Personal video recorder

From Academic Kids

The personal video recorder (PVR), also called digital video recorder (DVR) or digital personal video recorder, is a consumer electronics device that records television shows to a hard disk in digital format. Since first introduced by TiVo at the Consumer Electronics Show in 1999, PVRs have steadily developed complementary abilities, such as recording onto DVDs.

This makes the "time shifting" feature (traditionally done by a VCR) much more convenient, and also allows for "trick modes" such as pausing live TV, instant replay of interesting scenes, and skipping advertising. Most PVR recorders use the MPEG format for encoding analog video signals.

The most popular PVRs on the market are the TiVo and DNNA's ReplayTV, although most home electronics manufacturers now offer models. Many satellite and cable companies are incorporating PVR functions into their set-top box, such as with DirecTiVo, Motorola 6xxx from Comcast, Moxi Media Center from Charter, or Sky+. In this case there is no encoding necessary in the PVR, as the satellite signal is already a digitally encoded MPEG stream. The PVR simply stores the digital stream directly to disk. Having the broadcaster involved with (subsidizing) the design of the PVR, and directly recording encrypted digital streams can lead to fancy features - like the ability to use interactive TV on recorded shows, pre-loading of programmes; but can also lead to too much control by the broadcaster - like denying the ability to skip adverts and automatically expiring recordings after a time determined by the broadcaster.

Other entrants into the market include products such as Microsoft's Media Center.

In 2003, the Yakima, Washington Police Department began using PVRs in their patrol cars to record the activities of officers and suspects. Since then, many other police departments have followed suit, due to the increased reliability and decreased cost compared to analog video systems.

There are ways to make one's own PVR using software and hardware available for Microsoft Windows, Linux and Macintosh operating systems. There are even people working on turning the Xbox into a PVR with a modchip.


How a digital/personal video recorder works

Analog television

Analog television in NTSC, PAL or SECAM formats, analog cable, or regular VHS tapes use a signal that is fed directly to the electron beam within the television set. There are a number of details on how this is done, but in essence each line in each frame corresponds to a specific fraction of time within the signal.

To record an analog signal a few steps are required. A TV tuner card tunes into a particular frequency and then functions as a frame grabber, breaking the lines into individual pixels and quantizing them into a format that a computer can comprehend. Then the series of frames along with the audio (also sampled and quantized) are compressed into a manageable format, like MPEG-2, or WMF, usually in software. Some TV tuner cards like the PVR-250/350 or the TiVo chip deliver an MPEG-2 or other compressed stream directly to the computer, performing both the frame grabbing and compression in silico. This greatly reduces the load on the CPU allowing an overall cheaper implementation .

Analog Broadcast Copy Protection

Many mass-produced consumer DVRs implement a copy-protection system called CGMS-A (Copy Generation Management System--Analog). This encodes a pair of bits in the VBI of the analog video signal that specify one of the following settings:

  • Copying is freely allowed
  • Copying is prohibited
  • Only one copy of this material may be made
  • This is a copy of material for which only one copy was allowed to be made, so no further copies are allowed.

CGMS-A information may be present in analog broadcast TV signals, and is preserved when the signal is recorded and played back by analog VCRs, which of course don't understand the meanings of the bits. But the restrictions still come into effect when you try to copy the tape onto a DVR.

Digital television

Digital television are audio/visual signals that are broadcast over the air in a digital rather than analog format. Recording digital TV is generally a straightforward capture of the binary MPEG-2 data being received. No expensive hardware is required to quantize and compress the signal (as the television broadcaster has already done this in the studio). The MythTV PVR supports both European DVB signals and American ATSC signals while the HDTV Tivo supports the ATSC signals. In the US, the FCC attemped to place a road-block before digital PVRs with its "Broadcast flag" regulation. Personal Video Recorders which had not won prior approval from the FCC for implementing "effective" digital restrictions management would have been be banned from interstate commerce as of July 2005. The regulation was struck down on 6 May 2005.

Satellite or Digital Cable

Recording satellite or digital cable signals on a personal video recorder is more complex than recording analog signals or broadcast digital signals. This is so because the MPEG-2 stream is usually encrypted to prevent people from viewing the content without paying for it (usually via subscription to a valid satellite decryption box and a decoder card).

The satellite or cable decoder box does two things. First it decrypts the signal. Second, it decodes the MPEG-2 stream into an analog signal for play on the television. In order to record cable/satellite digital signals you would need to get the signal after it is decrypted but before it is decoded (between steps one and two).

An alternative is that some satellite/cable decoder boxes have a firewire port that can be connected to a computer. The MPEG stream could be relayed to the computer via this firewire port although there is as yet few, if any, current cards or devices that allow for a firewire connection to the computer from this box.

PVR software

There is PVR software available for Linux and Windows for people who make their own homemade recorders.


The two main PVR applications for Linux are MythTV and Freevo both of which are GPL open source software.


Elgato makes a PVR device called EyeTV (


Microsoft Windows has several free PVR applications including GB-PVR and Media Center Portal.

There also are several proprietary applications including SageTV, SnapStream Beyond TV, ChrisTV, Showshifter, Meedio, InterVideo WinDVR and Recordit Plus.

See also: Hard disk recorder, DVD recorder

External links


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