Digital Volume Control - Is It Good?

[kspv]

I have heard that digital volume controls are inferior to analog volume controls, and that they reduce the quality of sound reproduction from CDs, by typically about 1 bit for every 6 dB reduction in volume, i.e., if the volume is reduced using a digital volume control from maximum to a level 6 dB lower, the 16 bit CD format will sound like a 15 bit one.

It is claimed that some companies like Wadia overcame this limitation by employing a conversion technology, which converts the 16 bit CD output to 21 bit, so that no noticeable reduction in quality occurs with reduction in volume using a digital volume control.

My questions are:

1.) Whether there is truth in the statement that digital volume controls reduce the quality of sound reproduction from CDs, by about 1 bit for every 6 dB reduction in volume?

2.) If it is true, why does it occur?

3.) Is using a CD player's built in volume control, for managing the overall system's (=integrated amplifier, subwoofer, speakers etc.) volume, bad?

I would be grateful for a reply.

[b-square]

Yes, this is true in most cases. 6db is a factor of 2 change in volume (up or down), and each bit in a word is a factor of 2 change in range. Chop bits as digital voume controls typically do and you lose range.

CD players with variable outputs generally do the attenuation in the analog output section, not in the digital domain so there isn't the loss of range above.

[jefemeister]

Originally posted by kspv

My questions are:

1.) Whether there is truth in the statement that digital volume controls reduce the quality of sound reproduction from CDs, by about 1 bit for every 6 dB reduction in volume?

2.) If it is true, why does it occur?

3.) Is using a CD player's built in volume control, for managing the overall system's (=integrated amplifier, subwoofer, speakers etc.) volume, bad?

I would be grateful for a reply.

It is true that you lose a bit for every 6dB you attenuate. To reduce by 6dB is to cut it into half. The following explains how that equates to losing a bit: (2^n)/2 = 2^(n-1), where n is the number of bits and thus 2^n is the number of quantization levels. It is valid to increase the number of bits before applying the volume control. If a rate converter is being used, the output is most probably 24 bits anyway. Analog volume controls add their own degredation to the sound. There just isn't a neat word like "bit" that can be used to describe it. (Digital gets a bad rap sometimes in general for that reason.)

If you're using a source with digitally controlled volume coupled to a preamplifier (or intergrated amp) it is best to leave the source's volume at it's maximum level and use your analog pot.

[arniel]

Beware that there are also digitally controlled resistor arrays that are used as volume controls. In this case, the signal is kept in the analogue domain and the digital part is used to route the signal through a series of resistors.
Have a look at the Dallas (Maxim) website for examples.