2

DIGITAL DISK RECORDING

 

 

Videotape has remained the predominant form of recording and storage for moving images dating back to the 1950s when Ampex Corporation developed what would be the first commercial form of television recording. Within a decade following that introduction, alternative means of capturing and playing back media on formats other than videotape and film would start to emerge.

Those who worked in the broadcast industry during the late 1960s may remember the earliest form of commercial disk recorders—camouflaged under the hood as the “instant replay” device. Like what the analog videotape recorder of its day did for television programming and broadcasting, this analog video disk recorder with its stunt features of forward, backward, still, and other non-real-time capabilities would prove to be the catalyst that single handedly changed the viewing experience for televised live sports forever.

Another 20 years would pass before development of nonlinear recording and playback would again have a serious commercial and professional impact. For disk-based recording to succeed, a new set of core technologies plus a paradigm shift in workflow would be necessary in order for the ideas, concepts, and practicalities of today’s digital disk-based recording to occur.

Many of the digital disk recorder (DDR) developments have made their way well into the consumer space, changing the way we view television and moving media forever.

Another 20 years would pass before development of nonlinear recording and playback would again have a serious commercial and professional impact. For disk-based recording to succeed, a new set of core technologies plus a paradigm shift in workflow would be necessary in order for the ideas, concepts, and practicalities of today’s digital disk-based recording to occur.

Many of the digital disk recorder (DDR) developments have made their way well into the consumer space, changing the way we view television and moving media forever.

KEY CHAPTER POINTS

•A historical development of analog recording on videotape and disk

•The impact from analog instant replay, and how disk recording technologies evolved

•The core technologies responsible for advancements in moving media storage and applications

•Digital disk recorders as clip stores, still stores, and media recorders

•The JPEG and MPEG picture compression formats, with an introduction to the video and image compression technologies used in the DDR/DVR and early nonlinear editing systems

•DDRs and DVRs in the marketplace, professional and home media recorders, and server platforms, including audio

Recording Moving Images

Recording video images onto both magnetic linear tape and spinning magnetic storage surfaces had similar and parallel developments. Alongside, the development of magnetic recording tape, random-access video-on-demand, which had been predicted in 1921, was demonstrated in principle as early as 1950.

The concept of recording video onto a spinning platter would be shown just shortly before the 1951 John Mullin (Bing Crosby Enterprises) demonstration of an experimental 12-head videotape recorder (VTR) that ran at 100 in. per second. To put this into perspective, this development occurred about the same time as the first disk drive and the provisions for color NTSC (in 1953) were being introduced.

A rudimentary plastic video disk was demonstrated at the Salone Internazionale della Tecnica in 1957 by Antonio Rubbiani, and a few years later, technologists at CBS developed a procedure for a video disk recorder.

Origins of Videotape Recording

Recording video onto linear magnetic media (as videotape) was developed by a team of engineers at Ampex Corporation led by Charles Ginsburg, who began work on the videotape recorder (VTR) in 1951. Ampex would eventually manufacture both the magnetic tape and the recording systems that would support the process, but not at first.

Ampex demonstrated its first three-head system in November 1952 and, in March 1953, a second system, using four heads, was shown. However, problems continued with the “Venetian blinds” effect due to discontinuous recording from one head to the next. In 1954, Charles Anderson and the Ampex team, including Shelby Henderson, Fred Pfost, and Alex Maxey, were working on an FM circuit that debuted in February 1955. Later, Ray Dolby designed a multivibrator modulator, Maxey discovered how to vary tape tension, and Pfost developed a new sandwich-type magnetic head. The product’s technology would be dubbed the quadruplex (quad) videotape recorder.

A half-century later, in 2005, the original team would be awarded, some posthumously, the Lifetime Achievement in Technology Emmy for their contributions of what would be remembered as the “quad” VTR.

In preparation for the first public demonstration of video recording at the Chicago convention of the National Association of Radio and Television Broadcasters (NARTB) in April 16, 1956, an improved VTR, later to become the Ampex Mark IV, was shown to Bill Lodge of CBS and other TV people. The Mark IV, later renamed the VRX-1000, used 2-in. wide tape, which ran at 15 in. per second past a transverse track, rotating head assembly, and employed FM video and AM sound recording. Ampex took out a trademark on the name videotape for its recorder. In 1959, color videotape was debuted during the Nixon-Khrushchev Kitchen Debate in Moscow.

