Basic Quality Parameters

type of coaxial cables where the central or axial conductor is surrounded by two concentric and mutually isolated mesh-tubular conductors, which act also as radiation shields) that carry all relevant signals both ways in a multiplexed form. All modern cameras feature a number of other controls and corrections. Some of them are essential; some are an “added value” that differentiates one camera from the other on the market. The selection of a camera has to be based on the evaluation of objective quality parameters, operational features it offers, and price, but all have to be weighed against the intended use of that piece of equipment.

9.4 Basic Quality Parameters

To select one among many cameras offered on the market, or to select the most appropriate camera for a given application, it is necessary to assess its perfor- mance. This can be evaluated either by a subjective assessment of the quality of the produced picture or by an objective measurement of a number of critical parameters. Subjective evaluations are attractive but highly unreliable and dif- ficult to reproduce since they involve, by definition, highly variable subjective factors. In addition, the comparison of two subjective assessments of two cam- eras, or of the same camera, requires a very precise definition and control of all subjective assessment conditions, and that would be a very complex and deli- cate task. On the other hand, an objective measurement can be easily repeated as many times as necessary. However, meaningful and usable measurement results can be obtained only if measurements are conducted in accordance with known and well-defined procedures and methods. While the departure from the results of a subjective assessment is normal and practically expected, different results obtained from two objective measurements of the same parameter indicate imme- diately that something is wrong either with the measurement itself or with the measured piece of equipment. A careful check of the measurement procedure can easily give a quick and unambiguous answer to that dilemma.

A single, objectively measurable parameter offering a good representation of the quality of a given camera does not exist. Such an assessment can be obtained only by taking together the results of a number of measured parameters. While a good part of these results are directly related, or practically equal to the perfor- mances of the sensors, others are the result of a combined action of the sensors and the camera processing circuitry.

9.4.1 Noise level

Every electronic component generates a certain quantity of unwanted, chaotic, and uncontrolled electron movements in the form of noise accompanying the

signal. The noise in the case of the video signal is visible in the form of small points (unrelated to the picture) that randomly move across the screen and that are commonly called “snow” due to their resemblance to snow flakes. Once gen- erated, the noise will accompany the signal throughout its path. Other elements on that path will add their own noise, and every time the signal is amplified, the noise is amplified as well. Since the sensor is the first element in the picture chain, its own level of noise defines the minimum noise level achievable by a given system. Usually we do not measure the absolute level of noise, only the ratio between the peak-to-peak value of the signal and the mean value of the noise. The signal-to-noise ratio is expressed as a logarithmic computation, and its unit is a decibel (dB).

9.4.2 Resolution

The resolution is the capacity to differentiate small picture details and sharp tran- sitions. Higher resolution means finer and sharper pictures—in short, a better reproduction of the real world. The maximum picture resolution is limited by the basic parameters of a given television system—the overall frequency band- width and the number of scanning lines. The resolution of a sensor, or of the whole camera chain, is measured in a way that is similar to the measurement of resolution of photographic systems: by the number of parallel black lines on a white surface that the system is capable of reproducing as identifiable separate lines. Beyond the resolution limit, parallel separate lines will be reproduced as a uniform gray area.

The resolution of a sensor, or of a camera, is usually expressed as modulation

depth. To understand that parameter, suppose that a camera is pointed at a test

chart constituted by a number of “packets” of equally-spaced black lines on a white surface. The first packet is made of rather wide black lines and all following ones are made of thinner and thinner lines.

As shown on Figure 9.11, after the optoelectric conversion, an electric signal is obtained where wider pulses correspond to wider lines and, at the same time, these pulses have a higher amplitude. With the decrease of the line width, the pulses become narrower and their amplitude is reduced. This loss in amplitude is called loss of modulation depth and it is quantified as a percentage of the widest pulse amplitude, which is equal to the maximum amplitude featured by the television system. The smaller the pulse amplitude, the more difficult its reproduction on the screen will be. Therefore, a resolution expressed as 35% at 400 lines indicates that 400 black and white lines will be reproduced with an amplitude representing 35% of the maximum amplitude. The source resolution can be degraded by inadequate quality of the television production chain. Lines transmitted with a modest modulation depth may be identifiable as separate lines

9.4 Basic Quality Parameters 153

Figure 9.11 Modulation depth. A.) wide, high amplitude pulses corresponding to wide

lines; B.) medium pulses corresponding to medium sized lines; C.) narrow, low amplitudes pulses corresponding to thin, narrowly spaced lines.

on the monitor connected to the camera output. However, all subsequent elements in the video signal path will add their amount of noise to the original signal. At a certain point the overall noise level will mask the parts of the signal whose amplitude is small, and it will become impossible to identify separate lines. The area will appear to be a uniform gray. In practical terms, this means that at the end of the chain, the viewer will experience a picture with less resolution and less sharpness than the one observed on the monitor in the studio control room.

9.4.3 Spectral characteristic

Every sensor has its own spectral characteristic, that is, it offers different responses at different wavelengths of the visible spectrum. While in the time of black-and- white television the spectral characteristic was not of such importance, it became very significant with the advent of color. It is desirable today that the sensitivity of sensors to different parts of the visible spectrum coincides as much as possible with the equivalent sensitivity (or spectral characteristic) of the human eye, so that the reproduction of colors satisfies our appreciation of them.

9.4.4 Sensitivity

The sensitivity is defined by the lowest lighting level that allows the generation of optimum-quality pictures. In order to transpose this definition into a system of

objective, measurable entities, it is necessary to define • the meaning of “optimum picture quality”

• the conditions under which that optimum quality is assessed (light intensity, reflectance of the surface in front of the camera lens, settings of the camera optics, etc.)

The response of a camera depends on the level of the light illuminating the scene in front of it. Reducing the light level that falls on the sensor also reduces the amplitude of the electrical signal generated by it. A lower signal level means a decrease of the signal-to-noise ratio (S/N ratio). A lower S/N ratio is assessed by the viewer as increased noisiness of the picture and interpreted as a loss in quality. Consequently, the signal-to-noise ratio can be used as the necessary objective and measurable parameter expressing the picture quality.

In order to make measurements and assessments as objective and repeatable as possible, the measurement conditions have to be precisely defined. It is necessary to define not only the light level and color temperature but also the reflectivity of the surface at which the camera is pointed. Since the adjustment of optical parameters of the zoom lens determines the quantity of light projected on the photosensitive surface of the sensor, it is important to state exactly what the iris opening, or the f-stop was during the measurement.

Following is an example of the sensitivity of a given camera showing the parameters that should be expressed:

• 2,000 lux at a color temperature of 3,200 K • aperture at f/8

• uniform white surface with a 90% reflectivity • signal to noise ratio of 60 dB.

Many camera specifications give additional information with the sensitivity data, such as “minimum illumination” or “usable picture at XY lux.” The quoted illumination is usually strikingly low. However, that indication has to be taken with a great degree of caution since it usually corresponds to noisy pictures that can be obtained with a full iris opening. Such pictures can be used if the footage is aimed for a news program and if its topicality is extremely high. In short, the level of “usability” of such pictures depends on their “news value” and not on the results of any sort of quantifiable assessment method.

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