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The signal-processing part of the camera has to perform a number of operations whose ultimate goal is to produce the best possible output signal. The output signal, depending on the user requirements, can be analog component, analog composite, or digital component in its parallel or serial form. However, practi- cally all present-day cameras use digital signal processing (DSP). At the output of the sensors, and after preamplification (which is the first amplification stage and is usually an integral part of the sensor), the R, G, and B signals are digitized and all subsequent operations conducted with digital signals. The A/D conversion (see Section 5.2) is usually performed with a 10- or even 12-bit quantization that ensures high-quality signal processing. At the end of the processing chain, and before the output, the R, G, and B signals are converted to the standard digital components (Y, R-Y, and B-Y) complying with Recommendation 601. Compared with traditional analog signal-processing techniques, digital signal processing offers a number of advantages:

• considerable reduction of the influence of signal-processing circuits on the signal quality

• introduction of additional signal corrections, which are difficult or imprac- tical to perform with analog techniques

• easy-to-memorize settings and use of same settings on several cameras. CCD cameras with digital signal processing are extremely stable and require few setup alignments. In practical terms this means that a CCD camera will be ready to operate several seconds after being switched on. However, there are a number of corrections that have to be performed in order to optimize the output picture or to adapt the camera to the lighting conditions. Some of these corrections are automatic and automatically triggered. Others are automatic but have to be manually started, while some corrections are still fully manual.

9.3 Camera Processing Circuitry 149

The signal goes through a number of processing stages after preamplification: • The master gain (overall and simultaneous gain adjustment of all three R, G, and B channels) is manually controlled in order to increase the signal amplitude in case of extremely low light levels when the voltage of the signal generated by the sensor is too low for further processing. Increasing the gain results in an unavoidable increase of noise level.

• The fully automatic shading control is meant to correct the nonuniformity of response over the whole image surface, which could be the result of lens errors or of differences in sensitivity of particular cells of the sensor. • Color correction is used to adjust the correct colorimetry of the reproduced

picture. The spectral sensitivity of the R, G, and B camera channels has to be adjusted to match the spectral curves of the screen phosphors. Although it is possible to compute a theoretical optimum, many broadcasters have their own colorimetric preferences and consequently, camera manufactur- ers offer switchable preset values (e.g., EBU, BBC, RAI, etc.). At the same time, studio and outside broadcast (OB) cameras are equipped with manual gain controls that support matching the appearance of pictures delivered by several cameras in a multicamera production.

Like their predecessors the pick-up tubes, CCD sensors used today have a linear

transfer characteristic, that is, a linear relationship between the generated electri-

cal signal and the incoming light—throughout the whole range of illuminations, equal changes of the intensity of the incoming light will generate equal changes of the amplitude of the electrical signal. However, for correct color reproduction it is necessary that the whole production, transmission, and reception chain be linear. The last element in that chain, the cathode-ray tube, is not a linear device. Consequently, to ensure a linear operation of the system a precorrection has to be introduced at the front end of the chain. The linear characteristic of the sensor must be made nonlinear but complementary to the transfer characteristic of the display. The addition of the sensor and of the display characteristics will produce an overall linear response (see Figure 9.10). That precorrection, known as gamma

correction, usually has several manually controllable preset values.

The finite number of photosensitive cells in a CCD sensor as well as the pres- ence of the optical prefiltering result in a loss of resolution. To compensate for this loss, a special manual control called contour correction is introduced to artifi- cially improve the crispness of all transitions. This correction ensures better and sharper pictures but must be used with great care. Its application leads not only to an increase of the basic noise level but also to too much contour correction, which may produce unnatural-looking pictures in that they are much sharper than real ones.

Resulting linear response Camera

precorrection

Cathode-ray tube characteristic

Figure 9.10 Gamma correction.

As was explained in Chapter 2, white lights from different sources have charac- teristics, expressed as different color temperatures, which unavoidably influence the color rendition of photographed objects. Consequently, to ensure a faithful reproduction of the whole color spectrum under given lighting conditions, it is necessary to adjust the relative values of the R, G, and B channels accordingly. That adjustment is known as white balance and has to be performed prior to any shooting and whenever the lighting conditions change (shooting indoors vs. out- doors, etc.). To adjust the white balance, the operator has to point the camera toward a uniform white surface exposed to the ambient light and then manually initiate the automatic adjustment circuitry.

Studio or OB cameras usually have three separate subassemblies, which together represent the camera chain:

• camera head – optics, sensors, viewfinder, and the associated circuitry • camera control unit or CCU – containing the major part of the processing

circuitry

• operational control panel – remote controls of the iris, black level, white level, and individual R, G, and B channel controls, allowing a continual control and adjustment of the photographic quality of the output picture. A camera cable connects the camera head and the CCU. Since every cable introduces a degree of attenuation and distortion of the transmitted signal that is directly proportional to the length of the cable, it is necessary to introduce a precorrection that will compensate for the expected losses. The amount of pre- correction depends on the length of the cable, and there is a limit of length over which no correction is possible. That maximum length depends on the type of cable and on the type of camera, but generally it is around several hundred meters for multicore cables and several thousand meters for “triax” cables (special