Dawn of the Video Disk

The Minnesota Mining and Manufacturing Company (3M Company), which produced the first 2-in. wide videotape for the VRX-1000, showed a 1964 noise-plagued video disk, publicly demonstrating that this new disk format had a future. Although less than a year before the first demonstration of a random access, still-image generating, recording, and playback device, the demonstration utilized disk-drive recording technologies and changed the future of recording in a profound way. Some 20 years later, the concept of a “video serving device” would emerge, a breakthrough that took its roots from television sports, and began in the form of the instant replay.

Television Instant Replay

At the July 1965 SMPTE conference in San Francisco, MVR Corporation showed a 600-frame (20 second), black-and-white video recorder, the VDR-210CF model, that would record individual video frames. CBS used the device as a freeze-action video disk around August of that year. The MVR, with its shiny aluminum, nickel-cobalt-coated magnetic disk, was used in football telecasts to instantly play back short-action sequences in normal motion or freeze the motion on a single frame.

Ampex took a different approach creating an 1800 RPM spinning metal disk with a series of stepper motor-driven recording heads that moved radially across the platters, creating 30 seconds of normal video using analog recording technologies. The diskbased system recorded 30 video tracks per second, with each track holding one NTSC frame, giving a total of 1800 NTSC fields.

The heads could be rapidly moved to any location on the disk for replay at normal speed, or when the heads were slowed down and the same frame was repeated in multiple sets, a slow-motion playback stunt feature could be created. When the playback stepper heads were paused and the platters continued to spin, the same frames were repeated with a freeze frame continuously displayed.

In March 1967, the first commercially available video magnetic disk recorder with appropriate slow and stop motion, the Ampex HS-100, was placed into service. Effects included rapid playback at normal speed, forward and reverse slow motion, and stop action (freeze frames). The first commercial use of the Ampex HS-100 was at the World Series of Skiing program for the US Ski Championships in Vail, Colorado; it marked the dawn of the disk recorder for instant replay in live television broadcasting.

Digital Recording Supplants the
Analog Disk Recorder

The analog video disk recorder had a long and productive life. It would find use in video editing under computer control, in special effects creation, as a substitute for another VTR transport, and in other creative endeavors. Ultimately, this disk-based recording device proved to be not just profitable but expensive and marginally unreliable with highly specialized maintenance skills and a high cost of ownership. The disk recorder would be confined mostly to sports but did find its way into editorial and special effects in commercial postproduction.

As the dawn of digital technologies emerged, takeoffs of the disk recorder took on other aspects. The devices would find themselves embedded in special effects systems used for video graphics and compositing in the mid-1980s. By the end of the 1980s, there was sufficient demand and experimentation with digital video disk recording that true product lines would emerge. As hard disk drive technologies for the emerging computer data industry took off, the disk recorder would be used in the products for graphics and still images. Ultimately, the concepts of dedicated disk recorders would pave the way to the infancy of the videoserver and the era of digital media storage technologies.

Fundamentals of Digital Disk Recording

The broadcast and postproduction industry have seen the migration from transport-based linear tape recording to professional videoserver and nonlinear digital disk recording. Workflow, technology, and operational costs have all contributed to this paradigm shift. The transition from videotape to magnetic disk storage and server-based production is now well into its third decade. Along the way, we’ve seemed to have lost the developmental evolution of the mainstream disk recorder and the nonlinear editing (NLE) platform. We find that historically the concepts of the professional video disk recorder had been around long before NLEs and videoservers.

The videoserver is a descendent of the digital disk recorder (DDR). The earliest DDRs would use similar recording principles to those used in the D-1 videotape format that recorded video sampled as 8-bit (ITU-R BT.601) data as individual paired sets of Y-frames (luminance) and C_B/C_R frames (based on R-Y and B-Y color difference signals) onto predetermined track locations stored progressively and continuously around the magnetic surface of each respective Y and C_B/C_R disk drive pair.

In these early digital video disk recorders, paired sets of hard disk drives (from two to many) would be configured such that the component elements of the individual video frames were recorded to dedicated tracks so that when played back there would be minimal latency when searching for frames and almost no need for buffering of the video data for synchronization purposes.

