Transcript
VIDEO RECORDING TECHNOLOGY REFERENCE presented by
Copyright 2002 Peninsular Technologies. All rights reserved.
TABLE OF CONTENTS DISCLAIMER .............................................................................................. 3 PREFACE ................................................................................................... 3 TAPE-BASED RECORDING FORMATS ............................................................ 4 VHS...................................................................................................... 4 S-VHS .................................................................................................. 4 DIGITAL VIDEO (DV) FORMAT ................................................................ 5 NON-LINEAR VIDEO RECORDING................................................................. 6 THE PERSONAL COMPUTER ..................................................................... 6 DIGITAL VIDEO ENCODING PARAMETERS ................................................. 6 FRAME RATE ....................................................................................... 7 RESOLUTION (FRAME SIZE).................................................................. 7 BIT RATE............................................................................................ 8 MPEG VIDEO ........................................................................................ 9 MPEG-1............................................................................................. 9 MPEG-2........................................................................................... 10 MPEG COMPRESSION........................................................................ 11 MPEG DECOMPRESSION.................................................................... 12 NON-LINEAR DIGITAL MEDIA ................................................................ 13 DVD-R ............................................................................................ 14 RE-WRITABLE DVD .......................................................................... 15 CD-R ............................................................................................... 16 HARD DRIVES.................................................................................... 18 TRADEMARKS.......................................................................................... 18 REFERENCES ........................................................................................... 20
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DISCLAIMER This document is based on preliminary research conducted to answer frequently asked questions about video recording technology. Peninsular Technologies makes no warranty ensuring the accuracy of the information provided. Cost data was estimated based on retail offerings available at the time of authoring.
PREFACE This document is intended to compare traditional tape-based recording systems to recently developed non-linear video recording systems. Video recording technologies are often presented in the context of a sewer inspection, but the information applies equally well to video recording applications in any industry.
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TAPE-BASED RECORDING FORMATS Linear magnetic tape has been a reliable video recording media for decades. Cassette sizes and styles evolved as the technology developed. Regardless of design, all cassette recorders save video information on a continuous line of magnetic tape. In order to retrieve a specific piece of information, the tape must be physically advanced or retracted until the point of interest is accessible by the reader head. The disadvantage of this characteristic grows as the size of a tape-based video library grows.
VHS The venerable VHS tape preserves ~240 horizontal lines of resolution1. The majority of VHS VCRs cost $150 to $300. High-grade VHS tapes with 120 minutes of video recording time cost $1 to $2. The typical input/output connectors are composite RCA jacks.
S-VHS Super-VHS (or S-VHS) technology is similar to VHS, but is capable of over 400 lines of horizontal resolution. Most S-VHS VCRs cost $300 to $800. S-VHS tapes that support the higher resolution cost $6 - $8 each, and hold 120 minutes of video at standard recording speed. S-VHS units include s-video connectors and cables, which preserve the high resolution video better than standard composite RCA cable. Regardless of the connector, all S-VHS VCRs record analog video.
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DIGITAL VIDEO (DV) FORMAT DV is an international video standard that is occasionally confused with the generic concept of “digital video” recording. Digital video broadly refers to the representation of a source signal with discreet reproducible digital packets, while DV is a small subset of this basic concept. Although the DV format is capable of high resolution recordings (~500 horizontal lines 2) with excellent video quality, the vast majority of DV recordings are saved on linear tape. This disadvantage results in a cumbersome linear navigation process on par with VHS or S-VHS tape. High-quality DV recordings can be transferred to non-linear media without signal degradation, but this approach requires extensive digital storage space. One minute of DV format video occupies approximately 270 megabytes of disk space, making non-linear DV archiving impractical for most consumers. The DV format has spawned several variations and sub-standards beyond the scope of this document. Links to additional information and retail products incorporating DV technology can be found in the References section. 3
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NON-LINEAR VIDEO RECORDING In a non-linear video system, images are recorded on non-linear or random access media. With random access digital media, individual video frames can be located directly, without having to advance or rewind a long magnetic tape. Furthermore, each video frame can be directly linked to supplemental information that describes the situation depicted by the frame. This supplemental information can be searched and sorted to quickly locate a specific video clip from a vast library of recordings. This video indexing technology is described in U.S. patent number 6,175,380 held by Peninsular Technologies and is currently available in the PipeTech sewer TV inspection system.
