with an external DAC. In my simple logic the SBT should only be sending binary information to the DAC, which won't won't have any gain data with it. Is the SBT not sending a "pure" signal to the DAC, and if so how is the DAC coping/varying the gain.

[adamdea]

The SBT is acting as a digital volume control- it effectively calculates what the sample values would have been if the music had been been quieter. It works just the same whether the SBT is using an internal or external dac.

[vinylnvalves]

Yes - i now feel stupid asking the question - obviously not all frequencies play at the same time (doh) so there must be something in the signal (codec) that accounts for this. I will just go back under my rock and play that infinite sampling rate music, vinyl - which i fully understand.

[adamdea]

Yes - i now feel stupid asking the question - obviously not all frequencies play at the same time (doh) so there must be something in the signal (codec) that accounts for this. I will just go back under my rock and play that infinite sampling rate music, vinyl - which i fully understand.

It's not a stupid question- there is a very real and live debate about whether digital volume controls are better or worse than analog volume controls.

If you think about an anlog vc in a preamp- the volume controls changes the voltage level of the output which goes into the power amp. Fopr the sake of argument let's say that the preamp output varies from 0-2v which the powr amp will mutiply by 20 or so. ie 0-40 v

Now imagine that you have a dac going straight into the power amp. if the dac has the maximum possible digtal signal it will output 2v. If the value of the digntal signals is equivlanet to 1/2 max level (ie 1 bit less) it will output 1v. An anlog vc at 1/2 level would take the output of the dac (reating to any given digital signal level)and halve it (1v for full digtal signal; o.5v if the digtal signal is at 1/2 value) . A digital volume control at 1/2 volume level would simply shuffle the bits so that they now represent 1 bit less (efftively you add a 0 at the beginning and shunt off the bit at the end). so what was previously a full scale digtal signal (2v out) becomes 1/2 scale and now the dac itself outputs a lower level (1v). This could go straight inot the power amp without a pre amp and will produce the same output level as the full scale digtal signal with an analog preamp at 1/2 level.

[I just realise I may have confused the issue here- the digital vc in the dac would eb the same as a digtal vc in the SBT sending out a signal to the dac.]

for a good discussion of the relative merits of digtal and analog vc try looking at this http://resonessencelabs.com/?page_id=314
http://www.resonessencelabs.com/invi...tal_volume.pdf
[edit apologies I sent wrong link first time]