Computer Audio Asylum: RE: Basic Questions about USB Audio on PC by John Swenson

The USB bus is purely a digital bus so there can never be any analog audio on a USB cable. Does that answer your questions about analog or are you asking something different such as are the analog outs on a motherboard active at the same time as the USB port?

If you are using the builtin USB audio driver that comes with operating systems, it only supports 2 channel PCM. Some USB audio devices have their own proprietary drivers and these can do multiple channels. In order to do this you would have to explicitely select the USB device in the DVD player program and set it up for multi-channel support.

In general USB drivers only change sample rates when they have to. For example if you have a USB DAC that only supports 44.1 and you play a 48 file it will have to change the sample rate.

Whther the infamous K-MIXER resamples things depends on how you set things up. If you set the USB device as the default windows audio output device and just use the default output settings of a player program then you will get whatever windows throws in the path. But if you explicitely select the USB device as the output from the player program then the audio will go direct to the USB device. Of course you can only do this if the player program lets you choose the output device.

I'm not quite sure what your last question is trying to ask. It sounds like you are talking about a USB device that has an S/PDIF output, correct? Some devices have a "Passthrough" mode specifically designed for outputting Dolby Digital or DTS on the S/PDIF output. In this mode the DVD player program is in charge of making sure the multichannel audio is properly encoded, it just sends the encoded data as a regular two channel stream to the USB device which just outputs it on the S/PDIF port without changing it in any way. As long as the player program has done its job correctly a Dolby Digital or DTS receiver should be able to properly decode the data. In order to make this work you have to properly setup the DVD player program, as well as put the USB device in the "passthrough" mode. (the passthrough mode just means it doesn't change the data in ANY way, including applying volume control etc). Just make sure you don't listen to the analog outs from the USB device in this setup, they will be outputting high level noise which could easily damage speakers.

I hope this clears things up.

John S.