THE PERSONAL COMPUTER The PC is the cornerstone of mainstream non-linear video recording. In order to manipulate video with a computer, it must be digitized and recorded. This is achieved through a video capture device and a non-linear (random access) storage device. The hard drive in a PC is a non-linear device. CDs and DVDs are also examples of non-linear storage media and are discussed in more detail later in the document. In addition to digitizing a video signal, the PC is frequently used to compress the digital information for manageability purposes. Full-size, fullmotion, uncompressed video is impractically large for today’s PCs. The application of a compression algorithm to a video signal in real-time is frequently called encoding. The inverse process of rendering a video stream from a compressed source file is called decoding. PCs can also use software to convert or compress existing video files in a transcoding process.
DIGITAL VIDEO ENCODING P ARAMETERS Digital video encoding can result in a wide variety of picture qualities, even more so than tape-based recording systems. Digital video encoding equipment typically specifies video quality in terms of three 3 variables: frame rate, resolution, and bit rate.
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F RAME RATE Frame rate is usually specified in frames per second (fps). The standard TV signal in North America is broadcast at 30 fps. The European equivalent standard specifies 25 frames per second. Motion pictures are typically filmed at 24 fps. At 15 fps, the human eye will clearly recognize a sequence of discreet images, and the illusion of continuous motion is lost. However, there are some benefits to low frame rate video. The corresponding file size is reduced in proportion to the reduction in frame rate. For example, any video clip digitized at 30 fps will be 3 times larger than the same clip digitized at 10 fps. If the appearance of the 10 fps video is acceptable, the smaller file size will be beneficial when storing, transporting, or broadcasting the video over a computer network.
RESOLUTION (F RAME S IZE) In digital video terms, Source Input Format (SIF) resolution [352 x 240 pixels] is the closest approximation of VHS quality. Digital video frames can technically be any size, though most encoding devices only support a combination of the following choices at 30 fps. Resolution 175 x 120 352 x 240 352 x 480 640 x 480 704 x 480 720 x 480
Name (QSIF) (SIF) (Half D1) (VGA) (D1) (D1 alternate)
The frame size is an important factor determining the detail a video recording can preserve, but is only one of many variables that contribute to picture quality 4.
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B IT R ATE Bit rate, also known as data rate, is usually specified in bits per second (bps) or megabits per second (Mbits/sec). A reduction in bit rate increases the compression applied to the incoming video frames. The file size of an encoded video stream is directly proportional to the bit rate parameter of the encoding device. However, an increased bit rate does not translate directly into an increase in picture quality. In some cases, increasing the bit rate will have no perceptible effect on image quality. In other situations, a small increase in bit rate will greatly improve image quality.
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MPEG VIDEO MPEG is a professional group responsible for establishing international standards for video and audio compression. MPEG specifications are named using the MPEG acronym followed by a number (MPEG-1, MPEG-2, MPEG-4, etc). MPEG-1 and MPEG-2 are not specifications of video quality. While higher video quality is achievable with MPEG-2 standards, an MPEG-2 video file is not necessarily better than an MPEG-1. The numeric component of the MPEG-# designation is not related to the quality of the video whatsoever. MPEG-4 is actually optimized for low bit rate encoding. It is more appropriate to define digital video in terms of frame rate, frame size, and bit rate than by specifying an MPEG Video standard.
MPEG-1 The MPEG-1 definition theoretically supports an unlimited spectrum of video qualities and compression ratios. Casual use of the term “MPEG-1” often refers to a subset of the MPEG-1 video definition known as the Constrained Parameters Bitstream or CPB. The constrained parameters of the CPB are the frame size (352 x 240), frame rate (30 fps), and bit rate (up to 1.862 Mbit/sec). Although the CPB supports bit rates up to 1.862 Mbits/sec, the “nominal” MPEG-1 bit rate is 1.15 Mb/s and is considered optimal for most applications. The term VideoCD also describes a CD containing MPEG-1 video encoded at the following parameters: • 352 x 240 resolution • constant bit rate of 1.15 Mbits/sec • 30 frames per second Applications with a fairly consistent frame appearance, like a pipeline inspection video, can reduce the bit rate further without significantly degrading the video quality. Most MPEG-1 devices encode at 30 fps with a frame size no greater than 352 x 240. However, the resolution parameters are a limitation and specification of the device, not of the MPEG-1 standard.
