Going on record

I've seen articles in magazines where behaviourists have used recording equipment to see what is going on when owners are out. My video recorder only lasts for half an hour before dying, so can anyone tell me what they use and where I can buy/hire it?
My neighbour has just told me that one of my dogs is regularly barking when I am out, all is quiet when I return, and as I have four I really need to find out which and why, so I can address the problem before it becomes a complaint.
Any help would be appreciated.

Ann Button, by email