Case Study: TV Monitor
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GOAL
GHB Intellect was tasked with the review and verification of the technical performance of various LCD TVs and display monitors for potential use of certain patented technologies related to black frame insertion (“BFI”), gamma correction, pulse-width modulation, and luminance control of the displays.
BACKGROUND
Liquid-crystalline displays (“LCD”) use light-emitting diodes (“LED”) as a light source. To improve the appearance of motion, a display (such as that of a television set or a monitor) may utilize BFI by inserting “black frames,” i.e., a period with no LED light emission, within each frame time. This technique reduces trailing effects that might occur from fast motion video on the display.
Moreover, LCD display brightness has traditionally been adjusted by using amplitude modulation. More recently, pulse width modulation (“PWM”), in which backlight is rapidly turned on and off at a preset frequency, has been utilized in some displays. Depending on the period of time the backlight is on compared to the period of time it is not, the average brightness to the human eye is adjusted.
TEAM
For this project, GHB Intellect formed a team consisting of the following:
- an expert with substantial experience in TV displays
- a lab engineer with substantial experience in lab experimentations
APPROACH
GHB Intellect developed an initial test plan that was further modified as the capabilities/functionalities of the displays were further discovered.
The display settings were configured so that stable test conditions could be maintained for consistent measurements, as shown in Table 1.
| Parameters |
|---|
| Local Dimming |
| Picture Modes |
| Brightness |
| Backlight |
| Sharpness |
| Contrast |
| Color |
| Tint |
A photometer and oscilloscope were used to measure the visible light energy of the display and the real-time voltage output, respectively. The equipment set-up was used to observe and record various characteristics of the waveforms indicative of the operation of the display.
For various display modes, the display brightness was captured in real time with the photometer. We performed tests with a small white patch on a black screen (Figure 3a) so that PWM was readily detectable. We also utilized a color test image (Figure 3b) to infer nuanced performance metrics. A gray scale was also used.
RESULTS
Our team determined that some TVs/displays did not use BFI in any mode.
The testing also revealed that display brightness was changed by both current modulation and PWM in some displays. This method of changing display brightness was implemented differently for different picture modes.
Pulse amplitude is typically linear with brightness if only PWM is being utilized to change the display brightness. However, our test results revealed that for some displays pulse amplitude was changing as a function of brightness as well, as shown in Figures 4 and 5. Amplitude changing as a function of brightness suggests that LED current modulation was used to a greater extent (at least in some picture modes).
Figures 6 and 7 show that pulse width modulation at two different brightness settings. For some displays and certain picture modes, the pulse-width modulation pattern was more elaborate as shown in Figure 8.
A comprehensive test report supporting the discovered evidence was provided to the client. Claim charts were subsequently generated based on the test report.