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MPEG-2 MPEG-2 is an enhancement to the MPEG-1 standard, but supports an even wider array of encoding parameters. The MPEG-2 specification is divided into sub-sets called Profiles. Encoding parameters within a profile are defined as Levels. Levels define the parameters most closely related to video quality (sample rates, frame dimensions, coded bit rates etc.)5 The majority of MPEG-2 video is encoded using the Main Profile at the Main Level (abbreviated MP@ML). The parameters for this level follow the recommendation of International Telecommunication Union document CCIR-6016 for digital television broadcasts, resulting in a sampling dimension of 720 x 480 @ 30 fps. Within the MP@ML constraints, the range of available bit rates can still have a dramatic effect on image quality. MPEG-2 encoders support variable bit rates, whereby the compression of the video stream varies depending on content. Most MP@ML video streams average ~ 3.5 Mbits/sec. At 6 Mbits/sec the compressed file is nearly indistinguishable from the original video stream, even for fast-action content like sporting events. The maximum bit rate of the Main Level is 15 Mbits/sec. Most MPEG-2 encoding devices support the nominal MPEG-1 parameters and at least one higher resolution option (usually 720 x 480).
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MPEG COMPRESSION Early generations of digital video encoders used electronic circuitry to apply an MPEG compression algorithm through dedicated hardware. As CPU speed and power developed, it became possible to encode video signals through mathematical calculations performed by software running on a PC. Since software encoders are frequently bundled with video display hardware, it is sometimes difficult to determine the actual recording mechanism implemented in MPEG-enabled retail products. SOFTWARE ENCODERS MPEG compression is computationally very complex and software encoding can put a significant strain on a computer’s CPU. Inadequate processor speed, lack of RAM, and interruptions caused by other software can result in dropped video frames and degraded quality. For most applications, including pipeline inspections, real-time encoding is desirable. With a realtime encoding application, the video is compressed and recorded as the signal enters the PC video capture device. Some software-based systems cannot perform the calculations necessary for compression fast enough to keep up with a live video signal. In this situation, the file must be recorded uncompressed as it enters the PC then transcoded later over a period of time. Systems and software incapable of real time encoding are not recommended.
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HARDWARE ENCODERS Hardware encoders are internal or external physical devices that attach to the PC as peripherals. They compress the video signal internally and are much less dependent on the CPU and system RAM, but suffer from problems typical of complex PC hardware. Some devices are prone to unrecoverable recording errors that defy explanation, but others perform quite reliably. Since it is impossible to test all available devices on all possible system configurations, setup and installation problems can arise. In our experience, hardware encoders generally produce higher quality video with a lower probability of system failure than software-based encoders. With the devices supported by PipeTech, recording can be performed within a pipeline inspection system interface and video frames can be linked directly to observations describing the condition of the pipe. However, the performance claims of software-based encoders are currently being investigated, and we will continue to support the most effective encoding technologies available.
MPEG DECOMPRESSION MPEG-1 decoders are fairly common. Many popular media players including Windows Media Player and RealNetworks RealPlayer support MPEG-1 playback and are available for free. MPEG-2 decoders are less common and more sophisticated. Few if any are distributed freely. Computers with DVD drives frequently have MPEG-2 decoders preinstalled as part of the DVD playback system. Commercially available media players that include MPEG-2 playback generally cost between $20 and $30.
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NON-LINEAR DIGITAL MEDIA DVD DVDs are correctly defined in terms of storage capacity. DVDs can be used to store anything from spreadsheets to still photographs to high-quality audio. They frequently store MPEG-2 MP@ML encoded video signals and in some cases have incorrectly become synonymous with high quality video. Since the term DVD does not refer to a specific disc capacity, disc format, disc content, or recording standard, casual use of the term can be confusing. While DVDs are equally capable of storing high and low quality video, the phrase “DVD-quality” video usually refers to the MPEG-2 format, Main Profile at the Main Level (720x480 @ 30fps). A range of bit rates is supported at this level, but the compression is considered optimal at 6 Mbits/sec.
