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ANALOG |
DIGITAL |
How a broadcast transmission works:
A strong alternating current is run along a wire that is called an antenna.
This causes an alternating electric and magnetic field that gives rise to an
electromagnetic field radiating in all directions. The wavelength - and therefore
the frequency - of the signal is determined by the rate of alternation.
The receiver is a passive wire that when struck by the electromagnetic wave
produces an alternating current that mirrors the original signal. The circuitry
of the receiver enables tuning or resonance with a given wavelength.
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Modulation:
Transmission frequencies don't fall within the programming ranges. Their frequencies
are assigned by the FCC and are called carrier frequencies. Amplitude modulation
(AM) programming is modulated to variations in the carrier signal's amplitude.
Frequency modulation (FM) converts program material into variations of the carrier
signal's frequency.
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Broadcast NTSC
In 1945, the FCC allocated 13 VHF television channels. The NTSC in 1953 created
the standards for the signals. The standard dictates a frame rate of 30 frames
per second in an interlaced fashion. This divided each frame into two fields,
and therefore each second into 60 fields. NTSC standard has a visible resolution
of 484 scanlines. The horizontal resolution is 400. This standard was devised
for black and white television. The broadcast bandwidth was set for 4.5MHz.
To adapt the system for color transmission a chroma signal was modulated onto
the luminance signal as a subcarrier. The combined signals are known as composite
video.
The phase of the chroma signal determines hue, while the amplitude determines
saturation. The chroma information is modulated onto the subcarrier at two phases:
the I, or in-phase value at 0 degrees; and Q, or quadrature value at 90 degrees.
The luminance signal is called the Y signal. NTSC color video is referred to
as a YIQ signal.
NTSC is used in North America, Central America, Japan, and parts of the South
Pacific and South America.
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Other Broadcast Formats
PAL (Phase Alteration Line)
PAL offers 25 interlaced frames per second with 625 scanlines. It was developed after NTSC and has greater bandwidth for chroma modulation and therefore better color resolution. PAL is used in Great Britain, West Germany, and The Netherlands.
SECAM (Sequential Couleur Avec Memoire)
SECAM has the same frame rate and resolution as PAL, except that FM is used to encode the chroma. SECAM is used in France, the former Eastern block countries, and the Middle East.
HDTV (High Definition Television)
High Definition Television (HDTV) is part of the new digital television standard recommended by the ATSC and adopted by the Federal Communications Commission (FCC). In total, the new digital television standard is composed of 18 individual specifications for a variety of resolutions, refresh rates, aspect ratios and scanning methods (progressive versus interlaced). Of these 18 Digital Television (DTV) specifications, 12 are known as SDTV or Standard Definition Television while 6 are truly HDTV formats.
HDTV enjoys a number of advantages over traditional analog television. Chief among these advantages is the higher resolution of HDTV. Additionally, the standard sets a wider aspect ratio for televisions similar to the wide screens used in movie theaters. All the digital television formats, both SDTV and HDT, benefit from AC-3 digital sound. They also benefit from the nature of digital signals, which eliminate snow, color bleeding and other picture anomalies resulting in an image with excellent rendition of detail and color depth without distortion.
The digital television standard incorporates 18 formats. The 12 standard definition flavors of DTV make use of "low" resolutions essentially equal to those available with analog television using the best source material. There is a resolution of 640 pixels wide by 480 pixels tall available in progressive scan formats with refresh rates of 60, 30, and 24 Hz (hertz - cycles per second). The 640 by 480 resolution is also available in a 60 Hz interlaced format. All four of the 640 by 480 standards are available only in the 4:3 "square" aspect ratio used by the old NTSC analog television standard.
A slightly higher resolution of 704 pixels wide by 480 pixels tall provides aspect ratios of both 4 by 3 and 16 by 9. These formats are available in 60, 30, and 24 Hz refresh rates with progressive scanning and in 60 Hz with interlaced scanning. All eight of the possible 704 by 480 formats are standard definition.
True high definition signals are available in two resolutions and in six varieties. All HDTV formats use the wide 16:9 aspect ratio. The first three HDTV formats provide resolutions of 1,280 pixels across by 720 pixels high all in progressive scan formats with refresh rates of 60, 30, and 24 Hz. The highest resolution HDTV format creates an image using 1,920 pixels left to right and 1,080 pixels top to bottom. This ultra-high resolution format is available in 30 and 24 Hz refresh rates using progressive scanning and 60 Hz using interlaced scanning. There are also plans to introduce a 60 Hz progressive scanning format of the 1,920 by 1,080 HDTV format when the technology becomes available to sufficiently compress and deliver such a signal.
