How DVR Works


DVRs advertise anywhere from 35-hour to over 300-hour recording capabilities. It's worth noting that these units can record programs at varying levels of quality -- and the advertised capacity usually refers to the number of hours it can hold at the lowest quality setting.

­As an example, TiVo can record programs at four different quality levels: basic, medium, high, or best. A 40-hour TiVo unit can hold 40 hours at the basic quality level, but only about 11 hours at the best setting. The hard drive in a 40-hour TiVo is approximately 40 gigabytes in size; on the basic setting, one hour translates to 1 gigabyte, while at the highest setting one hour uses 4 gigabytes.

Newer DVR systems can record HDTV signals. The Series3 HD TiVo can record up to 300 hours of standard television (on the Basic quality setting) or up to 32 hours in HD format.

So what's the difference? If you've ever seen full-motion video on the Web, you know how images can get blocky and distorted. This happens on DVR recordings made at low quality levels, particularly if there is a lot of movement in the image. As a result, different quality settings are good for different types of programs: while an old black-and-white movie or a talk show will look just fine at the basic level, a fast-moving sports program or action movie will be almost unwatchable. So bear this in mind if you're thinking of buying a DVR primarily to support your sports habit -- better to go for the higher capacity unit.

File Sizes

In Sony's Giga Pocket system, the files for each recorded program are stored on the computer's normal hard disk. If you have a drive with 60 free gigabytes of space, then Giga Pocket can use those 60 gigabytes to store TV programs. The amount of space that a program consumes on the hard disk depends on both the length of the program and the recording quality. Giga Pocket offers three quality modes:

  • LP - Stored as a highly compressed MPEG-1 file, consuming 0.6 gigabytes in a one-hour program
  • SP - Stored as a moderately compressed MPEG-2 file, consuming 1.7 gigabytes in a one-hour program
  • HQ - Stored as a high-quality MPEG-2 file, consuming 3.35 gigabytes for a one-hour program

In other words, you can store about 100 hours of video on 60 gigabytes of space in LP mode. In SP mode, you can store about 36 hours. In HQ mode, you can store about 18 hours.

In terms of quality, LP mode has a noticeable grain to it, but it's watchable. SP mode looks good. HQ mode seems like overkill -- there's not a noticeable difference between SP and HQ when you are watching a program recorded from cable. Perhaps if you were recording an s-video signal coming in from a DVD player or camcorder and you wanted to preserve all the detail, HQ mode would be useful.

Just like on a computer hard drive, deleting a program from a DVR doesn't actually delete the program itself -- it simply erases the file system's reference to where it's stored and how long it is, making it effectively gone. The raw program data remains on the drive until it is overwritten by a new recording.