While DVD is the base of digital recording, the hard disk has been the real innovation in digital video recorders. DVD-only recorders clearly lack flexibility, and moving from a VHS VCR to DVD alone is not totally justified. But the inclusion of hard disks provides new functions that will soon seem indispensable.
The most exciting of these is time shifting, which lets you watch segments from a program over and over without worrying about the rest of the program, which continues to be recorded. You then watch the rest of the program slightly later, and also replay it when you want. Watching while time shifting also lets you instantly skip over commercials (see the "Time Shifting: Delayed Live" page). The hard disk, of course, is what stores the recorded programs. Flexibility is complete - either programmed in advance or recorded on the spot, programs are recorded one after the other and form a library of programs you can keep for future use.
For this article we chose to test models with 80 GB, 160 GB, and 250 GB hard disks. Units of the largest size can store over 400 hours of programming at the highest compression rate, and 100 hours in DVD quality. (The second choice is preferable, of course.) In addition, almost all models have a function that lets you change the compression rate as a function of the remaining storage space. This reduces image quality, but ensures that your program won't be cut off in the middle.