If one looked at the track mapping on the drives, you would find that the real estate allocated for a given frame would be consistent each time the frame was recorded or played out. For example, frame 39’s Y component always could be found in the thirty-ninth position of the Y drive, with the corresponding C_B/C_R color difference frame always mapped to the thirty-ninth position on the C_B/C_R drive. The two sets of disk drives, running in exact synchronization, were either written to (during record) or read from (during playout) one frame at a time. This concept allowed for repetitive stop motion, frame-specific replacement for editing, or precise sequential playback as video clips without necessarily recording time code or other counter information from the original video sources.

One early DDR recorded precisely 750 frames of luminance data on one drive and 750 frames of color difference data on the other, for a total of 25 seconds of NTSC (at 30 frames per second) or 30 seconds of PAL (at 25 frames per second). Using external or integrated frame buffer technologies, a frame could be read into memory, manipulated (e.g., painted or layered onto another image) externally, and then written back to the same position on the same disk drives. The result was frame-by-frame graphic image manipulation, the concept leading to the early days of digital or computer animation.

Purpose-Built Applications

Products would be introduced that used multiple sets of hard drives purposely configured as digital disk recorder drive sets and combined internally or with other external devices, such as compositors and digital video effects units. Disk recorders were first devised to record still images that could be used in live and postproduction graphics. Most of the applications were dedicated to the platform they were created on. Evolution led to the exchange of the data on removable spinning media. These types of DDR devices would eventually acquire extended capabilities that allowed a series of frames to be played back, thus generating what would be called the (short) clip, a term coined to represent a sequence of video frames assembled as either an animation or a string of video. In almost every case, the systems were proprietary although some companies devised a means to move the “files” between comparable devices over a network transport such as 10base2 and 10base5 (“thicknet”) Ethernet at rates well below 10 Mbits/second.

These innovations triggered the age of digital video graphics and animation on professional video platforms. In parallel with these video-specific applications, the emerging growth of personal computers and higher end computer graphics workstations would again change the working way of television production.

Professional video producers would utilize these high-quality, full-bandwidth video disk recorders and integrate them into editing systems or computer graphics imaging (CGI) systems. Broadcasters would still use the store devices for live news telecasts. However, it wouldn’t be until video compression technologies began to develop that there would be a significant deployment of disk drive storage technologies for nonlinear video editing systems. Any widespread adoption of disk recording systems for motion video would have to wait for motion-JPEG and MPEG compressions.

First String Players

Products from companies such as Quantel (Paintbox and Harry) and Abekas Video Systems (A42, A60, A62 and A64) would become commercially available in the 1980s. Abekas’s first product, introduced in 1982, was the A42 Digital Video Still Store. The product, generically called just a “still store,” recorded and played back still video images with an associated key signal for use in live broadcast operations. The A62 Digital Disk Recorder was dubbed the world’s first digital video recording machine with a built-in real-time digital video keyer.

Various other digital tape-based and disk-based products were introduced in both component digital (the D-1 format described through ITU-R BT.601) and composite digital (D-2) format. These devices gained acceptance within the broadcast production marketplace around the beginning of the 1990s.

However, the industry faced a problem. People needed more storage capacity. Moving video consumed a lot of data in D-1 or D-2 forms. Reliable, large-capacity hard disk drives in the mid- 1980s were still rare and very expensive. Modern storage systems even remotely similar to today’s storage area networks (SAN) or network-attached storage (NAS) were still years away. Large-scale video playout systems with record or playout times in excess of 200 seconds were far too costly to implement. Thus, to increase the amount of video storage, video compression technology would have to come to the rescue. The high-performance, high-bit rate compression technologies taken for granted today were still in the laboratory stage and slowly making their way through the standardization processes.

Core Technologies for Digital
Disk Recording

Three core technologies have aided in the development of the digital disk recording platforms used throughout the media and entertainment industry.

Spinning Magnetic Disks: The First Core Technology

The development of the spinning magnetic disk drive (and ultimately disk arrays) would be paramount to the success of digital video recording and storage. Although digital linear tape for professional video recording was available before disk drives of comparable storage capacities, the disk drive made possible nonlinear storage and retrieval—the “emerging technology” that promoted the successful development of digital media overall.

The spinning disk would, and still today, prove essential to the development of modern compressed and digital video recording. Even though there have been great successes with optical disc platforms, solid state and flash memory, and even to a lesser degree, holographic recording, the hard disk drive still remains the predominant storage media for moving images.

With that said, the early development of compressed digital images was governed by the storage capacity of the hard disk and by the processing technologies available during these early times. Much like the software that didn’t enter into widespread usage until the code could be stored reliably and efficiently, media would remain on videotape for many years while technology developed and market usage grew.

