Grass Valley PROFILE FAMILY v.2.5 - Manual (Page 25)

Video Compression
Profile Family 25
playback. By adding a slave disk recorder board, a Profile unit becomes a four-
channel JPEG system. The master board has a Intel i960 real-time processor
which controls compression and the data flows on SCSI-2 channels and JPEG
codecs. Master and slave EDR boards also control MPEG encoder and decoder
boards which are connected to the master and slave via a PCI interconnect
board.
Video Compression
The i960 on the master board is used to control data flow and compression
coefficient loading of the JPEG codecs and, if present, MPEG encoders and
decoders. The amount of JPEG video compression varies according to the
setting of the compression coefficient; the amount of MPEG video compression
varies according to the bit rate. Higher compression ratios or bit rates store
more video, but the result is lower quality video. On the other hand, lower
compression ratios or bit rates result in higher quality video and less storage
capacity. Audio, however, is not compressed.
Since the video compression ratio can be varied to change the video quality
given available storage time, the amount of storage depends on your choice of
compression ratio. A quick rule of thumb is that five minutes of JPEG
video—plus four channels of audio and two channels of timecode—is roughly
equal to one gigabyte of disk storage at 75,000 bytes per field in the 525
standard video format. For example, a PDX 208 Disk Expansion unit expands
storage up to twelve hours and a PRS200 RAID Storage System can bring it up
to approximately 96 hours. For video stored in the MPEG format at an average
24 Mbps, you can just about double these capacities.
In addition to video compression, the disk recorder boards also integrate the
digital audio data coming from the EISA bus, with typically four channels of
audio per channel of video (up to 32). These recorder boards communicate with
the SCSI-2 interface using a Direct Memory Access (DMA) interface. The
PDR 200 also supports the audio signal processing board (ASPB). This board is
capable of delivering 16 channels of analog, embedded digital, or AES/EBU
digital audio. The PDR 200 can be equipped with two of these boards, for a total
of 32 channels of audio.
Video and Audio Interface Boards
Video and audio interface boards receive incoming and send outgoing video
and audio data. These boards are responsible for converting the video and audio
to internal formats used by the video server.