digital video recorder

Digital Video RecorderWriten by Clarence Harrell

A digital video recorder (DVR) (or personal video recorder (PVR)) is a device that records video without videotape to a hard drive-based digital storage medium. The term includes stand-alone set-top boxes and software for personal computers which enables video capture and playback to and from disk.
History
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The first DVR was tested on July 8, 1965, when CBS explored the possibilities of instant freeze-frame and rewind for sporting event broadcasts. Ampex released the first commercial hard disk video recorder in 1967. The HS-100 recorded analog video onto a digital hard disk and could store a maximum of only 30 seconds.
Hard disk-based DVRs
The two early consumer DVRs, ReplayTV and TiVo, were launched at the 1999 Consumer Electronics Show in Las Vegas. Although ReplayTV won the Best of Show award in the video category, it was TiVo that went on to much greater commercial success. The devices have steadily developed complementary abilities, such as recording onto DVDs, commercial skip, sharing of recordings over the Internet, and programming and remote control facilities using PDAs, networked PCs, or Web browsers.
This makes the ime shifting feature (traditionally done by a VCR) much more convenient, and also allows for rick modes such as pausing live TV, instant replay of interesting scenes, and skipping advertising. Most DVRs use the MPEG format for encoding analog video signals.
The two consumer DVR brands in the United States are the TiVo and DNNA's ReplayTV. In the UK TiVo has a small presence; Thomson, Topfield, Fusion, Pace and Humax also supply digital terrestrial (DTT) DVRs. BSkyB markets a popular combined EPG and DVR as Sky+.South African based Africa Satellite TV beamer Multichoice recently launched their PVR which is available on their Dstv platform.
Many satellite and cable companies are incorporating DVR functions into their set-top box, such as with DirecTiVo, DishPlayer/DishDVR, Scientific Atlanta Explorer 8xxx, Motorola 6xxx from Comcast, Moxi Media Center by Digeo (available through Charter, Adelphia, Sunflower, Bend Broadband, and soon Comcast and other cable companies), or Sky+. In this case there is no encoding necessary in the DVR, as the satellite signal is already a digitally encoded MPEG stream. The DVR simply stores the digital stream directly to disk. Having the broadcaster involved with (subsidizing) the design of the DVRand directly recording encrypted digital streamscan lead to features such as the ability to use interactive TV on recorded shows, pre-loading of programs. It can, however, also force the manufacturer to implement non-skippable advertisements and automatically-expiring recordings.

Software is available for transferring programs recorded on DVR hardware to or from PC's (e.g., TiVoToGo and DVArchive).
PCs
Software and hardware is available which can turn personal computers running Microsoft Windows, Linux and Macintosh into PVRs, and is a popular option for home-theater PC (HTPC) enthusiasts.
Linux
There are many open source and GPL-licensed DVR applications available for Linux:

• MythTV
  
• Freevo
  
• DVR
  
• VDR
  
• KnoppMyth - provides an out of the box MythTV based PVR solution
  

Commercial Linux version available from SageTV based on Gentoo Linux. Brightbox, a consumer electronics device, uses SageTV.
Macintosh
Elgato makes a series of DVR devices called EyeTV (400 / 500 / DTT etc.). The software supplied with each device (but available separately) is also called EyeTV. Elgato have recently released version 2.3 of their EyeTV software, with a redesigned interface, better program guide (built into the program) and a 'One Click' to iPod video feature. Miglia also makes DVR hardware that runs the EyeTV software, as well as DVR devices that use other software.
Other DVR products include myTV.PVR from EskapeLabs and ConvertX PVR from Plextor. MythTV (see above) also runs under OS X, but most recording devices are currently only supported under Linux. Precompiled binaries are available for the MythTV front-end, allowing a Mac to watch video from (and control) a MythTV server running under Linux.
Apple provides applications in the FireWire software developer kit which allow any Macintosh with a FireWire port to record the MPEG2 transport stream from a FireWire equipped cable box (e.g., Motorola 62xx, including HD streams). Applications can also change channels on the cable box via the firewire interface. Only broadcast channels can be recorded as the rest of the channels are encrypted. iRecord is a free scheduled-recording program derived from this SDK.
Windows
Microsoft Windows has several free DVR applications including GB-PVR, Got All Media and MediaPortal. DScaler has also DVR support in works.
There also are several proprietary applications including SageTV, SnapStream Beyond TV, ChrisTV, Showshifter, Meedio (now a dead product - Yahoo! bought most of the company's technology and discontinued the Meedio line, and rebranding the software Yahoo! Go - TV, which is now a free product), InterVideo WinDVR, Recordit Plus and the R5000-HD.
There is also a separate version of Microsoft Windows called Windows XP Media Center Edition which has DVR capabilities.

