When done with the contrast detect method, you can say it is software-based. That is, the camera reads the sensor, computes contrast and moves the lens according. It repeats the process until focus is locked... and then repeats it when contrast drops to reacquire focus.
This is what most cameras due for autofocus while recording and is rather annoying as the movement of lens back and forth to lock focus is disturbing.
When done with the phase-detect method, you need to have a hardware component. This is that happens with SLT cameras and why they can focus much faster and without the back-and-forth movement of the lens. In this case the translucent mirror reflects light to a special phase-detect sensor while video is recording. That sensor measures how far off focus is and moves the lens to that position directly.
That being said, even the phase-detect method is not ideal since you do not have control over what the camera is following. Professional videos are made using manual-focus because you can control where focus is and when it changes. See this question for someone asking to slow-down the AF of the SLT-A55 because it keeps following the wrong subject.
If you absolutely have to have autofocus while recording video, my advice is to buy an A55 (or one of its follow ups coming soon). It is a nice camera and will be compatible with all the lenses you use on you A580.