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DacMagic Plus volume control - how does it work?

H

hammeredklavier

owner of two very cheap hi-fis
Just for my personal edification, can someone with a technical knowledge of DACs tell me what this means please?

"the volume control [...] isn’t an analog or bit-reduction digital controller but a DSP driven digital domain gain stage using Anagram Q5 adaptive time filtering to accomplish gain reduction without bit-reduction."

Read it here:

http://www.avguide.com/review/cambr...usb-dacpreampheadphone-amp-playback-55?page=1

Sounds very impressive but how on earth do you make a digital attenuator that doesn't rely on reducing the bit depth?
 
hi,
asked a similar question of the Minidsp on the Minidsp tech site.(Diyaudio).

Quote:
My question is does the volume control degrade the sound in any way?

This is really hard to answer (you asked: in any way...)
Theoretically there is possible degradation of the sound quality if you use a lot (say 40dB) of digital attenuation (by the way, also analog attenuation will degrade S/N / resolution), however as lduarte already said, practically it works fine and I also do not recognize any significant impact on the sound quality. End quote.

I know very little about digital but as far as I have read I think it may be best to have any digital attenuation kept to a minimum and then use an analogue potentiometer.:)
 
Indeed, but I'm wondering what this rather wizzo-sounding method of gain control that Cambridge Audio have apparently come up with is all about...
 
Ironically the datasheet of the Q5 does not even mention volume control.

It is probably a digital attenuator with heavy noise shaping, taking
advantage of the large available ultrasonic band that stems from
the Q5's upsampling to 384kHz.
 
Werner said:
Ironically the datasheet of the Q5 does not even mention volume control.
Click to expand...
Yes, I noticed that myself, which was why I was a bit puzzled. The bumf about Q5 on Cambridge's website says nothing about attenuation either:

http://www.cambridgeaudio.com/content.php?COID=93

It is probably a digital attenuator with heavy noise shaping, taking
advantage of the large available ultrasonic band that stems from
the Q5's upsampling to 384kHz.
Click to expand...
I suppose if you can shift the noise far enough away from the audible spectrum then you can attenuate all you like... would still be interested to know what that volume control really does!
 
Sounds like pure marketing bollocks to me.

It's a digital attenuator that performs before the dac exactly as Werner proposed. Basically a 32 bit volume control.

If it actually discards less than one bit of resolution (at a 16 bit level ) over it's entire attenuation range then maybe they can say it does it without bit reduction- if you get my meaning.
 
sq225917 said:
If it actually discards less than one bit of resolution (at a 16 bit level ) over it's entire attenuation range then maybe they can say it does it without bit reduction- if you get my meaning.
Click to expand...
Which would, nonetheless, amount to bit reduction...!
 
Not pure.

If by using NS you build a digital attenuator that confines its *isolated* contribution to SNR to better than 16 bit equivalent at all levels and below 20kHz, then you could sort of rightfully state that the attenuator *performs as if* without loss of resolution. From this to the above blurb is but a small step.



If they are not doing as described above then we are morons. Can we unpost/unread this thread and run to the patent office?
 
Werner said:
If by using NS you build a digital attenuator that confines its *isolated* contribution to SNR to better than 16 bit equivalent at all levels and below 20kHz, then you could sort of rightfully state that the attenuator *performs as if* without loss of resolution. From this to the above blurb is but a small step.
Click to expand...
So IF the SNR is still better than 96 dB over the attenuator's full range within the audible spectrum...?

Presumably with higher input bit-depths the claim would still certainly be BS?
 
hammeredklavier said:
"the volume control [...] isn’t an analog or bit-reduction digital controller but a DSP driven digital domain gain stage using Anagram Q5 adaptive time filtering to accomplish gain reduction without bit-reduction."

Read it here:
http://www.avguide.com/review/cambr...usb-dacpreampheadphone-amp-playback-55?page=1

Sounds very impressive but how on earth do you make a digital attenuator that doesn't rely on reducing the bit depth?
Click to expand...

Why don't you ask the reviewer who wrote it?
 
hammeredklavier said:
So IF the SNR is still better than 96 dB over the attenuator's full range within the audible spectrum...?

Presumably with higher input bit-depths the claim would still certainly be BS?
Click to expand...

That's what I was thinking. It's either very, very carefully and well written marketing blurb, or bollocks. ;-)
 
Although I wouldn't want that to put anyone off buying the DAC. It's very nice and works like a charm.

The manufacturer has to earn a living from these things. I might add, in fairness, that the suspect BS that I quoted was from an independent review and not CA's website. (Although it was written in a way that seemed to me to imply that the info had been supplied by the manufacturer - it seems a strange thing to make up!)
 
What's interesting about the Dac is it doesn't seem to change in character that much when the volume is set low (allowing for the Fletcher–Munson curve), or so it seems, where other Dacs ive tried with digital volume control do seem to change their characters at lower and higher volume, of course it could be me imagining it! ;)
 
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