Figure 2.1 Chronological evolution of spinning magnetic disk drives from 1956 through 2000.

Digital Image Compression: The Second
Core Technology

The technology that leads to the success of digital media recoding and storage development began with those standards developed out of the Joint Photographic Experts Group (JPEG) who, in 1992, standardized a tool kit of image processing based on a lossy compression format used in the capture and storage of photographic images. The JPEG standard specifies how an image is compressed into a stream of bytes and subsequently decompressed back into an image. The JPEG standard specifies only the codec and not the file format used to contain the stream.

Following the JPEG (lossy) standards development, the committee later revisited the lossless coding mode within JPEG. The JPEG-LS mode was a late addition to the standard, and in the baseline form of JPEG (not using arithmetic coding) the algorithm used was not a ‘state of the art’ technique. However, this eventually led to the discussions from which came the JPEG 2000 standard prominently used today. Table 2.1 shows the JPEG and JPEG-LS development phases. Table 2.2 shows the JPEG 2000 standard.

Motion JPEG

The principles of the JPEG codec were independently adapted such that a series or string of individually captured frames from a moving video image could be stored to a disk drive, linked as a single group of pictures, and then played back sequentially to reform the original moving image. This notion was dubbed “motion-JPEG” (a nonstandardized implementation) and was widely used by nonlinear editing manufacturers, most notably Avid Technologies, which was founded in 1987 by William J. Warner. The use of MJPEG during these early years of product deployment was well before the formal approval of MPEG encoding, which was developed by the Motion Picture Experts Group and standardized under the International Organization for Standardization (ISO). Motion-JPEG is still used in many types of imaging systems including surveillance and some video editing or graphics systems.

Extensions of these proprietary implementations of JPEG formats continued and were applied in many products including still stores, clip stores, videoservers, and surveillance recorders, with many still being used today.

JPEG 2000

The development of JPEG 2000, a wavelet-based image compression standard and coding system, also created by the Joint Photographic Experts Group committee in 2000, was intended to supersede their original discrete cosine transform-based JPEG standard. The JPEG 2000 codec is one of the founding compression formats for scalable, high-resolution imaging such as that used in digital cinema applications, as well as high-quality security systems, where analysis of the captured images as still frames is extremely important. (see Table 2.2)

As requirements for extended recording capabilities continued, a trade-off would be necessary. Either disk drives would have to get a lot larger in order to store the files or motion-JPEG codec technologies would need to be fine-tuned so that the images could be stored on the hard disk drives available at that time. While this requirement was being contemplated, MPEG compression technologies were being developed that would address the growing needs of standard video and audio formats, streaming video applications, and lower bit rates.

MPEG-1

Video applications eventually begin to use a new compression format designed for moving image applications. MPEG would be that new compression format. The standards process began with the formation of a subcommittee in the International Standards Organization (ISO) which would be called the Moving Pictures Experts Group (MPEG). The first meeting was held in May 1988 and was attended by 25 participants. The first MPEG-1 codec would be built on H.261, the ITU-T video coding standard originally designed in 1990 for the transmission of video and audio over ISDN lines.

The MPEG target goals included specifying a codec that produced moving video that could be carried at a low 1.5 Mbits/second bit rate (i.e., T-1/E-1 data circuits) and at a data rate that would be similar to those used on audio compaq discs. The MPEG-1 standard strictly defines the bit stream and the decoder functionality. Neither MPEG-1 nor its successor MPEG-2 defines the encoding process or how encoding is to be performed.

Drawbacks to MPEG-1 would become evident as the development of MPEG-2 began.

MPEG-1 Weaknesses

•Limitation of audio compression to two channels (stereo)

•No standardized support for interlaced video

•Poor compression quality when used for interlaced video

•Only a single standardized profile (Constrained Parameters Bitstream)

•Unsuitable for higher resolution video

•Only a single color space of 4:2:0

The limitations of MPEG-1 made it impractical for many professional video applications. Hence, another solution was necessary, which could provide better processing for higher resolution images and enable higher data rates.

MPEG-2

Recognizing that compressed video and audio systems would be the enabling technology for high-quality moving images and that the broadcast industry would be facing the development of a digital transmission system, the MPEG committees went back to work on an improved implementation of the discrete cosine transfer and their associated systems.

