[earplay]

The Realtek audio device on my Compaq Presario CQ56-115DX Notebook PC gives output options up to 24/196. This seems unusually high for such a low-end machine. 

I wonder if this is an actual output thru the USB port. Anyone here that can shed some light on this issue? The HP website gives specs for this machine, but not in enough detail to answer my question.

[srb]

When using USB to connect to an external DAC, the external DAC is the "audio device", not the Realtek audio chipset.  The Realtek chipset has nothing to do with the USB port.
 
Most Realtek chips themselves can do 24/192.  If you are looking at the audio devices, do you have more than one Realtek audio device?  If you only have one listed, it is likely listed as Speakers - Realtek High Definition Audio which would be the analog sound output either through the internal speakers or the headphone jack, and would have the capability to process 24/192 internally.
 
If your laptop happens to have a combination headphone/digital S/PDIF optical jack (?), you could also have a second Realtek audio device, and it would be called Realtek Digital Output.  Although there may be a supported format checkbox of 192KHz, that is generally not supported over TOSLINK optical, and there are almost no laptops with a coaxial S/PDIF output, which can support 24/192.
 
So back to your USB port.  Windows native USB drivers only support up to 24/96 out of the USB port and anything higher will require require (a) USB 2.0 port - which your laptop has, (b) an external USB DAC capable of 24/192 and (c) a third-party or proprietary USB driver for that DAC.
 
Steve
 
 

[earplay]

See! This is what makes AudioCircle a great resource. People like srb (Thank you) and everyone else that is willing to share their time and knowledge.