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The luminance signal is recorded in its original form ( albeit with some reduction of bandwidth ) but the PAL chrominance signal of about 4. 4 MHz is too sensitive to small changes in frequency caused by inevitable small variations in tape speed to be recorded directly ( NTSC suffers from the same problem ).
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luminance and signal
When analog television was developed, no affordable technology for storing any video signals existed ; the luminance signal has to be generated and transmitted at the same time at which it is displayed on the CRT.
The timing of the luminance signal must allow for this.
The video carrier is demodulated to give a composite video signal ; this contains luminance, chrominance and synchronization signals ; this is identical to the video signal format used by analog video devices such as VCRs or CCTV cameras.
The color space is three dimensional by the nature of the human vision, so after subtracting the luminance, which is carried by the base signal, the color sub carrier still has to carry a two dimensional signal.
Typically the red ( R ) and the blue ( B ) information are carried because their signal difference with luminance ( R-Y and B-Y ) is stronger than that of green ( G-Y ).
Because the color information was designed to be a cheap, backwards compatible addition to the monochrome signal, the color signal has a lower bandwidth than the luminance signal, and hence lower horizontal resolution.
If the signal data are not constant, then can be calculated from data where the signal is zero or relatively constant .</ ref > Notice that such an alternative definition is only useful for variables that are always non-negative ( such as photon counts and luminance ).
VHS tapes have approximately 3 MHz of video bandwidth and 400 kHz of chroma bandwidth, which is achieved at a relatively low tape speed by the use of helical scan recording of a frequency modulated luminance ( black and white ) signal, with a down-converted " color under " chroma ( color ) signal recorded directly at the baseband.
The frequency modulation of the VHS luminance signal is limited to 3 megahertz which makes higher resolutions technically impossible even with the highest-quality recording heads and tape materials, but an HQ branded deck includes luminance noise reduction, chroma noise reduction, white clip extension, and improved sharpness circuitry.
Subsequently, the video head erases and re-records the video signal ( combined luminance and color signal ) over the same tape surface, but the video signal's higher center frequency results in a shallower magnetization of the tape, allowing both the video and residual AFM audio signal to coexist on tape.
Altering the input signal by gamma compression can cancel this nonlinearity, such that the output picture has the intended luminance.
The following illustration shows the difference between a scale with linearly-increasing encoded luminance signal ( linear input ) and a scale with linearly-increasing intensity ( i. e., gamma-corrected ) scale ( linear output ).
The color video signal is a linear combination of the luminance of the picture, and a modulated subcarrier carries the chrominance or color information, a combination of hue and saturation.
Dot crawl is a defect that results from crosstalk due to the intermodulation of the chrominance and luminance components of the signal.
luminance and is
As it passes each point the intensity of the beam is varied, varying the luminance of that point.
The compromise can improve text appearance when luminance detail is more important than chrominance.
For example, the human eye is more sensitive to subtle variations in luminance than it is to variations in color.
* the power by which the luminance of an image is increased in gamma correction
Another use is for backward compatibility and graceful degradation: in color television, encoding color via a luminance-chrominance transform domain ( such as YUV ) means that black-and-white sets display the luminance, while ignoring the color information.
The SI unit for luminance is candela per square metre ( cd / m < sup > 2 </ sup >).
The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd / m < sup > 2 </ sup >.
This means that for an ideal optical system, the luminance at the output is the same as the input luminance.
For real, passive, optical systems, the output luminance is at most equal to the input.
The light at the image plane, however, fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens.
: is the luminance ( cd / m < sup > 2 </ sup >),
One application is to formulas for washing and conditioning grey or blonde hair, where the brightener can not only increase the luminance and sparkle of the hair, but can also correct dull, yellowish discoloration without darkening the hair.
In a Bayer filter arrangement, green is given twice as many detectors as red and blue ( ratio 1: 2: 1 ) in order to achieve higher luminance resolution than chrominance resolution.
The color information is called chrominance or C for short, while the black and white information is called the luminance or Y for short.
Fortunately, the human visual system is similar in design: it perceives changes in luminance at a higher resolution than changes in chrominance, so this asymmetry has minimal visual impact.
luminance and recorded
The modulated audio carrier pair was placed in the hitherto-unused frequency range between the luminance and the color carrier ( below 1. 6 MHz ), and recorded first.
For PAL, however, the bandwidth between the chroma and luminance carriers was not sufficient to allow additional FM carriers, so depth multiplexing was employed, wherein the audio track would be recorded in the same way that the video track was.
In digital recording, digital audio and digital video is directly recorded to a storage device as a stream of discrete numbers, representing the changes in air pressure ( sound ) for audio and chroma and luminance values for video through time, thus making an abstract template for the original sound or moving image.
The video signal is recorded using a method called " time compression integration " which " records separated component video, luminance and color signals offset by time in alternating parts of the video track ".
luminance and its
This is usually seen when chrominance is transmitted with a high bandwidth, and its spectrum reaches into the band of the luminance frequencies.
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ( or saturation, chroma, intensity, or excitation purity ).
The xyY space is a cross between the CIEXYZ and its normalized chromaticity coordinates xyz, such that the luminance Y is preserved and augmented with just the required two chromaticity dimensions.
For example in North America, NTSC standards dictate that CH2 is a 6 MHz wide channel with its luminance carrier at 55. 25 MHz, so the modulator for channel 2 will impose the appropriate input signal on to the 55. 25 MHz frequency to be received by any TV tuned to Channel 2.
To convert any color to a grayscale representation of its luminance, first one must obtain the values of its red, green, and blue ( RGB ) primaries in linear intensity encoding, by gamma expansion.
Since the eye is an optical system, radiance and its cousin luminance are good indicators of how bright an object will appear.
When exposure is determined from average luminance measurements, the exposure of any given scene element depends on the relationship of its reflectance to the effective average reflectance.
This is a different concept from contrast, which by itself refers to one object's difference in color and luminance compared to its surroundings or background.
Scattering adds the sky light as a veiling luminance onto the light from the object, reducing its contrast with the background sky light.
Dot crawl is most visible when the chrominance is transmitted with a high bandwidth, so that its spectrum reaches well into the band of frequencies used by the luminance signal in the composite video signal.
Taken further, since 537. 6 lines is equal to 268. 8 cycles, and given 576i50 has an active line period of 52µs, its luminance signal requires a bandwidth of 268. 8 / 52 = 5. 17 MHz.
Furthermore, many people find luminance noise less objectionable to the eye, since its textured appearance mimics the appearance of film grain.
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