The MPEG-2 system, formally known as ISO/IEC 13818-1 and as ITU-T Rec. H.222.0, is widely used as the format for digital signals used by terrestrial (over-the-air) television broadcasting, cable, and direct broadcast satellite TV systems. It also specifies the format of movies and other programs that are distributed on DVD and similar discs. As such, TV stations, TV receivers, DVD players, and other equipment are often designed to this standard.

MPEG-2 was the second of several standards developed by the Moving Pictures Experts Group (MPEG). The entire standard consists of multiple parts. Part 1 (systems) and Part 2 (video) of MPEG-2 were developed in a joint collaborative team with ITU-T, and they have a respective catalog number in the ITU-T Recommendation Series.

While MPEG-2 is the core of the most digital television and DVD formats, it does not completely specify them. The ITU-T Recommendations provide the study groups and ongoing work related to most of the currently employed, and industry accepted, coding formats. Extensive technical information and descriptions related to implementations using MPEG-2 and MPEG-4 coding systems, as well as applications of JPEG and its moving image extension, are available from a number of sources.

Networked Services: The Third Core Technology

Along with many other systems, digital video technologies, storage systems, and their interfaces tend to depend on the principle and structures of networked systems. The components of networking contribute to real-time and non-real-time operating systems, file transport and interchange, and content managementor distribution. Most of the IT-based networking technologies developed for data-centric and computer systems are applied in the structure and feature sets found in today’s videoservers, their storage subsystems, and the intelligent management of media and associated metadata.

Networked services are the key to workflow collaboration, to high-speed interconnects, to content delivery mechanisms, and to the internal architectures of secured and protected data storage systems. Even as the world evolves through the 1- and 2-Gb Ethernet topologies, to the 4-, 6-, and 8-Gb Fibre Channel systems, and on toward 10-, 40-, and 100-Gb technologies, media storage infrastructures will keep pace with these developments in order to support the continually growing demands on video, audio, and data.

DDR, DVR, or Videoserver

The question on the minds those who saw digital disk recording rise in acceptance would become “Is videotape dead?” That question may never fully be answered, but while the volume of videotape diminishes, the growth in linear data tape storage (e.g., Linear Tape-Open or LTO) continues.

The 1990s saw a gradual and steady shift to disk-based recording as the requirements for faster access increased and as the dependencies on robotic videotape libraries decreased. Today, there is reluctance by some to use tape drives for archive, based on a combination of the cost basis for the library slots and on the continually evolving capabilities of disk drives as a substitute for tape.

At first introduction, disk recording technologies did not become a mainstream component, at least for the broadcaster, until the development of the professional videoserver began in the mid-1990s. Disk recorders saw most of their uses as storage platforms for still images. Once the images could be recovered fast enough from a disk to be “frame sequential,” that is, they could be assembled at 30 frames per second (and interlaced), the value of disk storage became evident.

Beginning with the purpose-built digital disk recorders (DDRs) and the move to nonlinear editing with its compressed video systems that stored video (as files) on internal disk drives, there were only a few applications that made practical and economical sense. Those applications were mainly in the postproduction industry where special effects or high-end graphics with multilayer compositing was a thriving business for those with the right hardware (see Fig 2.2).

The disk recorder inside a nonlinear editor was seen by many as the up-and-coming alternative to linear-based videotape editing.

Figure 2.2 Basic input and output flows for a digital disk recorder.

Bit Buckets

When the device that stores the image files from a video and the sound in an audio stream is used principally as a bit bucket, it is simply a repository for the bits that make up the audio/video or transport stream file. One would not necessarily call that kind of a device a digital disk recorder (DDR).

For a device to function as a DDR, it must be configured to accept a real-time live video stream, as analog video or as digital video per SMPTE 259M (standard definition) signal, or per SMPTE 292M (high definition) signal. The device must process the signals, formulate them into data elements that can be stored on a disk platform, and then reassemble at playback in real time.

For definition purposes, we’ll make the repeated distinction that a DDR (sometimes known as a “digital video recorder” or DVR) must be capable of capturing real-time video and audio signals, storing them, and then playing them back as video and audio in at least a linear fashion. In doing this, the quality of the audio and video should not suffer any appreciable degradation.

