Why not use onboard computer volume control?

[Ultralight]

I'm running a DAC without volume control.

 

So there are two ways to get volume from my Win 7 laptop:

 

1. Use the Win 7 volume control.

2. Use the playback software volume control.

 

I'm told for some reason that both are not idea. Something about throwing bits away. Can someone explain this to me?

 

Is there a playback software that allows volume control without this happening so that I don't have to purchase a preamp or something to physically control the volume?

 

Thanks!

UL

 

[syganymede]

When you turn down the volume digitally it is like changing from 16 bit to 15 or less bits. For example you might just shift the 16 bits to the right putting bit 16 into 15 etc. Thus you throw away the LSB, i.e. bit 1.

 

To overcome this some software does two things, uses 24 or 32 bit to increase resolution, andh aving done that randomises, or dithers the LSB.

 

This is why the volume control in iTunes on a Mac, sounds better than the volume control on an iPad.

 

If you don't want to lose resolution (i.e. number of bits representing your sound) then use an analog volume control. You do not need an active pre-amp with any gain, provided the output of your DAC is big enough to drive your power amp, thus the volume control only has to attenuate the signal. So what you need is just a potentiometer in a box! Say 10$ or so...

 

[PeterSt]

... and kill the sound by means of *that*. Haha.

 

[aps]

Peter

 

I don't understand. What are you saying? And, out of interest, how is volume control managed within XXHighEnd?

 

APS

 

[Viola_Player]

The input voltage to a DAC is fixed. A DAC is nothing more than a pile of resistors with on/off switches ahead of them. The software controls which resistors the current is allowed to flow through generating different voltages. To make the numbers easy think about it this way: if I turn on resistor #1 I get a 1 volt signal, if I turn on resistor #2 I get a 2 volt signal, resistor 3-4 volts etc. By turning on various combinations (16 sets) you get 2^16 levels of voltage. However, say you want to turn down the "volume". To do this you rescale the signal so that instead of 2^16 levels you're not going to allow the full range, say 2^15, so by reducing the maximum allowed voltage you've reduced the resolution of the system. If you could change the input voltage you could reduce volume without reducing resolution but DAC's don't typically work this way. Having an external volume control: potentiometer (potentially noisy and not so easy to balance between channels) or a network of resistors like in some preamplifiers. This way you maintain the high level of precision (2^16 for red book CD) and can still control the volume. You can buy decent potentiometers with gold plated windings and wipers and, while not the best possible, it may work just fine for you.

 

I hope that helps.

 

Ray

 

[esldude]

Built a sort of digital analog volume control box once.

 

I had a series resistor, and then 8 resistors connected with a good quality silver contact switches going to ground. These eight resistors could be connected in parallel. I arranged them in cascading values. They were not quite like 8 bit digital steps, but close enough. Gave me 255 volume steps. The upper steps were quite close the lower steps about three tenths of a dB. I forget the lowest setting, but it was something close to -60dB. You got excellent channel to channel balance with the ability to set each channel independently. It actually didn't take much use of this for me to simply adjust volume using this system without thinking about it. A bit of overkill though it worked very well.

 

Nevertheless, if you have at least 24 bit volume control digitally, I don't think you have much to worry about. A DAC directly driving a power amp with good digital volume control will work great, and pretty much defeat any active pre-amp with analog volume.

 

[barrows]

with digital volume control.

 

Consider a 24 bit volume as typically in a DAC:

 

Each 6 dB of volume reduction results in a loss of 1 bit of resolution, this means one has about 4 "free" bits to use, as no one's system achieves better than 20 bit performance anyway. Therefore, as far as resolution goes, one can use 4 (bits)*6(dB)=24 dB of volume reduction without real losses. But, there are two other problems-with a typical analog volume control, one reduces the signal and the noise as attenuation is applied. With a digital volume control only the signal is reduced when attenuation is applied. In most cases, while this is a problem in theory, in the real world it will not matter (with the possible exception of very unusual systems with poorly matched gain structures needing high levels of attenuation).

The second problem is quantisation noise. My understanding is that this can be a real problem for digital volume controls (real digital experts, please feel free to make this more clear and/or correct my layperson's understanding). Quantisation noise occurs as a result of the applied digital attenuation, and this noise includes some spiky, relatively high level artifacts-which might be audible as signal degradation. The solution for this is to apply dither noise, which smooths out the spikes, and distributes the noise evenly, usually to the point where it is (theoretically) inaudible. A further refinement of dithering is noise shaping, where the dither noise is shifted higher in frequency, up and out of the audio band.

Remember, that analog volume control is not problem free either-it comes with its own added noise/distortion. A really well implemented digital volume control, combined with a DAC which has a beefy enough output stage (many DACs do not) to properly drive the input of the amplifier can outperform an analog preamp. This is all system dependent, and the details matter.

Theoretically, a digital volume control in a 32 bit DAC chip could be better (at least at very low levels). And dither/noise shaping if done properly will be an advantage. Like most things in audio, one needs to pay attention to the details, and listen to the results in their own system to decide which approach offers the best performance.