Digital video recorder
The first consumer digital video recorders (DVRs) were launched at the 1999 Consumer Electronics Show in Las Vegas. Apart from improved picture quality, DVRs provide advanced features in terms of time shifting, paus- ing live TV, instant replay of interesting scenes and chasing playback where a recording can be viewed before it has been completed. Most DVRs use the MPEG-2 for encoding analogue video signals.
Video signals may be available in a digital format directly from a digi- tal source via an HDMI or a DVI port. In such cases, the MPEG packets are recorded directly on the hard disk. Alternatively, where the video signal is analogue RGB or YCRCB signals, analogue-to-digital conversion and data compression must first be performed by the DVR before recording can take place.
Many satellite, cable and terrestrial DTV decoder manufacturers are incorporating an in-built DVR in the form of a hard disk into their set-top box such as the Sky + box and the twin-tuner terrestrial box. In these cases, encoding in not necessary as the signal is already digitally encoded. The in-built DVR simply stores the digital stream directly to the disk. With this arrangement, interactive functions can continue to be provided on recorded programs. Today, Satellite DTV set-top boxes feature two sepa- rate LNB inputs allowing two separate recordings to take place simultaneously while a recorded program is being played back. The same facility is also available in twin-tuner terrestrial set-top boxes without the need for a dual aerial sockets.
Recording files
Data (video, audio, etc.) are recorded on the had disk in the form of files. The interleaved audio and video data forms a container (AVI) file with extension .avi which can be written to the hard disk. A container is a computer file format that can contain various types of compressed data. Simpler container formats can contain different types of audio data, while more advanced container formats can support audio, video, subtitles, chapters, and control/information data known as meta-data or tags along with the synchronisation information needed to play back the various streams. Windows Media Video (WMV) is another VC-1/MPEG-2 derived codec specification which may be combined with the Windows Media Audio (WMA) into an AVI with extension .wmv or Advanced Systems Format (ASF) container (extension .asf) or a Matroska container (extension .mkv). WMV is a popular codec for video streaming on the internet together with its high- definition companion, WMV HD.