Thursday, 9 October 2008

Introduction to Video Recording



Q1 Define Video Recording and identify some of its advantages.

Ans) Video Recording is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.

Its advantages:
The quality of video tapes programs is indistinguishable from the original picture and sound with an excellent broadcast quality. The tapes can be reused and also duplicated without loss of quality in picture or sound. The video tapes can also be replayed immediately and the recording can be analyzed by the technicians and the directors. This leads to saving of time. Another major advantage is that the video tape’s picture and sound can be edited or modified separately. Unwanted or faulty sections can be deleted and also replaced by some other material. Video Taped Programs can be easily stored as an entire program or single sections or even as still shots that can be reused and manipulated for further usage. The tapes are also not prone to damage therefore have longer lives.

Q2 What are the different types of Video Systems in use and explain there working.

Ans) There are three types of systems used: BETAMAX, VHS, HI8

Betamax: In the Betamax system, the video tape is guided along the head drum in a U-shape for all tape guidance functions, such as recording, playback and fast forward/backward. When the cassette is inserted, the tape is guided around the head drum (called threading). Threading the tape takes a few seconds, but once the tape is threaded, shifting from one tape function to another can be achieved rapidly and smoothly.

VHS: JVC's VHS System was introduced one year after the launch of Betamax. In VHS, the tape is guided through in an M-shape; the so-called M-tape guidance system. It is considered simpler and more compact than the U-system. Threading is faster and is done every time the tape guidance function is changed. It is therefore somewhat slower and noisier than the U-system. This problem is being solved by "Quick-start" VHS video recorders, which allow fast and silent changes in tape guidance functions. To avoid excessive wear, M-tape guidance system recorders are provided with an automatic switch-off feature, activated some minutes after the recorder is put on hold, which automatically unthreads the tape. An improvement of the basic VHS system is HQ (High Quality) VHS.

In the VHS system different starting points were used than in Betamax, such as track size and relative speed. VHS has rather wide video tracks, but a slightly lower relative tape speed, and that also counts for the audio track. In general, the advantages of one aspect are tempered by the disadvantages of the other. The end result is that there is not too much difference between the sound and image qualities of both systems.

HI8: As a direct addition to the Video-8 camcorders, there is a third system: Video Hi8, which uses a smaller cassette than VHS and Betamax. The sound recording takes place digitally, making its sound quality very good. When using the special Hi8 Metal Tape, the quality of both image and sound are equivalent to that of Super-VHS. The Video-Hi8-recorder can also be used to make audio recordings (digital stereo) only. Using a 90 minute cassette, one can record 6 x 90 minutes, making a total of 18 hours of continuous music. The video Hi8-system also allows manipulating digital images, such as picture-in-picture and editing. Video Hi8 uses a combination of the M- and U-tape guidance system.

Q3 Describe the process of Sound Recording.

Ans) In case of a mono video recorder, the audio signal which corresponds with the image is transferred to a separate, fixed audio head. As in an audio cassette deck, this head writes an audio track in longitudinal direction of the tape. This is called linear or longitudinal track recording.

The video recorder has two erase heads. One is a wide erase head covering the whole tape width which automatically erases all existing image, synchronization and sound information when a new recording is made.

The other erase head is smaller and positioned at the position of the audio track. With this erase head, the soundtrack can be erased separately, without affecting the video information. In this way, separate audio can be added to a video recording. This is called audio dubbing, and can be particularly useful when making your own camera recordings. The linear audio track does have some restrictions. Due to its low tape speed, it is not suitable for hi-fi recordings. Moreover, the audio track is so narrow (0.7 mm for VHS and 1.04 mm for Betamax) that not even stereo sound can be recorded properly.

The frequency range is limited as is the dynamic range (which relates to the amount of decibels), and the signal-to-noise ratio is not very high.(The signal-to-noise ratio relates to amount of noise compared to the total signal. The higher this ratio, the less noise and the better the signal will be).

Hi-fi video recorders were developed for improved sound quality. In the case of hi-fi, the audio signal is also put on tape via revolving heads similar to the video signal, not on the linear track.