DVD-V IDEO VS . DVD-ROM The comparison between DVD-Video and DVD-ROM closely parallels the comparison between audio CDs and CD-ROMs. A wide variety of software is distributed on CD-ROM for use in personal computers. Not surprisingly, audio CD players designed specifically for audio discs do not recognize these CD-ROMs. CD-ROM drives can recognize and read audio CDs, but audio CD players only recognize properly formatted audio discs. An increasing variety of software and reference materials are distributed on DVD-ROM for use in personal computers. Not surprisingly, DVD players designed specifically for high-quality movie playback do not recognize these DVD-ROMs. DVD-Video discs are formatted specifically for use in these single-purpose DVD movie players. DVD-ROM drives can recognize and play DVD movies, but typical DVD players only recognize properly formatted DVD-Video discs. DVD recorders are a popular solution for archiving relatively large amounts of digital information. DVD recorders are frequently bundled with mastering software to help with the complexities disc formatting, menu building, and chapter/title grouping. Depending on the drive manufacturer, bundled disc mastering software may support the creation of DVD-Video discs, DVD-ROMs, or both. If the digital archive is made up exclusively of Filename: Digital Video Technology Reference.doc v2.0 Copyright 2002 Peninsular Technologies. All rights reserved.
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high-resolution video, the DVD-Video format is a sensible option. If the video is one portion of a broader archive that includes additional data files, a DVD-ROM format is appropriate.
DVD-R Two generations of DVD recorders have been released since the DVD-R standard was introduced. The first generation recorded 3.95 GB per disc. The second generation improved compatibility with existing DVD readers and increased the disc capacity to 4.7 GB. The DVD-R specification also branched into two subcategories, DVD-R for General use and DVD-R for Authoring. Authoring media and drives are specifically designed to create Master DVD discs for large-scale replication. DVD-R for General media and drives are far more common devices designed for all-purpose consumer use. DVD-R media can be used to create both DVD-ROM and DVD-Video discs.
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RE- WRITABLE DVD (-RW, +RW, -RAM) DVD-RW and DVD+RW are re-writable DVD drive specifications. Both technologies promise to record DVD media compatible with most standalone and PC-based DVD readers. Both specifications are well supported by a broad coalition of industry heavyweights. The standards are only incompatible at the recording stage with respect to media. DVD-RW discs cannot be recorded by DVD+RW drives, and vice-versa. Since both formats share a common set of basic features and adequately address the issue of backward compatibility, it may take unusually long for a single standard7 to emerge. DVD-RW entered the mainstream consumer market first with Pioneer Electronics release of the DVR-A03 in March 2001. Pioneer offered a standalone version of the drive to end users, while Compaq, Sony and Apple quickly made an OEM version available in built-to-order multimedia PCs. The drive was popular among multimedia enthusiasts and a worthy high-capacity successor to CD-RW technology. In August of 2001 Hewlett Packard introduced the industry’s first mainstream DVD+RW drive, superceding the original advantage of DVD-RW: availability. DVD+RW is generally considered a superior technology and compares favorably to DVD-RW in recording speed, recording and playback reliability, and compatibility with existing players. After numerous delays and several announcements amounting to vaporware, DVD+RW is now a legitimate DVD recording solution. DVD-RAM has been in the marketplace the longest of any DVD recording technology, but has suffered from both standardization and compatibility issues. Disc capacities and media types are manufacturer-specific, and discs may not be readable in standard DVD drives. The use of DVD-RAM for the creation of a permanent video library is not recommended.
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CD-R/RW CD-R is technologically very similar to DVD-R. The primary differences are capacity and cost:
Media Capacity: Equivalent Capacities: MPEG-1 (1 Mb/sec) MPEG-2 (4 Mb/sec) Recorder Cost:
CD-R 650 MB
86 min 21 min ~ $250 ~ $0.75 each Media Cost: ($1.15 per GB)
DVD-R 4700 MB Over 10 hrs 2½ hours ~ $800 ~ $10 each ($2.10 per GB)
NOTES: •
DVD media can include a second layer that adds an additional 3.8 GB of capacity. DVD media may also be recordable on both sides, which doubles disc capacity. In a double-sided double-layer configuration, a single DVD can hold approximately 17 GB of data. Most DVD playback devices and all PC-based recording hardware currently lack support for the second data layer, but future generations will take full advantage of the DVD media specification.
•
Time estimates are based on 100% use of available disc space, which may not be possible in applications with only a few large video files per disc.
The re-writable CD industry was able to agree on a single standard (CD-RW), resulting in a widespread and reliable technology not nearly as confusing as re-writable DVD. CD-RW media is more expensive than CDR, but CD-RW discs can be erased and re-written as needed.