Basic Functionality of a DDR

To be considered a DDR (or DVR), there would be certain minimal functions that the device must be consistently capable of doing:

•Ingesting the moving media through conventional input ports, in real time

•Recording while playing

•Browsing of and indexing or cataloging of the files (clips) on the system

•Accessing the clips in any order for near instantaneous playback

•Sequencing and controlling of playout and record by either its own applications or by external control, such as from a protocol like VDCP

•Playing back of synchronization audio, video, and ancillary data (closed captions or time code) on demand and without degradation

Secondary Capabilities of a DDR

Generally accepted features that are inherent in a videotape transport plus additional functionality including:

•Audio and video splitting for separate playback

•Video and audio synchronization to a house reference signal (i.e., be able to genlock)

•Jog/shuttle features for video or audio locations

•Ability to accept multiple audio- and video-source configurations including, but not limited to,

•analog and digital video inputs and outputs

•standard-definition and high-definition video inputs and outputs

•embedded or discrete AES audio

•Stunt features including reverse, slow motion, and freeze frame/stills playout

•Protection of the storage subsystem data elements through RAID-based architectures

Remote control and addressable catalogs through protocols or by third-party interfaces, such as VDCP, are recommended in order for the DDR to function in a production environment.

Additional Functions and Capabilities of a DDR

Through networking and file transfer services, a DDR should allow for the interchange of data files over a network connection, plus optional feature sets such as follows:

•A second alpha (luminance) channel used for key channel information

•External storage in a mathematically protected (RAID) system, or the equivalent

•Protective features such as redundant power supplies, fans, system disks, or control systems

•Ability to specify differing recording and/or compression formats

•Ability to record and play back transport streams

How the actual video and audio data is stored on the device is irrelevant to the baseline functionality of a DDR. What is the most important is that the video and audio elements are as easy to access and use as its linear videotape equivalent.

Comparing DDRs to Videoservers

When the DDR adds some or all of those items identified previously as “additional features or capabilities,” especially the ability to function in a file-based domain (network connected, multiple records or playback channels, or the interchange of files with other devices), the DDR has gone beyond a simple VTR-replacement mode and has become essentially a videoserver.

When speaking strictly of a recording device that primarily accepts linear serial digital video inputs (SMPTE 259M or SMPTE 292M), the videoserver has a very similar media structure to that of the videotape recorder, with certain exceptions:

•Physically, the media is of a disk-based or solid state memory format.

•The recorder/server has nonlinear or random access playout capabilities.

•Reproduction of the video slip is nearly instantaneous.

•There are starting, stopping, cueing, and still capabilities from any segment or video clip.

•There is random on demand access to video clips.

•Images are stored as files and may be shuffled, moved, or transported by means of other than a conventional baseband video/audio interface.

A DDR or DVR is a close cousin to a videoserver, but the server has much more sophistication and may be its own platform that calls the media content from ancillary storage. The DDR will intrinsically be self-contained, with the exception that it may have storage that can be externally attached or moved.

DDR Storage

At a higher level, the media storage properties for a disk recorder will most likely be in a protected architecture, typically using conventional spinning hard disk drives in a RAIDbased (mathematically protected) configuration or a duplicative structure such as mirroring or replication. The content may be stored in a native (full-bandwidth) format, using the coding structures of the baseband video. Such an example would be the D-1 component digital format, whereby the bit stream of a Y’C_RC_B digital video signal is recorded to the disk drive and when played back does not suffer from lossy compression properties.

The moving images on a DDR may be stored in an entirely proprietary manner if it is in a closed system. The DDR may take files created by another application or from other PC-like workstations (e.g., finished clips from Adobe After Effects) and then reformat them for playout on their particular platform.

Another, more familiar, and today highly common means of digital video storage will compress the incoming serial digital bit stream (video and audio, independently or otherwise) by means that are standardized per industry recognized and accepted encoding practices, such as MPEG or similar formats. In this example, after compressing the video, the DDR now saves data as, for example, MPEG video and audio files to the hard disk and on playout decodes those files to baseband digital video and AES audio for presentation to other video productions or signal systems.

Clip Servers

The DDR/DVR and its successor the videoserver in their simpler forms may be configured into scaled-down versions that are designed for the recording and playing back of short segments of video as “clips.” there are now dozens of various products that, since the early years of digital or analog still store devices, capture both still images and short-segment clips for use in news, sports, and video loops for displays. this market space is being extended to other storage and coding platforms that are deeply integrated into peripheral or mainstream video production equipment.

The evolution of the DDR, as the videoserver took off as a more complete platform for moving video, began to take on the forms called clip or image servers. In the consumer space, the DVR has appeared by the millions as part of set-top boxes for cable or satellite broadcasting. PCs and workstations have included the capabilities of these DVR feature sets, as have iPods and other similar video recording and playback systems and mobile devices.

