The evolution of
the video server
the video server is one of the key enabling technolo- gies behind the move from videotape recording to file-based production. The server provides the bridge between content in
the digital domain and real-time video. The device has come a long way
from its early days as a VTR replacement, and it now forms an essential
component in the carriage of video
from acquisition to air.
The roots of the video server can be
traced back to the DDR. These allowed
editors in a linear suite to bounce effects back and forth without adding
tape generations, in the analog days a
quality loss. The capacity was limited
to minutes, but it was easily sufficient
as a temporary store for short effects
clips. The video was usually handled
uncompressed, sampled to Rec. 601.
The breakthrough was to use video
compression and to leverage the ever-increasing capacity of hard drives.
Compression gave higher storage capacity, measured in hours, and eased
internal data read/write speeds to the
disks. (See Figure 1.) The Tektronix
Profile and the Hewlett-Packard
MediaStream were successful early
examples from the mid-1990s. The
former used motion JPEG compression, and the latter used MPEG-2.
Since then, the servers have evolved
so that the video server is now more
of an edge device providing an I/O
port between real-time streaming
video and the file domain.
recording, the video input must be converted to a stream of data and recorded
to disk without any loss of video data.
Conventional computing was best
effort. Writing to disk took as long as
the process took. Similarly, to play back
files, the server had to create a continuous stream of video with no dropped
frames. Video demands a deterministic system, driven by frame sync.
Even a low-cost DVR can achieve
this today, but in the 1990s there
were many obstacles to overcome.
Standard operating systems like
Windows were not deterministic, so
specialist real-time operating sys-
tems were used to manage the video
data path, with Windows providing a
management overlay. That has since
changed, and servers today run on
Windows, Linux and Mac OS.
Commercials to
program playout
The servers immediately found application for the playout of commercials. The constantly repeating playout caused wear and tear to VTRs,
and expensive library management
systems were needed to store and load
tapes to play the spots.
The move to play out programs as
well as commercials from disk required
much higher disk capacity. With program duration 60 or so times larger,
manufacturers had to wait until disk
Digital disk recorder
270Mb/s
SDI
Uncompressed video
601
270Mb/s
SDI
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Video server
270Mb/s
SDI
Compress
Compressed video
M-JPEG, MPEG-2
Decompress
270Mb/s
SDI
The video server
The challenge for the early video
server designers was to build a device
that would interface between the bursty
random access of the drive and the
constant flow of video frames. When
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