If USB is digital, how does it power my little light and PC speakers?

[idc]

So USB is a means of carrying a digital signal to a DAC. So how does it manage to power my Logitech PC speakers and the nifty USB light I have? I take it that digital coaxial and optical outputs cannot power speakers or lights, or can they?

 

[Andrew Everard]

USB carries both data and power. That's how you can charge a mobile from your laptop, or run one of these

 

[Dave_]

Because USB has four conductors/two circuits, two conductors/one circuit which carries data(D- and D+) and two conductors/one circuit that carries power(+5v and ground).

 

[laserman16]

Andrew Everard:

USB carries both data and power. That's how you can charge a mobile from your laptop, or run one of these

Official What Hi-Fi Sound and Vision issue for use at work?

 

[idc]

Does that mean optical cables are used to power these......?

 

[idc]

Serious follow on question. If USB carries both data and power, what happens, when you plug a USB cable into a USB DAC, to the power? Does the power part stop sending or is it sent on elsewhere when it gets to the DAC or is part of the power for the DAC?

 

[Anonymous]

oh dear

 

[PJPro]

idc:Serious follow on question. If USB carries both data and power, what happens, when you plug a USB cable into a USB DAC, to the power? Does the power part stop sending or is it sent on elsewhere when it gets to the DAC or is part of the power for the DAC?
For a mains powered DAC I suspect it isn't used ie the power connections on the socket are disconnected. In fact, it's the power side of the USB cable that is probably responsible (in some instances) for carrying noise from the computer to the DAC and onto the amp. With optical cables there is no power connection between the computer and DAC, hence no noise / ground loops.

 

[idc]

Thanks PJPro. So the USB connector in the DAC is missing a couple or whatever of its connectors so there is no circuit for the power. So a product such as the Fireye II USB DAC, which can powered by either the mains or USB can switch between the two, somehow.

 

[Andrew Everard]

idc:So the USB connector in the DAC is missing a couple or whatever of its connectors so there is no circuit for the power. So a product such as the Fireye II USB DAC, which can powered by either the mains or USB can switch between the two, somehow.

Yes, using the switch clearly visible in that picture. The USB connector in that DAC has all its connections, and the switch selects the power source.

By the way, HDMI cables also carry power as well as signals, which is how some switchboxes and repeaters are powered, although most also have a power input, too.

 

[idc]

Thanks Andrew. I have been reading up and see that Firewire can also carry power, USB's 5v to Firewires up to 30v and USBs up to 2.5watts to Firewires up to 60watts.

Interesting that digital coax, optical and USB are such different formats and yet can all transmit a digital signal without any apparent problems and debateable difference to sound. Maybe.