Clip servers may be as simple as the DDR or may take on much more sophistication such as those used in sports replay systems (e.g., the EVS [XT]2 + and the Grass Valley K2 Solo Media Server). High-performance replay systems are built around those concepts that are found in the video/media server platform but include specialized user interfaces that allow rapid recue, segment collection and repackaging, exportation of the media as files to dedicated or other third-party systems, and highly networked, user programmable systems aimed at professional and broadcast applications. An example of a complete workflow with replay DDRs, editing, and network storage is shown in Fig. 2.3;.

Figure 2.3 Workflow for disk- based replay and playout system typical for a combined live and postproduction operation for sports or other live venues.

Clip servers, as image storage devices, have found their way into video production switchers integrated with playout and effects channels that can store from a few frames to hundreds of seconds of high-definition video. Usually, this form of clip server uses some form of solid state- or flash memory-based storage and is built into the video production switcher directly. The feature sets for these forms of video clip servers may be limited to the functionalities of the production systems and may lack export capabilities or external streaming playout unless as a part of the video production switchers, mix effects, or program buses.

Clip Server File Formats

Highly compact clip servers, for example, compact (1 RU) offthe- shelf IT-like servers, are not uncommon platforms for clip servers. When equipped with HD/SD-SDI inputs and outputs, solid state drives (SSD), or integral disks for storage, these devices may record and play MXF-wrapped video files in profiles that are compatible with prominent manufacturers of video production and recording equipment such as the Panasonic P2/P2 HD (as DVCPro) or Sony XDCAM formats.

For other than live, baseband ingest, clips may be transferred into the server and/or saved over gigabit Ethernet to external storage devices, or interconnections to other serving platforms, including PC or Mac workstations. These servers can be highly versatile and reliable and may be used in a variety of applications for image ingest, synchronized playback, and even full-length program playout using a third-party playout control system.

Applications for these devices run the gamut from digital signage for theme parks, broadcasters, news organizations, production companies, and arena or stadium replay control rooms. On a much broader scale, streaming media servers (a related type of clip server) may be utilized for video on demand or similar applications where the number of streams is not on the order of those required for services such as YouTube.

The architecture of these more dedicated forms of clip servers allow, even during video recording or playback, for clips to be added to the local storage via a network connection. Provided there is sufficient local storage on board, there is no time lost because of the switching disks or because of external media such as tape, memory cards, USB sticks, and so on.

Most applications will use the file formats for video and audio that are used by many of the industry accepted standard encoding schemes, both for professional video and (depending on the coding software of the clip server) possibly other “nonbroadcast” formats or wrappers. The listing of available file formats for clip servers and DDRs is dependent on what the particular manufacturer of the devices has selected for their usually software-based codecs. Dedicated platforms that do not offer or have a requirement for external interfaces or file exchanges may effectively utilize any form of encoding that makes their platforms perform, which could include wavelet compression or even conventional PC/MAC-based formats.

Clip Server Storage

When using commodity-based server and storage systems for these clip servers or DDR applications, the manufacturer may select to use disk drives or solid state memory systems (including SSD) that are integrated with the hardware platform they have built their systems on. Given that the purpose of these devices is aimed at short-term, limited duration media files, the likelihood of requiring costly high bandwidth, resilient storage platforms is minimal. As such, a vendor may select to use low-cost SATA drives that can be easily interchanged should a drive fail. The drives employed in these applications could be commodity off the shelf plug-and-play drives or third party disk arrays.

Drive bandwidth requirements will probably not be a limiting performance factor. With today’s disk drives, most of the purposebuilt systems are able to support single or dual streams of 50–100 Mbits/second files (such as DVCPRO50 or DVCPRO HD) without too much difficulty. However, once there becomes a requirement for the recording of two or more streams combined with FTP network transfers between devices and simultaneous multistream playouts, most users will find they need a server-based system rather than a standalone single disk recorder platform.

Videoserver platforms may generally be used as clip or image servers, with the caveat that the ability to access multiple frames or very short-duration clips based on a continual “back to back” may be encumbered. In such applications, a professional videoserver may need some additional buffering time (varying from a few frames to a second or two) to charge the video playback coding engines and to make ready the next clip for a seamless transition between the last frame of one clip and the first frame of the next. For this, and other cost-related factors, videoservers that are designed for mission critical program or commercial length playback may not be advised for use in this application.

