ALIASING TEST CHART

         REFLECTANCE

The TE173 is designed for measuring aliasing.

Aliasing effects are moiré distortions caused by interferences of two rasters. Independent 

of the type of camera aliasing effects can arise by interferences of motive rasters and the 

TV line structure e.g. The well known venetian blind effect or textile stripe patterns.

CCD cameras have a special immanent aliasing problem caused by the spatial image 

sampling. The spectrum of the signal obtained at the camera output is compromised 

of a basic spectrum repeated around multiples of the CCD sampling frequency. The 

CCD sampling frequency depends on the CCD size and the number of the pixels

per CCD width. For some high spatial frequencies of the image, the condition 

dictated by sampling theorem is not met, so that the basic spectrum and duplicated 

spectrum can be superimposed and give rise to inter-frequency beats. The result is 

moiré in the pictures.

Moiré visibility depends on the type of analyser, on the camera's low pass optical 

filtering and on the spatial frequency of the test pattern analysed.

The TE173 consists of rectangular bars the spatial frequencies of which depend 

on the picture width used. On the top and bottom, outside of the picture area, 

horizontal lines are drawn on which are marked the framings that are to be 

carried out in order to modify, by zooming, the spatial frequency seen by the camera.

Measurement procedures can be made with a spectrum analyser or a wide-band video oscilloscope.

Measurement conditions

Gamma correction: OFF

Contour correction: OFF

Colour correction: ON

Spectrum analysers measurements

The measurement principle consists in locating, on the spectral analyses of the luminance

signal Y or coded Y, and successively R, G, B; the interference lines resulting from

sampling. The camera is aligned on the test chart at a given spatial frequency. The

camera output is connected with the input of a spectrum analyser. The iris is 

adjusted so, that the white level corresponds to a vision signal of 700mV / 75 Ohms.

The pedestral is set at 0mV. In case of the analyses of a signal including mixed sync 

pulses, it is recommended that these pulses, which render interpretation of the spectrum 

analyser curves more difficult, be suppressed. A measurement example is given in figure 

2 below.

Figure 2

If the number of horizontal CCD pixels is known, it is possible to calculate the sampling 

frequency, which is easily located on the analysers screen:

Sampling F (MHz) = No. of horizontal pixels / 52

The frequency of the interference line is given by the difference between the sampling 

frequency and the test pattern frequency:

Interference F = Sampling F - test pattern F

The measurement involves determine the difference in level between the useful signal(s)

at test pattern frequency and the signal at interference frequency. The measurements 

have to be repeated for each of the spatial frequencies and made several times 

(approx. 10 times) to determine an average result.