And Microsoft Windows Vista will include in its standard version the Media Center functionalities.
Source video
Television and video are terms that are sometimes used interchangeably, but differ in their technical definitions. Video is the visual portion of television, whereas television is the combination of video and audio modulated onto a carrier frequency (i.e., a television channel), so that multiple frequencies (i.e., multiple channels) may be transmitted at the same time.
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 DVR-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 Player.
Digital television
Digital television is 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 DVR supports both international DVB signals and American ATSC signals while the HDTV Tivo supports only the ATSC signals. In the U.S., the FCC attempted to place a road-block before digital DVRs with its Broadcast flag regulation. Digital video recorders which had not won prior approval from the FCC for implementing effective digital rights management would have been banned from interstate commerce as of July 2005. The regulation was struck down on May 6, 2005.
DVD-based PVRs available on the market as of 2006 are not capable of capturing the full range of the visual signal available with high definition television (HDTV). This is largely because HDTV standards were finalized at a later time than the standards for DVDs. However, DVD-based PVRs can still be used (albeit at reduced visual quality) with HDTV since currently available HDTV sets also have standard A/V connections.
Satellite or Digital Cable
Recording satellite or digital cable signals on a digital video recorder is more complex than recording analog signals or broadcast digital signals. This is so because the MPEG-2 or MPEG-4 stream is usually encrypted to prevent people from viewing the content without paying for it (usually via subscription).
The satellite or cable set-top box does two things. First, it decrypts the signal. Second, it decodes the MPEG stream into an analog, DVI, or HDMI signal for viewing on the television. In order to record cable/satellite digital signals you must get the signal after it is decrypted, but before it is decoded (between steps one and two); this is how DVRs built into set-top boxes work.
An alternative is that some satellite or (more commonly) cable set-top boxes have a FireWire port that can be connected to a computer. The recorded MPEG stream can be relayed to the computer via this FireWire port; though it can be done live, this is more commonly used for transferring shows from a set-top box with built-in DVR. (For instructions on doing this on a popular set-top box with DVR, please see the Wikibook entry How to use a Motorola DVR; some of the ideas there may apply to other set-top boxes as well.)
DVD
Many DVD-based DVRs are equipped with two DVD drives or an additional internal hard drive. This arrangement can be used to copy content from a source DVD, which is disallowed in the US under the Digital Millennium Copyright Act if the disc is encrypted. Most such DVRs will hence not allow recording of video streams from encrypted movie discs.
Digital camcorders
Some DVD-based DVRs incorporate a Firewire connector which can be used to capture digital video from a MiniDV or Digital 8 camcorder, possibly recording a simple DVD as the camcorder is played back. Some editing of the resulting DVD is usually possible, such as adding chapter points.
Security applications
Digital video recorders configured for physical security applications record video signals from closed circuit television cameras for detection and documentation purposes. Many are designed to record audio as well. DVRs have evolved into devices that are feature rich and provide services that exceed the simple recording of video images that was previously done through VCRs. A DVR CCTV system provides a multitude of advanced functions over VCR technology including video searches by event, time, date and camera. There is also much more control over quality and frame rate allowing disk space usage to be optimized and the DVR can also be set to overwrite the oldest security footage should the disk become full. In some DVR security systems remote access to security footage using a PC can also be achieved by connecting the DVR to a LAN network or the internet.
Security DVRs may be categorized as being either PC based or embedded. A PC based DVRs architecture is a classical personal computer with video capture cards designed to capture video images. An embedded type DVR is specifically designed as a digital video recorder with its operating system and application software contained in firmware or read only memory.
Hardware Features
Hardware features of security DVRs vary between manufacturers and may include but are not necessarily limited to:

• Designed for rack mounting or desktop configurations.
  
• Single or multiple video inputs with connector types consistent with the analogue or digital video provided such as coaxial cable, twisted pair or optical fiber cable. The most common number of inputs are 1, 4, 8, 16 and 32. Systems may be configured with a very large number of inputs by networking or bussing individual DVRs together.
  
• Looping video outputs for each input which duplicates the corresponding input video signal and connector type. These output signals are used by other video equipment such as matrix switchers, multiplexers, and video monitors.
  
• Controlled outputs to external video display monitors.
  
• Front panel switches and indicators that allow the various features of the machine to be controlled.
  
• Network connections consistent with the network type and utilized to control features of the recorder and to send and/or receive video signals.
  
• Connections to external control devices such as keyboards.
  
• A connection to external pan-tilt-zoom drives that position cameras.
  
• Internal CD, DVD, VCR devices typically for archiving video.
  
• Connections to external storage media.
  
• Alarm event inputs from external security detection devices, usually one per video input.
  
• Alarm event outputs from internal detection features such as motion detection or loss of video.
  

Software Features
Software features vary between manufacturers and may include but are not necessarily limited to:

• User selectable image capture rates either on an all input basis or input by input basis. The capture rate feature may be programmed to automatically adjust the capture rate on the occurrence of an external alarm or an internal event
  
• Selectable image resolution either on an all input basis or input by input basis. The image resolution feature may be programmed to automatically adjust the image resolution on the occurrence of an external alarm or an internal event.
  
• Motion detection: Provided on an input by input basis, this feature detects motion detection in the total image or a user definable portion of the image and usually provides sensitivity settings. Detection causes an internal event that may be output to external equipment and/or be used to trigger changes in other internal features.
  
• Lack of motion detection. Provided on an input by input basis, this feature detects the movement of an object into the field of view and remaining still for a user definable time. Detection causes an internal event that may be output to external equipment and/or used to trigger changes in other internal features.
  
• Direction of motion detection. Provided on an input by input basis, this feature detects the direction of motion in the image that has been determined by the user as an unacceptable occurrence. Detection causes an internal event that may be output to external equipment and/or be used to trigger changes in other internal features.
  
• Routing of input video to video monitors based on user inputs or automatically on alarms or events.
  
• Input, time and date stamping.
  
• Alarm and event logging on appropriate video inputs.
  
• Alarm and event search.
  
• One or more sound recording channels.
  
• Archival.
  
• Remote control.
  

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