HDTV signals at 60 Hz require around 19 megabits per second to carry all the necessary information to recreate the video. This 19 Mbits is achieved by compressing the pure signal through a special high definition subset of the MPEG-2 video compression scheme and Dolby's AC-3 audio compression scheme. In fact, the uncompressed HDTV data is about sixty times larger than the compressed 19 Mbit signal.
Format Comparison:
Current TV standard - 525 X 700, 30fps, 60 fields/sec, 4:3 (w:h)
HDTV standard - 720 X 1280 or 1080 X 1920, 30fps, 60 fields/sec, 1.78:1 (w:h)
VHS quality - 320 X 240, 30fps
Broadcast quality - 720 X 480, 30fps
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Video Specifications
Video uses an additive color system, mixing Red, Green, and Blue
Video Equipment
Different quality equipment yields different quality analog video
Video Connections
Traditional TV antenna connections are 300-ohm. The cable coming into your
home is a 75-ohm F -connector.
All cables used to carry video signals are coaxial to shield against RF.
On consumer products RCA plugs are used for both audio and video connections.
Professional video equipment more often employs twist-locking BNC connectors for video, and XLR's for audio.
Component Video (S-Video)
Broadcast signals suffer from the conversion to Radio Frequency carrier waves,
and are susceptible to RF interference. These RF signals are received by an
antenna and are converted back to composite video by the tuner or receiver.
They are further decoded into RGB signals for the CRT. Each stage in the process
can degrade the signal.
Excluding the RF stage of video results in a much better signal. Even so, composite
video is still several steps away from the RGB signal needed by the CRT. There
is still interference from modulating the chroma onto the luminance signal.
A solution in video production work is component video, which separates
luminance and chrominance signals. The chroma channel retains the hue and saturation
information in one component, called C. The luminance component is still Y,
although recorded at a higher frequency, making it possible to exceed 400 line
resolution. The Y/C signal is used by Hi8 Camcorders and S-VHS equipment.
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Signal Sync
Video waveform is actually three signals at once: luminance, chrominance, and
sync. A voltage range describes the luminance from black (minimum) to white
(maximum). The calibration method which is system independent is called the
IRE levels. In NTSC, one volt peak to peak corresponds to 140 IRE. In NTSC,
the black level, or pedestal level, is 7.5 IRE; and white level is 100 IRE.
Blanking is controlled directly from the video waveform- that is the guns are
turned off anytime the level drops below 7.5 IRE. Timing of all video equipment
takes the form of sync pulses corresponding to -40 IRE.
The other signal in the video waveform is the color burst. This takes the form
of nine consecutive cycles with absolute peaks of 20 IRE and -20 IRE. It serves
as a color sync signal and communicates proper hue to the video monitor. Vertical
sync keeps the picture from flipping. Horizontal sync keeps the image from being
skewed. Color sync ensures that the proper color is displayed. Unlike audio,
sync must be maintained for video signals to work together.
50dB represents better than 200:1 signal to noise ratio, considered good. At
40 dB snow, or video noise, becomes noticeable.
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Video Formats
Betacam SPDeveloped by Sony, perhaps the most popular component format for both field acquisition and post production today. Betacam uses cassettes and transports similar to the old Betamax home video format, but the similarities end there. Tape speed is six times higher, and luminance and chrominance are recorded on two separate tracks.
U-Matic
U-Matic SP (in common lingo "3/4" after the tape width in inches) is still a popular production format for those not wealthy enough to use Beta SP or similar. Although U-Matic doesn't appear much better than Super VHS on paper, the higher colour resolution and much better signal-to-noise ratio make the picture far more enjoyable. The U-Matic tape transport is also much faster in changing modes, which makes editing less frustrating.
LB and HB U-Matic tapes are often used for archiving because of the relatively low tape costs and low recording density, which makes the tapes robust against aging.
DV/DVCPRO
DV (formerly DVC) is backed by manufacturers such as Sony, Philips, Thomson, Hitachi, Matsushita (Panasonic) and others. It was the first digital recording format in the reach of consumer markets. DV uses 5:1 compression based on DCT. Depending on the image contents, the encoder adaptively decides whether to compress picture fields separately or combine two fields into a single compression block. As such, DV coding can be thought of as something half-way between Motion JPEG and MPEG.
