USB and Volume Adjustment

[PJH]

Hello,

 

I was surprised to find out that when I plugged my DAC in to my Powerbook via USB that I was able to adjust the volume with my Powerbook. I know that I am supposed to have the volume at maximum but I am curious about what is happening to the "bit perfect" digital when turning the volume down on the Powerbook and is it really bit perfect even at full volume? It must be normal to be able to adjust the volume via USB since my DAC also has a headphone output. I am using a Blue Circle USB Thingee. Could someone explain how volume control is achieved via USB and what is happening to the digital signal?

 

PJH

 

[The Computer Audiophile]

Hey PJH - This is one of those things that I kind of understand but am nowhere near good at explaining it. Hopefully Gordon will pop over here to share some of his knowledge.

 

When I connect a DAC to my MacBook Pro the system volume gets locked automatically at 100% I can still adjust the level within iTunes and from what I understand this has no negative effect on the audio stream. I don't think this is a general statement that holds true for many other applications.

 

I think Gordon's new Proton DAC is going to have the volume level only adjustable via an application that comes with the DAC. I'm willing to bet this one will be bit perfect :-)

 

[rom661]

I'm probably out of my element here, but since the output is digital from the USB of the computer, I can't imagine that the volume control is not digital. Leaving aside all other issues, I have used two expensive pieces, the Meridian 518 and a Z-Systems digital EQ unit that both can function as preamps. This was in the day of separate transports and DACs and both of these pieces, when outputting 16 bit signals, noticeable degraded the sound as you turned them down.

 

I have tried the volume control in Itunes and, at least in my opinion, I heard the same degradation of quality, somewhat similar to a lossy compression sound.

 

I was already predisposed to go that direction and I didn't spend a lot of time experimenting, but there's my two cent's worth.

 

Interestingly, back in the day, when my Levinson DAC was upgraded to 24 bit, I set both of the above mentioned devices to convert the 16 bit input to a 24 bit output. I know this is something there is a lot of debate about but leaving that aside, the ability to use the digital volume control became viable at that point. I didn't hear noticeable degradation unitl I was getting down into background music levels.

 

[radar2886]

Benckmark wrote a good white paper on this subject a while back. I have not read it in a while, but I think they basically recommended setting the volume at max and making sure that the computer was not doing any converting.

 

[The Computer Audiophile]

from the Benchmark website:

 

 

"The Volume Control in iTunes v7 is very-high quality under both operating system. It is very well designed and operates at 24-bits for audio devices that support 24-bit operation. The end user should not hesitate to use the volume control in iTunes 7.x under either operating system."

 

[Wavelength]

Guys,

 

There can be two different vol controls in any operating system. The system controller is the optional one. The application volume control is the one that really has to be put a full volume.

 

The system one can by the USB device declare an internal volume control. This usually controls the volume control in the dac chip itself. Many of these have some sorta mechanism for setting the volume and this would have nothing to do with the bit resolution.

 

Enumeration is what tells the computer what the device can do. If you open up the Sound preferences and select the dac if it shows a volume control there then this is capable of doing volume control without losing bits. If it is greyed out then the application volume control would be the only one present and that one WILL change the bit resolution going to the dac.

 

Thanks

Gordon

 

[Innertuber]

Volume control? You mean you can turn it down? Now, why on earth would you want to do that? Sounds like a conspiracy by the neighbors.