As there is no space between the video tracks, as the video tracks lie right next to each other with no space in between, the audio tracks need to be recorded in the same place as the video tracks. The way this is realized is by recording the audio signal under (deeper than) the video signal. In hi-fi video recorders, the audio signal is modulated to a high carrier frequency. This is realized via FM modulation, with the right channel stereo signal at a slightly higher frequency than the left channel.

The corresponding video and audio signals are written to tape immediately after each other. First the FM audio signal is registered at a deep level in the tape's magnetic coating. Straight after the audio signal, the video signal is recorded. As the frequency of the video signal is higher than the audio signal, it will not register as deep in the tape coating as the audio signal. The video signal erases the audio signal in the top layer and records the video signal instead. Thus, the audio and video signal tracks are written in the same magnetic layer, separately, one on top of the other. A hi-fi video recorder is also suitable as a high-quality audio recorder, not only because of the professional recording quality, but also because of the long play possibilities and the low recording costs. A hi-fi-video recorder needs to be tuned very accurately. As the two rotating audio heads function alternately, the recorded sound consists of successive particles and need to fit together perfectly. If they do not, the result is rumble, which is a humming sound. In high quality, well-tuned hi-fi video recorders you will not hear this sound.

Q4 What are Camcorders and what are the formats being used by them?

Ans) A camcorder is a portable electronic device for recording video images and audio onto an internal storage device. The camcorder contains both a video camera and (traditionally) a videocassette recorder in one unit. Camcorders are often classified by their storage device: VHS, Betamax, Video8 are examples of older, videotape-based camcorders which record video in analog form. Newer camcorders include Digital8, miniDV, DVD, Hard drive and solid-state (flash) semiconductor memory, which all record video in digital form.

MiniDV is now the most popular format for tape-based consumer camcorders, providing near-broadcast quality video and sophisticated nonlinear editing capability on consumer equipment. MiniDV storage allows full resolution video (720x576 for PAL, 720x480 for NTSC), much unlike the analogue video standards before. Digital video doesn't experience colour bleeding or fade. There has been a trend, largely spearheaded by Hitachi, Panasonic, and Sony, to sell consumer camcorders based on optical discs rather than tape. Most common are DVD recordable camcorders, which are common among point and shoot users due to the ability to take a disc out of the camcorder and drop it directly into a DVD player, much like VHS-C on the analog side. However, professionals consider DVD media to be too inflexible for easy editing.

Q5 What are the two different image capture formats?

Ans) Digital video cameras come in two different image capture formats: interlaced and progressive scan. Interlaced cameras record the image in alternating sets of lines: the odd-numbered lines are scanned, and then the even-numbered lines are scanned, then the odd-numbered lines are scanned again, and so on. One set of odd or even lines is referred to as a "field", and a consecutive pairing of two fields of opposite parity is called a frame. A progressive scanning digital video camera records each frame as distinct, with both fields being identical.

Thus, interlaced video captures twice as many fields per second as progressive video does when both operate at the same number of frames per second. This is one of the reasons video has a “hyper-real” look, because it draws a different image 60 times per second, as opposed to film, which records 24 or 25 progressive frames per second. Progressive scan camcorders such as the Panasonic DVX100 are generally more desirable because of the similarities they share with film. They both record frames progressively, which results in a crisper image. They can both shoot at 24 frames per second, which results in motion strobing (blurring of the subject when fast movement occurs). Thus so, progressive scanning video cameras tend to be more expensive than their interlaced counterparts. (Note that even though the digital video format only allows for 29.97 interlaced frames per second [or 25 for PAL], 24 frames per second progressive video is possible by displaying identical fields for each frame, and displaying 3 fields of an identical image for certain frames.

Q6 What is Video Compression and explain its different types.

Ans) Video compression refers to reducing the quantity of data used to represent video content without excessively reducing the quality of the picture. It also reduces the number of bits required to store and/or transmit digital media. Compressed video can be transmitted more economically over a smaller carrier.

Digital video requires high data rates - the better the picture, the more data is ordinarily needed. This means powerful hardware, and lots of bandwidth when video is transmitted. However much of the data in video is not necessary for achieving good perceptual quality, e.g., because it can be easily predicted - for example, successive frames in a movie rarely change much from one to the next - this makes data compression work well with video. Video compression can make video files far smaller with little perceptible loss in quality.