DVR Marketplaces

The consumer and home viewing sector has come to realize the impact of the digital video recorder (DVR). Albeit the DVR has been branded almost universally as a “TiVo,” and the word has almost become a verb, the functionality of the DVR as built into the platforms ranging from satellite receiver systems to cable settop boxes (STBs) and more recently into the home PC or television display itself is not unlike that of the professional digital disk recorder or clip server.

In STB configurations, most DVRs will record the incoming signal not as files but as transport streams. The signals from the cable and satellite systems are generally delivered as MPEG-2 or MPEG-4 streams. Seldom will these STB devices allow the recording of discrete video and audio signals, unless equipped with selected video and audio input cards. For the carrier-based DVR, this practice reduces the component count and in turn makes them less costly when produced in mass proportions for cable system and satellite uses.

The impact of DDRs and video-media servers has produced paradigm changes in all aspects of life—from television production workflow to home television viewing. The significance is that these devices and their associated counterparts have taken what was once a dominant videotape-based marketplace and shifted it away from linear functionality, not unlike what the DVD has done to the home VHS recorder. Even as we’ve entered the second decade of the new millennium, videotape continues to have a significant place in the capture and retention of the moving image—but that too is beginning to fade as solid state devices capable of holding more data than a lineal videotape are being incorporated into cameras, mobile devices, and other media delivery platforms. All of these next-generation systems have functionality not unlike that found in the home DVR.

Home Media Network Disk Recorders

The dividing line between a dedicated DDR/DVR, and when DDR functionality is integrated into alternative platforms (e.g., a home media network), is a fine line segmented by cost, performance, scalability, and feature sets. Limiting factors that differentiate the groups include resolution, performance, storage capabilities, and media format awareness.

One benefit of this new era of digital media distribution is that users can store all of their music, movies, TV shows, and videos as data on hard drives rather than have them stacked on shelves or in the basement box collection. However, with high-definition television, and higher quality files, added to the ever-growing larger collections of media, this can still add up to a lot of storage.

Finding an appropriate storage solution suited for the users’ needs can be complicated, especially as one tries to balance expandability with performance. Not to mention, there is still a need for backup, network-wide access, permissions, security of the assets, and more.

For example, Microsoft, as well as others, have developed home media server platforms that have the sophistication of professional media content servers, yet are focused on what the consumer needs to organize, back up, protect and secure, plus exchange those files over local and Internet network services. This, of course, raises the same level of questions and concerns as the users of professional serving products-insufficient storage and issues with storage management complexities. Faced with this dilemma, but on a much wider scale, Microsoft looked at these issues and over time developed a new storage technology that enables users to use both internal and external hard drives of any size. This takes away the dedicated platform approach that other makers of home media networking systems have taken.

Protection, scalability, and extensibility are important requirements for the sustainability of media asset platforms. Microsoft found this and developed their Windows Home Server Drive Extender. This product, as a concept, has a built-in protection application as one of its prime features, enabled by folder duplication that is set up for specific user-specified shared folders on home servers. It functionally works by maintaining two copies of a shared folder on separate hard drives, protecting against the failure of a single hard drive on a single device. This not only improves content management but also adds elements of protection and security heretofore not easily available at a consumer level on PC platforms.

This is just one of the many enabling technologies integrated into a moving media storage that is extensible to DVR systems coupled with home networking, and in turn, we are seeing similar kinds of functionality in the professional and industrial market spaces at far less cost.

Audio

Not to be left out, the radio and the news industries rely heavily on closed or purpose-built systems for the recording and playing back of commercial content, reporter stories, music, and other audio media. These devices, although less complex in terms of handling video, offer precisely the same capabilities as the DDR or videoserver, but for audio only.

Instant start or stop capabilities and repeatability are the two main functional requirements for an audio clip server implementation. In studios where on-air operations are closely coupled with various other radio production requirements, these systems will be networked or directly interfaced with digital audio workstations (DAWs), audio consoles, amplifiers, and other control surfaces to allow for a complete production workflow.

Radio has been using forms of audio clip serving devices long before the popularity of video or media serving systems. Television sound effects that follow video animation clips all use these clip players during live productions to enhance their graphics or the start of an instant replay sequence. Clip servers or audio disk recorders have essentially replaced the older legacy audio carts by the hundreds in news operations and production studios.

Further Readings