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CTX S700

Our first participant is a display from CTX. This is a well-known manufacturer of mainstream displays – both CRT and LCD. The CTX S700 display is the junior model from the 17” family and doesn’t offer any extraordinary characteristics: the specified brightness is 250nit (1 nit = 1 candela per square meter), contrast ratio is 350:1, and both horizontal and vertical viewing angles are 140 degrees. The specified response time equals 25ms.

Designers took it easy when making the case: it won’t become a major attraction of your office or room. The base only allows adjusting the display tiling, but it can be replaced as it complies with the VESA standard.

The matrix has rather wide spaces between pixels and you can see separate strings. Overall, the display looks zebra-like striped. Viewing angles are really moderate: when viewed from below, the display gets very dark, from above – it shows wide dark horizontal lines. Brightness is unevenly distributed from top to bottom – it can be clearly seen at work. Backlight brightness was also small according to today’s standards – I had to set both brightness and contrast of the display to 60% at work.

Response time as measured in the test noticeably exceeded the specified value (33mm pixel rise time plus 3ms pixel fall time equals 36ms total). I won’t conceal the fact that the image produced by the display was more “blurred” than when 25ms matrixes are used. Moreover, you can see in the oscillograms certain brightness fluctuations corresponding to the refresh rate (I set the refresh rate to 60Hz and 75Hz – the brightness fluctuation frequency changed, too). It’s not clear why there is such an effect. It may be that the manufacturer reduced the capacity of Cs capacitors (see the picture below) to reduce response time. These capacitors are connected in parallel to each cell and maintain its state until the matrix is refreshed again. If their capacity is low, the cell gets partially discharged and changes its brightness before the refresh (the picture with the sub-pixel structure is taken from the Samsung Electronics website).


Sub-pixel structure for LCD displays

Theoretically, such effects can be the result of the LCD display working on alternating current (and this is a must for normal work of any LCD display, starting from wristwatch and calculator displays). The polarity can be changed in three ways: for the whole screen at once, for a line of pixels or for a dotted line (checkerboard pattern). The latter variant is the most complex one as concerns its electronic implementation, but the other two make the whole screen or separate 1-pixel-wide lines flicker (the scheme of polarity change algorithms is taken from the Samsung Electronics website).

When working with this display, I couldn’t set it up for the eyes to feel good, although this was hardly because of the flicker: its amplitude was too low to be perceived by the eye.

Pixel rise time

Pixel fall time

 
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