DVCPRO is a professional variant of the DV by Panasonic. The only major difference is doubled tape speed, which is needed for better drop-out tolerance and general recording robustness. It is also capable of 4x normal speed playback.
As for the picture quality, all these variants are nearly broadcast quality, DV being available at nearly consumer prices. For newsgathering and other similar uses, the quality is certainly enough, especially considering that typical postproduction will be done digitally, which will not degrade the quality any further. Compression is mild enough to keep artifacts away in all but problem scenes. The quantization will be visible if you try something like chroma keying, however.
Digital Video
Analog signals from a video source are converted to digital values via an ADC.
The digital information is converted back to analog by a DAC to be viewed on
a monitor.
The major issue in digital video is the disk space it takes up, and the corollary
issues of transmission, throughput and display. A video image that is 640X480
with a 24 bit resolution at 30 fps represents a little over 26MB per second,
not including audio. Given that, a 1 GB disk can only hold about 38 seconds
of digital video.
Compromise is required to incorporate digital video in multimedia. Frame size
can be reduced, as is often done in games and other applications to provide
menu and control space on screen. Pixel resolution can be sacrificed, and 24bit
video can be dithered down to 8bit. Finally, the frame rate can be reduced.
Below 16fps, however, flicker becomes very noticeable.
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FireWire
FireWire is a computer serial bus more accurately known as IEEE 1394. It is bi-directional (meaning it can carry signals in both directions). Although was not developed specifically for digital cameras it was designed with applications like video in mind.
FireWire can transfer video at 4 times real time for rapid transfer to a computer or edit station. FireWire is the only computer bus connector provided on any DV format camcorders. A DV recorded signal can also be transferred via analog composite or S-video connectors as well as digitally via Firewire.
FireWire supports 63 devices on a single bus (SCSI supports 7, SCSI Wide supports 15) and allows busses to be bridged (joined together) to give a theoretical maximum of thousands of devices. It uses a thin, easy to handle cable that can stretch further between devices than SCSI which only supports a maximum 'chain' length of 7 meters (20 feet).
Data Transfer Rate
| Port | Megabytes per second |
| Serial | .01 |
| Parallel | .115 |
| USB | 1.5 |
| SCSI-1 | 5 |
| SCSI-2 | 10 |
| Ultra SCSI | 20 |
| FireWire | 12.5-50 |
| Wide Ultra SCSI | 40 |
Video compression
Most compression schemes are lossy, and many are adaptive. That means they
can be implemented so that the compression algorithm can be optimized for an
image or series of images.
One approach is to average color areas so that redundancy compression will
be effective. This is called quantization of the image. It can be applied in
increasing steps.
Another scheme is to use motion compression. In scenes where the action is
limited, significant compression is achieved by only redrawing the pixels that
change from one frame to the next. The utility of this method is limited when
the scene changes or the camera zooms or pans. In a related scheme, rapid motion
scenes, which the eye perceives as a blur, can be more heavily compressed, and
when the action slows down, a less lossy codec can be used.
Compression techniques take advantage of redundancy or coherence in images
Compression techniques may apply to color bands individually or together, and may be either fixed or adaptive.
Codecs
:
Cinepak is a vector quantization based codec developed specifically to deliver 24-bit video in quarter screen (320 X 240 pixel) windows from files restricted to single-spin CD-ROM data rates. Vector quantization stores information about differences between frames of video by quantifying the magnitude and direction of a pixel's movement. Cinepak's decompressor then uses a CLUT (color lookup table) to recreate the color of each pixel in a frame.
Motion JPEG:
JPEG is a well-established still-image compression codec that removes the redundancies in individual frames. Motion-JPEG is an adaptation of the still-image standard to video. Motion-JPEG uses the same algorithms as JPEG to create I-frames (compressed intraframes), and then successive frames are compressed by holding the compression parameters constant, to keep up with the video data stream in real-time. There are various, non-compatible versions of Motion JPEG from different manufacturers. Using fast codec acceleration hardware available from several manufacturers, I-frames are coded and decoded symmetrically in less than one-thirtieth of a second.
MPEG:
MPEG was designed for digital video. MPEG uses the same algorithms as JPEG to create one I-frame, then removes the redundancy from successive frames by predicting them from the I-frame and encoding only the difference from its predictions. This is called interframe compression. The MPEG committee created two standards, MPEG I that can play back from a single speed CD-ROM (150 KB per second) at 320 X 240 at 30 fps and MPEG II with enough data (1.2 MB/second) to encode studio-quality video at 704 X 480 at 30 frames per second.
