Digital Volume controls.. -XDb = - Y Bits?

taff

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Evening all!

I've been wondering about digital volume controls for a while, especially the one on my Matrix Mini I DAC - which I use in Pre mode.

Firstly an assumption - I'm pretty sure the volume is digital, and therefore removes bits to reduce the volume?

If this is true, I'm trying to work out where the big degradation in quality is, it sounds like 0 to -15 Db is marginal in terms of SQ, then down to -30 or so is a fairly big decrease in Sq. Most notably in treble detail. The biggest noticeable difference in the last 10 db is almost treble balance increasing and does not seem to massively effect overall volume.

So.. my question is really, is there a formula I can use whereby minus so many Db is equal to minus so many bits?

Cheers!

James

 

Purite Audio

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A properly dithered digital attenuator is as good as any analogue unit, the only difference is that a analogue attenuator attenuates noise as well as the signal,a digital attenuator only attenuates the signal, it is good practise not too use too much digital attenuation..

Do you know if yours is dithered.

Keith.

 

john dolan

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I have lots of gain in my system so when I use the M-dac direct the volume is turned way down and found it sounded better running full gain through a AS passion passive preamp.

 

AmDismal

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Apr 22, 2007
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1 bit is 6dB, so 16 bit has dynamic range of 96dB, 24 has 144dB. Obviously these are theoretical maxima.

Dithering is important - merely truncating a 24 bit signal to 16 bits causes quite a significant degradation, I have found. Most modern volume controls are fine though.

J River has a bit depth simulator you can try if you like :)

 

i_should_coco

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The dynamic range is still reduced though, all dither does is ensure the nose is random and therefore as benign as possible. The loss of resolution still happens and, at some point, will become noticeable.

 

john dolan

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JW the designer of the M-dac is working on his own brand Westlake and the power amp is being designed to self adjust sensitivity when used with the M-dac so the volume control will always be working at its optimal no matter its volume setting so he must be well aware of how digital volumes can degrade the sound when run at low settings.

Im running my M-dac atm full volume into my TVC that's set at the 12 oclock position then that goes into my AS Passion.

Adding the TVC into the loop when used with my SS amps give the same sound as the Passion into my WAD valve amp and also the volume steps which I hate can be fine tuned to be very close.

 

AmDismal

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Generally, if you are using a digital volume control then you want to use attenuators so that 0dB digital is the loudest that you ever listen, and you will normally be within 20dB of this; this should be pretty transparent. I use 25dB attenuators to this effect - 0dB is pretty loud, usually I am in the -10 to -5 zone. It doesn't go that loud, but this is the second system. Ideally I'd use 15dB attenuators, but I had the 25s handy.

It would be nice to have a stepped attenuator with about 5-6 5dB steps, which you could then use for coarse attenuation and just stay close to 0dB on the digital.

 

SergeAuckland

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Does anyone know if the volume control on the SBT is dithered, and if so, what sort of dither is used?

I've looked on the SBT forum and can't find any definitive statement of this.

S.

 

john dolan

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Serge ive seen it said you want the volume on the SBT at 80% or more of full volume or the sound is degraded so if your running say 50% or less id suggest run it through a simple passive pre and disable the volume on the SBT that's how I used to use mine before I ran it into a M-dac and now I run m-dac with preamp disabled into my AS Passion passive as it does sound better to my ears but I have lots of gain in my system so was running the digital volumes way down low.

 

SergeAuckland

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Serge ive seen it said you want the volume on the SBT at 80% or more of full volume or the sound is degraded so if your running say 50% or less id suggest run it through a simple passive pre and disable the volume on the SBT that's how I used to use mine before I ran it into a M-dac and now I run m-dac with preamp disabled into my AS Passion passive as it does sound better to my ears but I have lots of gain in my system so was running the digital volumes way down low.
John,

Thanks for the reply, but it doesn't answer my question. I use my SBT with the volume at 100% through a conventional pre-amp. I was interested more academically than practically what sort of digital volume control the SBT had. Anyone know?

S

 

SergeAuckland

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Thanks John, that's really useful, although I could have done those for myself if hadn't been too lazy. :(

I looks like it's been dithered given that the linearity is maintained even at low levels, although I wonder why the frequency response drop off at low volume, although 0.5 dB is hardly significant!

S

Edit: I've just noticed that was for the Squeezebox3 rather than the Touch! Wonder how much difference there is. I'll just have to get my test kit out.

S

 

NAM

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Serge ive n the SBT at 80% or more of full volume or the sound is degraded so if your running say 50% or less id suggest run it through a simple passive pre and disable the volume on the SBT that's how I used to use mine before I ran it into a M-dac and now I run m-dac with preamp disabled into my AS Passion passive as it does sound better to my ears but I have lots of gain in my system so was running the digital volumes way down low.
I've seen it said that there is no degradation until below 17 (and whether that is audible or not is another thing) I use the squeezebox volume control direct into active speakers and use the 50-100 range generally. I don't notice any problem with any of the volume settings I generally use tbh

I don't think this is where I saw it originally but is mentioned here http:// http://forums.slimdevices.com/showthread.php?89228-Touch-Volume-Control-Quality

 

adamdea

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Thanks John, that's really useful, although I could have done those for myself if hadn't been too lazy. :( I looks like it's been dithered given that the linearity is maintained even at low levels, although I wonder why the frequency response drop off at low volume, although 0.5 dB is hardly significant!

S

Edit: I've just noticed that was for the Squeezebox3 rather than the Touch! Wonder how much difference there is. I'll just have to get my test kit out.

S
Do let us know your results. I would be gob smacked if the Touch vc is not dithered. I have in my mind that it uses 24 bit precision so you have some scope for attenuation before you get any reduction in the signal to quantisation noise ratio.

 

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