by Garystrat » 06 Jun 2012, 23:08
Hi Bojan
You are correct, the majority of "Digital Audio Interfaces" provide both input and output to speakers/headphones. A computer can only work with digital signals, so there is an ADC (Analogue to Digital Converter) and a DAC (Digital to Analogue Converter), the computer is performing its tasks between these two stages. In basic terms this is what an internal computer sound card does, but they are mostly designed to work at line level on both input and output, which is possibly not an ideal match for something like a High Z guitar output or certain types of microphone.
Most audio interfaces provide both ADC and DAC functionality in their own right, which means you don't actually need a computer sound card providing it is fully functioned (there are cut down ones that just provide input, like the IK Multimedia Stealth Plug). They normally connected using a USB connection or Firewire from the interface to the computer and are switched to the relevant device in software (effectively it becomes a dedicated sound card in its own right). It is also possible to use an audio interface in conjunction with the computers sound card, i.e. input via the audio interface and output via computer sound card, combinations can be switched in the computers input/output sound setup software.
When you go to the computers system sound settings, you will see all the usable audio devices and it is a question selecting the combination for your needs, in practice I tend to have everything set to go to the audio interface which is usually higher quality and more likely to provide better signal path matching. Both the input from the guitar and speakers/headphones are attached to the interface, the only audio connection to the PC is either USB or Firewire (although there are some new standards of connection coming along).
Audio interfaces are designed to match both instrument level (guitar) and microphone XLR connections (often with phantom power if required for the mic), normally Db switching and pots allows better control over the input signal level and they will have output level controls as well. On the older computer systems latency was a potential problem (the incoming signal was heard milli seconds slower when being converted back and forth by the ADC/DAC whilst being monitored) so it was common practice for them to be able to send the incoming signal to both the computer (for say recording) and directly to any external speakers/headphones (monitoring) attached to the audio interface in parallel.
Hope this helps.
Regards
Gary