Digital Volume Control for fixed output DACs
Worryingly, my lovely wife has started taking an interest in the technical side of computer audio.... It appears she has been reading random websites about digital music, and even a couple of blogs. I am not sure this is entirely a good thing – for either her or me, but it did inspire me to write my own blog. I think I shall use her (usually extremely basic) questions as a start.
So picture the scene as we sat down to coffee this morning
(Weanie Beans from London – great coffee). There was a John-Lee Hooker track
playing quietly in the background as I’m doing a bit of music software
comparison work today.
She asked me:
“They say that you need to turn the volume to maximum on
your computer. Does that mean you shouldn’t be controlling the volume on that
thing?”
“That thing” is the iPad, which we use as a remote to
control the Mac Mini computer which runs our music software.
I didn’t pursue the question of who “they” are but I thought
it best to encourage her interest in How Things Work by giving my opinion of
the topic. It took a while, but here’s what I explained.
Quite a few digital music commentators say that digital
volume control should never be used.
They say this because digital volume control lowers the resolution of
the signal. They are correct - digital
volume control or "attenuation" does indeed reduce the resolution. But I still believe this is outdated advice. We must remember that volume control in the
analogue domain also adds distortion.
The best audiophile music software these days has far more
sophisticated volume control than in the past, and no longer impacts the sound
quality as it used to. Modern day
software uses a technique called "dithering" that reduces the audible
effects dramatically. I have spent a
considerable amount of time comparing the sound quality of digital
"dithered" attenuation versus analog attenuation. If the gain between your DAC and power
amplifier is such that you are at a comfortable listening level somewhere
around -10dB to -6dB setting on your music player then in my opinion dithered digital
volume control is at least as good as the best analogue attenuation.
Secondly, by controlling the volume in the digital domain,
you can output your DAC directly to your amp, entirely bypassing the pre-amp. Now this is where things get interesting! The 3 main reasons we use pre-amplifiers are:
1) Allowing switching between multiple sources into one
amplifier
2) Impedance matching between source and amplifier
3) Volume Control
A pre-amplifier cannot "improve" the signal
quality from the source that is connected to, and in my experience most
pre-amplifiers degrade sound quality.
If your DAC has the proper output impedance
to work directly with your power amplifier you can plug the DAC outputs directly into the
power amplifier and use the digital volume control. If you have a DAC (or any
other source) that can do this I would ask that you give that a try one evening
and let me know how much better your music system sounds. Be careful that you start off with the volume
setting as low as possible!
The electronics of most pre-amps does far more damage to the
sound quality than does the digital volume control. The advantages (direct
transmission) far outweigh the small disadvantages that digital attenuation
causes.
Here is some good information on dither:
http://www.izotope.com/tech/mbit/
Here is some good information on dither:
http://www.izotope.com/tech/mbit/
Of course, pre amps are around because most of us also want
to have access to all of our sources.
But if you are looking for absolutely the best sound
quality possible give it a try!