SMPTE Time Code
SMPTE time code is the universal reference for synchronizing audio and video.
SMPTE readouts are in this form: HOURS:MINUTES:SECONDS:FRAME
This reference allows each frame to be identified and relocated. SMPTE also
specifies frame rates in use around the world: 24 fps for film, 25 fps for non-NTSC
video, 30 fps for NTSC black and white video, and 30 fps drop frame for NTSC
color video.
Because of the issues regarding modulation of chroma onto the luminance signal
the actual frame rate equals 29.97. This actually means that the nominal 30
fps is off by 108 frames per hour, or 3.6 seconds. Drop frame compensates for
this problem by omitting reference to 2 frames per minute except the tenth minute.
A pulse wave carries the reference to SMPTE time code. These pulses can recorded
onto videotape using a time code generator, in a process called striping a tape.
The pulse waves fluctuate between 2400Hz and 4800Hz.
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Digital SMPTE
A SMPTE reader translates the 2400Hz pulse into a digital 0, and the 4800Hz
pulse to a digital 1, and converts the bit stream into meaningful information.
This yields 80 bits of data for each frame that can even record user data such
as the recording date, reel number, etc.
There are two ways of recording this data to tape: LTC and VITC.
LTC (Linear Time Code) resides on a linear track, such as the audio track of
a video tape. LTC can't be read a transport speeds much higher or lower than
playback.
VITC (Vertical Interval Time Code) is stored in the video blanking interval
of the video signal itself. The advantage of this method is that the time code
can be read at any playback speed, even freeze-frame. One disadvantage is that
VITC can't be striped onto a tape- it must be recorded when the video is recorded.
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DVD
DVD is essentially a bigger, faster CD that can hold cinema-like video, better-than-CD audio, and computer data. DVD aims to encompass home entertainment, computers, and business information with a single digital format, eventually replacing audio CD, videotape, laserdisc, CD-ROM, and video game cartridges. DVD has widespread support from all major electronics companies, all major computer hardware companies, and all major movie and music studios. With this unprecedented support, DVD has become the most successful consumer electronics product of all time in less than three years of its introduction.
It's important to understand the difference between the physical formats (such as DVD-ROM or DVD-R) and the application formats (such as DVD-Video or DVD-Audio). DVD-ROM is the base format that holds data. DVD-Video (often simply called DVD) defines how video programs such as movies are stored on disc and played in a DVD-Video player or a DVD computer. The difference is similar to that between CD-ROM and Audio CD. DVD-ROM includes recordable variations DVD-R/RW, DVD-RAM, and DVD+R/RW . The application formats include DVD-Video, DVD-Video Recording, DVD-Audio, DVD-Audio Recording, DVD Stream Recording. There are also special application formats for game consoles such as Sony PlayStation 2.
Audio/Video Specifications:
| Data Transfer Rate: | Variable speed date transfer at an average rate of 4.69 megabits/ second for image and sound |
| Image Compression: | MPEG-2 digital image compression |
| Audio: | Dolby AC-3 (5.1 ch), LPCM for NTSC and MPEG Audio, LPCM for PAL/SECAM (A maximum of 8 audio channels and 32 subtitle channels can be stored) |
| Running Time (movies): | 133 min./side (At an average data rate of 4.69 megabits/ second for image and sound, including 3 audio channels and 4 sub-title channels) |
Streaming Video Over the Internet
Streaming video across the Internet has its own specific issues that must be addressed. The Internet was not designed for real time streaming. The Internet is a shared medium and uses a best effort delivery mechanism, Internet Protocol (IP) to deliver content. There is no dedicated path between source and the sink. IP breaks content up into self contained packets and these packets are routed independently. Limited bandwidth, latency, noise, packet loss, retransmission and out of order packet delivery are all problems that can affect real time streaming over the Internet.
All Internet streaming technologies get around this by buffering a certain amount of content before actually starting to play. The buffer irons out the natural traffic variations inherent on the Internet. Many seconds worth of content can be buffered and in excess of 30 seconds worth is not uncommon. Note that after the initial buffering the streamed broadcast will start to play at the same time as more content is being downloaded. This is an improvement over earlier technologies where the whole file had to be downloaded before playing could commence.
Over the last couple of years cable and DSL access has been increasing allowing bandwidths between 128kb/s to 512kb/s to be available to end users. At this bit rate near-VHS quality rich media can be achieved through modern compression techniques and sophisticated codec technology.
