Video Servers: From RAM to Digital Discs

It's hard to avoid hearing the comments that have been circulating about the servers. The truth is that the concept of the server has been around since the beginnings of video storage. The things that have changed are the speed with which we can access the information and the format of the signal that is being stored.

Let's see what a video server actually does:

- Archives audio and video material for playback.

- Access the material for use randomly and instantaneously.

- Reproduces the material in its original form.

If one stops to think, any studio that has a video library and the equipment to play the tapes would qualify as a video server. Automated video boxes that use robots to swap cassettes or records have been around for decades. The first commercial version of the video automation box was an old Ampex ACR-25 from the seventies, a four-tape cartridge system that could be programmed to serve all the commercials the TV station needs. The most recent video library management systems have used a tape for small format commercials as a file medium.

Older systems have delays of up to several seconds between the time a program is ordered until it is available for playback. Today's video servers have reduced random access time to a fraction of a second. These systems are computer-based, and consist of processors, hard drives, and ancillary equipment used to archive and retrieve video programming, and software and firmware that controls the hardware (or equipment) and compiles demographic and listing information.

The terms video-on-demand and near-video-on-demand gives an idea of how fast current servers can have programming available. In essence, video-on-demand refers to a system that can access program material without any noticeable delay between the time of ordering it until availability for playback. Near-video-on-demand means it will take a few seconds to find and submit the required material.

The cost of storing and processing audio and video signals is a big factor in determining the configuration of the video server system. Servers for video-on-demand applications use computer-style memory and hard drive archiving technology to achieve their very fast access times. Servers for near-video-on-demand systems, on the other hand, frequently use conventional television tape or disk systems, with computerized optimization of random robotic operation. Some systems archive digitized versions of video and audio signals; others archive analogous versions.

Random access memory (RAM) has long been used in the world of digitized video signal. Systems with time-based correctors, frame and field files, and digital video effect systems, for example, use RAM to archive a video signal until it is needed. The signal stays in these devices for a very short time—usually for tenths of a millisecond—because new information is written on top of the old information.

RAM chips can also be used to archive digitized video programs for a long time, but this requires a different approach: instead of reusing the same memory location, as in the RAM temporary archive, the information in each memory location remains constant until the program is no longer needed.

Servers using high-tech RAM chips provide the fastest retrieval of archived signals, with access times measured in tenths of nanoseconds (1 ns = 0.000000001 sg). Although RAM chips have been dropping in price over the past 10 years, they are still the most expensive method of storage and the size of digitized video files make servers with high-tech memory a very expensive proposition.

Most video server systems store digitized video programs on computer-style hard drives, which are rotating disks of magnetic material on top of which a series of magnetic heads move. Hard drives designed to compete with the demand for information output from digitized video signals cost considerably less than high-tech RAM. They are also slower, usually retrieving information in about 10 milliseconds (10 ms = 0.001 sg). This, however, is acceptable for many applications.

In some systems, a group of hard drives is connected over time to increase the amount of information that can be taken off the server or to improve the reliability of the information flow. In a redundant pool of low-cost disks (RAID), information is pumped from one or more disks into the same circuit, which helps replenish the relatively slow response of a hard drive by working alone. Often, RAID systems do not use all hard drives at the same time, allowing the remaining disk to serve as a backup system.

Not all hard drives are created equal. For audio and video applications special discs have been developed that can provide continuous output of required audio and video information. Normal drives interrupt their output to compensate for fluctuations in temperature that may occur while the hard drive is in use; AV drives and their control systems postpone thermal recalibration until the continuous information delivery process is suspended.

Even more expensive than RAM chips and hard drives are video discs. For servers designed to handle uncompressed analog information in applications where a couple of seconds of delay between the program order and the time it is played are not critical, video discs are an attractive storage medium. In servers built around a single disk, access time can be minimized using multiple laser readers on a playback machine. Servers that must archive multiple programs use various playback machines, as does a robotic system that archives and retrieves from a group of disks. The introduction of robotic elements increases the access time, but no disk is more than a few seconds away.

A new format, DVD, was released in 1996 and is a good candidate for use in near-video-on-demand applications. A standard-sized DVD can archive up to 17 gigabytes of information, enough space for theatrical programs.

Videotape has been the archival medium for many applications. It is not as amazing as the latest in file computing systems but they are not as expensive and are very reliable and reproduce on machines, which are known to almost every technician.

There are now two major markets for video servers: professional production and post-production companies and companies that offer pay-per-view and video-on-demand services. Based on these markets, video server producers have found that they must consider two different schools when it comes to audio and video quality. So with these possibilities available in the market it is better to know where you are going to know what type of server fits our needs and budget.