Re: Digital volume control question....

Keith G wrote:
A week or two ago I bought a little cheapo ss amplifier from Argos for fun/summer/curiosity/all day long radio, MP3s &c. and wuz so taken with it I bought a couple more (similarly cheap) components from the same range to go with it:

http://www.apah69.dsl.pipex.com/show/outbreak.JPG


(OK, I admit it - I was driven by the 3-way, multi-purpose remote control!! :-)

Anyway, the clarity I'm getting from this little bugger from any number of different sources is quite exceptional and I'm wondering if the 'digital volume control' has anything to do with it?

I know the speakers (firewood horns - Pinkies) are 'on song' now and will be contributing mostly to the sound quality and I am convinced that normal (carbon wiper) volume pots do the sound no favours whatsoever, but is there any reason the 'digital volume' (much like a computer soundcard, I guess) is likely to be helping in a significant way?

(If it is, I wonder why more manufacturers don't use them?)

As far as I know there are two sorts of "digital" volume control. One is a digitally controlled analogue attenuator, that changes its attenuation according to the numeric code it is given. This can be linked to a physical rotating control, driven from up-down counters etc. The second sort is a DSP function that multiplies the digital audio signal by varying co-efficients and outputs a digital signal who's values are an attenuated (or can be amplified) version of the input. I would imagine that the volume control of your amplifier is the former as it takes in and gives out analogue. The latter would require A-D and D-A conversions. Your Marantz CD player with the variable output could be of either sort but I suspect more the latter, as changing the digits before the D-A conversion would mean that the variable output would work on both the digital and analogue outputs simultaneously.

As Don Pearce mentioned, the provision of a "digital" volume control is often done for reasons of cost. Digital volume controls track left and right channels virtually perfectly (comfortably within 0.1dB) from full output to extremely quiet, and don't generate any significant noise when changing levels. Normal carbon pots can be relatively quite noisy, and even conductive plastic pots will find it difficult to track both channels to better than 2dB at high attenuations. Carbon pots can be as much as 6dB out at low levels. This will move the stereo image around as you change volume settings. An electronic attenuator chip is a lot cheaper than a conductive plastic pot, and the actual mechanical control can be a cheap device, as all one is sensing is position.

As to audio quality, a conductive plastic pot will have zero effect on audio quality. It is virtually pure resistance, and that doesn't have non-linearities or generate noise (other than Johnson noise, which is not terribly relevant.) A digital attenuator will have a finite level before overloading, and will generate some noise and distortion. However, that can be so low that it is essentially swamped by the inherent noise and distortion of the rest of the amplifier circuit. The only disadvantage of a "digital" volume control I can think of is that volume changes can only be made in discrete steps, typically 1dB at low volumes, perhaps 2 or even 3dB at high volumes.

S.
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