Streaming CodecsThere are a variety of compression systems used today. The Motion Picture Experts Group (MPEG) has three open (ISO/IEC) standards that can be used for streaming.
MPEG-1, originally developed for VHS quality video on CD-ROM in 1988 and has its optimal bit rate at about 1.5Mb/s for quarter screen TV (352x240) at 30 frames/sec. MPEG-1 is mainly considered as a storage format, however it does offer excellent streaming quality for the bit-rate it supports.
MPEG-2 was ratified in 1996. It was designed for use in digital TV broadcasting and is best known for DVD encoding. Its target bit-rate is between 4 and 9Mb/sec but it can be used in HDTV for resolutions up to 1920x1080 pixels at 30 frames/sec which will witness average bit rates up to 80 Mb/sec. As an Internet streaming technology it is probably not useful as it uses bit rates higher than those to which almost everyone has access.
MPEG-4 was ratified in 1999 and is a new standard specifically developed to address Web and mobile delivery. Its optimal bit rate is between 385 to 768 Kb/sec. There is still active work continuing on this standard but a number of groups are putting some heavy research and development efforts behind making MPEG4 the standard on the Internet. Codecs are currently available from Microsoft and Apple.
Despite the open standards of MPEG most people use one of the big three proprietary formats. These are RealMedia, Quicktime and Windows Media. All three have specific advantages which have allowed them to gain ground in the market - mainly because they are free, and support the Real Time Streaming Protocol (RTSP).
RealMediaQuicktimeA very popular player which is very widely distributed and available for all major OS platforms. RealNetworks claim over 70% of the Internet streaming market with the player being installed on over 90% of home PCs.
RealSystem 8 supports over 40 media formats. Surestream is an automatic multi bit-rate technology that will adjust the streamed data rate to suit the client's connectivity. In practical terms this means that a single encoding will suit all users from dial-up to corporate LAN. Also supported is Synchronised Multimedia Integration Language (SMIL) which allows mixed multimedia content to be delivered in a synchronised way.
Windows Media PlayerOriginally developed in 1991 QT now claims more than 100 million copies distributed world-wide. Quicktime's major advantages are its maturity and the large number of codecs available for it. It features an open plug-in feature to allow third party codecs to be added. MPEG1 and MPEG4 codecs are currently available.
The plug-in feature has allowed over 200 digital media formats to be supported with companies such as Sorenson Labs producing very impressive codecs. As with RealPlayer, SMIL is available and RTSP is also supported.
Windows Media Player is the newcomer to the streaming world. Because of this there are fewer codecs available for it. There is an MPEG4 codec and Microsoft's proprietary but very good ASF codec. Microsoft have put some work into their RTSP implementation and it is considered more efficient than others. SMIL is supported, but only at a basic level.
QuickTime VR
Panoramas:
QTVR Panoramas are in essence the view from a single point in space out to a surrounding environment. From the central observation point, called a node, a viewer can look in any direction and may zoom in or out from a particular view by changing the zoom angle of their view. Panoramas can be created in a number of ways. The most common method is to capture a series of source images around a single point of rotation, then digitize them into source Pict files. These source Picts are then stitched together to create a single panorama image that represents a cylindrical view from the point of rotation. Alternatively, many panoramic cameras create a single panoramic image directly on film, and these may also be converted into a QTVR panorama.
The source images for panoramas can also be created via a 3D rendering or CAD application. Most rendering programs can export a 360 degree panoramic Pict from any point in the scene. Even if a 3D program cannot do this, all rendering applications can generate a series of individual Pict files around a given point.
Finally, QTVR panoramas support the creation of hot spot areas, which function as an invisible yet detectable mask on the final panorama. Developers can use these hot spots to link panoramas to other QTVR panoramas, QTVR objects, QTVR scenes, or other media such as graphics, text, videos, and sounds via an authoring environment. Alternatively, the same hot spots can be used to reference World Wide Web sites when the QTVR panorama is included on a web page.
The old Multimedia Lab.
Objects:
While panoramas are represented by a 360 degree view from a single point in space, QTVR objects are essentially the reverse: the view from multiple points in space onto a single point, or object. Where QTVR panoramas are composed from a single cylindrical Pict image, objects are composed from a number of individual views that have been captured or rendered. The individual views are not stitched together, but instead are placed in an ordered sequence that enables a user to shift rapidly from view to view. Depending on the number of views that have been captured and assembled, the object movie can be manipulated to provide a full or partial range of vertical and horizontal motion.
PalmPilot/barcode scanner from Symbol Technologies.