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MULTI -SESSION D ISCS & P ACKET WRITING Several new features have been introduced in the evolution of disc recording technology to provide a convenient and seamless recording experience. Unfortunately, these new technologies and corresponding features add convenience at the expense of compatibility. The first generation of disc recorders required an entire disc layout to be prearranged before recording could begin. Once a disc layout was defined, the data was “burned” to the disc en masse. With multi-session technology, data can be added or removed from the disc incrementally in sessions. Most but not all disc readers recognize multi-session discs, so the decision must be weighed against the need for compatibility with existing equipment. Disc recording became extremely popular with the advent packet writing technology enabled by the UDF file system. Packet writing simplifies disc creation by allowing recorders to behave somewhat like standard hard drives in the Windows environment. Files can be added, copied, renamed, or removed through standard application interfaces like Windows Explorer, bypassing the need for dedicated disc mastering software. Before a packetwritten disc can be read by a standard drive, the disc must be closed using the ISO 9660 file format. Again, some early generation readers may not recognize discs recorded in this fashion. The greatest compatibility with existing readers results from single-session, closed discs created with dedicated disc mastering software, like Roxio CD Creator. For convenience, a packet writing utility like Roxio DirectCD is worthwhile.
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H ARD DRIVES There are advantages and disadvantages to storing video on an array of hard disk drives (HDDs). The most appealing feature of a dedicated video server is accessibility: all video is readily available without the need to change discs. Video servers are most advantageous in situations where archived video footage is accessed frequently and the process of locating and inserting specific discs is considered inconvenient. Since video servers are in essence an array of hard disk drives, they also come with disadvantages. HDDs are made of moving mechanical parts with a potential for failure, so a redundant system of drives for backup purposes is recommended. This adds significant cost to the system. Video discs also travel better than hard drives and are more easily duplicated. As the size and cost of hard drives decrease over time, they will become an increasingly appealing media for digital video archiving. Ordinary hard drives may be suitable for archiving and backing up relatively small digital video libraries. Beyond 120 GB, the data mirroring and network connectivity advantages available in RAID servers provide a more reliable and convenient solution.
TRADEMARKS PipeTech is a registered trademark of Peninsular Technologies, LLC. IEEE 1394 ports, devices, and cables are sometimes identified by the trade names iLink and FireWire.
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APPENDIX 1: RECORDING TIME The recording time available for a given disc depends on disc capacity and bit rate:
2 23 × C d Tr = 60 × (BRv + BRa ) TR: CD : BRV: BRA:
VARIABLE NAMES: RECORDING TIME (MINUTES) DISC CAPACITY (MEGABYTES) VIDEO BITRATE (BITS/SECOND ) AUDIO BITRATE (BITS/SECOND)
CONVERSION FACTORS: 223 BITS = 1 MEGABYTE 60 SECONDS = 1 MINUTE
Calculations based on standard CD media and common bit rate settings for digital encoding devices establish an audio/video recording time of slightly over 80 minutes:
2 23 × 650 Tr = = 80.56 60 × (1,000,000 + 128,000 ) By slightly reducing the video bit rate, eliminating audio, and using highcapacity CD media, nearly two hours of full-motion video will fit on a single disc:
2 23 × 700 Tr = = 115.14 60 × (850,000 ) Using the bit rates defined in the VideoCD specification in conjunction with DVD-R media, over 8 ½ hours of A/V recording is available per disc:
2 23 × 4700 Tr = = 514.17 60 × (1,150,000 + 128,000 ) Filename: Digital Video Technology Reference.doc v2.0 Copyright 2002 Peninsular Technologies. All rights reserved.
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REFERENCES 1
http://www.toshiba.com/taisisd/projectors/dlpresolution.htm; http://eshop.msn.com/softcontent/softcontent.asp?scmId=709 http://www.epinions.com/content_1552261252 3 http://www.adamwilt.com/DV-FAQ-tech.html#DV%20formats http://www.rzanerutledge.com/dv/dv.html http://www.panasonic.com/pbds/subcat/Products/vtrs_vcrs/f_ag-dv1000.html http://www.jvc.com/product.jsp?modelId=MODL022175 http://www.sonystyle.com/di/prd.jsp?pid=768 4 http://www.elitevideo.com/new2.htm 5 http://www.crs4.it/~luigi/MPEG/mpeg2.html 6 http://www.crs4.it/~luigi/MPEG/ccir601.html 7 http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=30514 http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=30470 http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=34214 2
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