Some forms of data compression are lossless. This means that when the data is decompressed, the result is a bit-for-bit perfect match with the original. While lossless compression of video is possible, it is rarely used. This is because any lossless compression system will sometimes result in a file (or portions of) that is as large and/or has the same data rate as the uncompressed original. As a result, all hardware in a lossless system would have to be able to run fast enough to handle uncompressed video as well. This eliminates much of the benefit of compressing the data in the first place. If the inverse of the process, decompression, produces an exact replica of the original data then the compression is lossless.

Lossy compression, usually applied to image data, does not allow reproduction of an exact replica of the original image, but has a higher compression ratio. Thus lossy compression allows only an approximation of the original to be generated.

The size of the data in compressed form (C) relative to the original size (O) is known as the compression ratio (R=C/O). For image compression, the fidelity of the approximation usually decreases as the compression ratio increases.The success of data compression depends largely on the data itself and some data types are inherently more compressible than others. Generally some elements within the data are more common than others and most compression algorithms exploit this property, known as redundancy. The greater the redundancy within the data, the more successful the compression of the data is likely to be. Fortunately, digital video contains a great deal of redundancy and thus is very suitable for compression.

Q7 List the major differences between a film and a video.

Ans) Exposure Latitude - A key difference between DV and film is exposure latitude, which affects contrast and detail. Color negative has a usable exposure range of 7 stops, with normal exposure approximately in the middle. Most stocks provide 4 stops overexposure and 3 stops underexposure where detail is still visible.

Video has a usable exposure latitude of 5 stops, providing 2 stops overexposure and 3 stops underexposure where detail is still visible. Exposure beyond the -/+ limits results in tonal compression and is reproduced as either pure white or pure black, respectively. Obviously, there is a loss of detail as well.

Motion Blur - Film yields a slight blur in moving objects. This is known as motion blur and it results in a distinct fluidity of movement-- a prime contributor to the "film look." Motion blur is caused by film's relatively low frame rate of 24 frames per second. A telltale sign of video is its extreme sharpness and lack of motion blur. There are two interlaced fields for every frame of video, so the effective rate is actually 60 images per second (= 30 fps x 2 fields). This virtually eliminates motion blur, creating an image that is a bit too sharp and devoid of fluidity (the dreaded "video look").

The answer to this is a technical breakthrough called progressive scanning, where each frame is scanned once. In other words, the frame is scanned as a single field, with no interlacing. The lower image rate reproduces motion blur comparable to film. Another benefit of progressive scanning is a dramatic increase in resolution. This occurs because progressive scanning eliminates interlace artifacts (combed edges in movement) and interline flicker (noise in fine patterns).

Resolution - The final difference between video and film is resolution. Many filmmakers erroneously assume that film is far superior across the board. Arguably, the disparity in resolution has less of an impact on the look of DV than exposure latitude and motion blur. It is not noticeable to the average audience, except of when aliasing rears its ugly head. Aliasing can be minimized by avoiding fine patterns, particularly checkered and striped clothing.

DV has an interesting advantage over film that may, in part, make up for its lower resolution. It can "see" in low light almost like the human eye and captures beautiful images during sunrise and sunset.

Q8 What is the major difference between Analog and Digital Video Recording?

Ans) The analog recording method stores signals as a continual wave in/on the media, rather than the discrete numbers used in digital recording. The wave is stored as a physical texture on a phonograph record, or a fluctuation in the field strength of a magnetic recording. In an analog system the continuously varying voltage magnetizes tape particles in a continuously varying pattern that mirrors the signal. On playback, the tape particles create a continuously varying output signal that continues to mirror the original.

Every transfer of the picture information is an imitation--or, more precisely, an imitation of an imitation, with consequences that we'll see shortly.In a digital system, by contrast, the first thing that happens to the original continuous signal is that it's fed through an analog/digital converter chip. That chip looks at the signal hundreds of thousands of separate times per second and assigns each discrete sampling a numerical value that corresponds to the strength of the signal at that precise moment in time. These numbers, rather than the signal itself, are copied and recopied throughout the rest of the process.

