why does digital volume control cause loss in info

I took a quick look through the CS3310 datasheet. It looks like a pretty nice device, although I'd expect that designing with it, and achieving good results, would be a non-trivial challenge because of the presence of both digital and analog signals and power on the same device. And I note that its datasheet was initially released in 1991, so there may be newer and better comparable devices available now.

Digikey charges $6.57 for it in quantities of 1000. I believe that a generally accepted rough rule of thumb for equating parts cost with final assembly selling price is to apply a factor of 5. So it would have about a $33 impact on the selling price of an audio component, or perhaps say $50 to $100 factoring in possible requirements for additional surrounding circuitry to interface to it, and additional circuit board real estate that may be required. Not a huge cost, but certainly significant.

A purely digital volume control, on the other hand, could perhaps be implemented within a custom digital chip that is required in the design for other purposes, resulting in negligible impact on recurring cost.

Thanks for pointing out this device, ElDartford and Shadorne.

Regards,
-- Al

It should be kept in mind, though, that in concept the loss of resolution with a digital volume control at lower volumes is no different than what occurs with an analog volume control, although there will certainly be differences in degree.

Al, the main difference is that the analog noise floor is uncorrelated noise (not related to the signal), and the quantization error of a linear PCM system at lower levels is strictly correlated to the signal, so is audible as distortion, not simply noise.

But on the other hand, virtually all DAC chips these days will accept a 24-bit input . . . so in practice if we have 16-bit source material, a digitally-implemented volume control can provide about 48dB (6dB per bit) of attenuation range without adding any additional quantization error, simply because the DAC has more resolution than the source. And 48dB is pretty workable, if the gain structure of the rest of the system is well designed. There will of course still be the DAC's uncorrelated noise floor on top of this, which is the same type as that produced by analog methods.

Now regarding the CS3310, which I have tested extensively . . . it's a very good "off the shelf" digitally-controlled analog attenuator. Its main limitations are that since it's a monolithic circuit, there are some significant limitations to the quality of the resistors that can be fabricated on the same die as the rest of the chip, as opposed to discrete resistors. It has an onboard opamp with gain-ranging, which doesn't provide a constant bandwidth and transient-response regardless of volume-control setting. And while its noise performance is pretty good (though not nearly as good as a low-impedance discrete-resistor attenuator), it's limited to +/- 5v power supplies, so there's not a lot of headroom if the designer wants to optimize the gain structure for lower noise.

Excellent comments, Kirk. Thanks.

Re your first point, to make sure its clear to others who may read this I'll add that the fundamental purpose of dither is to convert quantization distortion into low level analog-like broadband noise. Or even better, into "shaped" noise, that predominantly occupies parts of the spectrum to which the ear is relatively insensitive.

Re your second point, which I hadn't thought of when I wrote my earlier posts, that would say that the bad rap digital volume controls justifiably got a decade or two ago would seem to be much less applicable, and in some cases completely inapplicable, to today's designs.

Best regards,
-- Al

Have you noticed that the lowest settings on digital volume controls actually play the music pretty loud for the lowest setting? For example going from -50 dB (mute) to -49.5 dB is not a tiny little increase in volume. It's something that's a bit too loud for 2 a.m. in the morning drifting off to sleep music. I've noticed this behavior on several digital volume controls.