Q9 What are the advantages of Analog over digital and vice versa?

Ans) Analog over Digital

ScalabilityAll video, analog and digital, tends to look sharper and clearer on a smaller screen; it's the natural result of squeezing the same amount of visual information into a smaller space. All but the highest quality digital video, however, suffers greatly from enlargement. When you blow up your digitized image onto a huge home-theater TV screen, for example, all of those invisible digital compression artifacts become quite noticeable--straight lines become jaggy, curves look blocky, etc. Analog video, on the other hand, is much better at filling larger screens with sharp-looking images.

SeamlessnessIn the audio world, some purists have returned to analog (vinyl LP) recordings because they can hear the fact that digital recordings only sample the signal at intervals instead of copying the whole thing. To them, CDs sound hollow and brittle in consequence.

Digital over Analog: Instead of copying the video signal, digital duplication transcribes the numerical code that describes that signal. If you transcribe it accurately (and computers are outstanding at chores like that), you can decode the result into a daughter signal that is essentially indistinguishable from the parent.

Freedom from Noise

Noise is any disturbance in an electrical current that is not part of the signal, and every current carries a certain amount of this electrical disturbance. Since an analog dupe is an imitation, it copies the noise right along with the parent signal, while adding new noise in the process. That means that in each generation, the noise level relative to the signal (signal-to-noise ratio) increases and the quality decreases proportionately. In digital recording, noise is not a problem because the signal consists entirely of current pulses carrying information like Morse code: power on = 1; power off = 0. If the voltage level of the "power on" part of the signal is well above the noise level, then the transcribing (copying) system can be set to respond only to current at that level and ignore the noise entirely. So even if the process adds a small amount of its own noise, it never copies the parental noise--nor does it pass on its own noise to the grandchildren. The result is that digital video can be copied through many generations without appreciable quality loss. This is a massive improvement over analog video (and even over cinematic film, which is another analog medium).

Computer Compatibility

By far the biggest advantage of digital video is that a computer can process and store it. Computers are astonishingly powerful but they cannot work with pictures, or more accurately, with the continuously varying wave forms that record them. Before you can get your computer to handle or even recognize video input, you have to digitize the video. For many years, professionals have digitized video, not only to take advantage of loss-free duplicating, but also to perform image processing. Image processing means superimposing titles, compositing multiple images, and adding effects like dissolves and wipes. In image processing, digital is an ephemeral state: an analog signal is digitized, massaged for a few microseconds at most, and immediately reconverted to analog.

But as hard drives got bigger and faster, and as image compression techniques improved, it became possible to digitize the signal and then keep it in that form indefinitely by storing it in the computer.

Q10 Describe the process of Helical Scanning.

Ans) Helical scan or striping is a method of recording higher bandwidth signals onto magnetic tape than would otherwise be possible at the same tape speed with fixed heads. It is used in video cassette recorders, digital audio tape recorders, and numerous computer secondary storage and backup systems. In a fixed head system, tape is drawn past the head at a linear speed. The head creates a fluctuating magnetic field in response to the signal to be recorded, and the magnetic particles on the tape are forced to line up with the field at the head.

As the tape moves away, the magnetic particles carry an imprint of the signal in their magnetic orientation. If the tape moves too slowly, a high frequency signal will not be imprinted — the particles' polarity will simply oscillate in the vicinity of the head, to be left in a random position. Thus the bandwidth capacity of the recorded signal can be seen to be related to tape speed — the faster the speed, the higher the frequency that can be recorded.

Video and digital audio need considerably more bandwidth than analog audio, so much so that tape would have to be drawn past the heads at very high speed in order to capture this signal. Clearly this is impractical, since tapes of immense length would be required. (However, see VERA for details of a partially-successful linear videotape system.) The generally adopted solution is to rotate the head against the tape at high speed, so that the relative velocity is high, but the tape itself moves at a slow speed. To accomplish this, the head must be tilted so that at each rotation of the head, a new area of tape is brought into play; each segment of the signal is recorded as a diagonal stripe across the tape. This is known as a helical scan because the tape wraps around the circular drum at an angle, traveling up like a helix.

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