Home > News & Trends > Industry News

Industry News

LED video screen refresh rate

The cost of LED video screen high refresh

Irrespective of the PWM generation method they all have common features. PWM generation operates on a certain clock rate Fpwm. Let us assume that we have to generate a certain number N of brightness levels. In that case refresh rate Fr cannot exceed Fpwm/N.

Here are some examples to illustrate the above statement:

PWM clock rate Brightness levels Refresh rate
Fpwm=10 MHz N=256 (8 bit per channel) Fr=39 kHz
Fpwm=10 MHz N=1024 (10 bit per channel) Fr=9.8 kHz
Fpwm=10 MHz N=2048 (11 bit per channel) Fr=4.9 kHz
Fpwm=10 MHz N=65536 (16 bit per channel) Fr=152 Hz
Fpwm=20 MHz N=65536 (16 bit per channel) Fr=305 Hz

These numbers demonstrate that each LED of video screen follows some independent PWM generation process, i.e. PWM generation method is programmed directly into IC drivers.

With simple and cheap IC drivers, PWM is generated on a controller for the LED display screen. We should then consider how many drivers are linked consecutively and are serviced by one PWM generation process. If one PWM generation scheme requires M 16-output channel drivers, the refresh rate may not exceedFpwm/(N*M*16, otherwise it leads to significantly lower refresh rate or the need to increase clock frequency.

In case of time division (interlace scanning) the refresh rate falls in proportion to division coefficient.

Thus, to increase refresh rate on LED video screens the following options are available:

  • Use of “intelligent” (expensive) drivers;
  • Increase of clock rate in the PWM generation process;
  • Reducing the number of brightness levels (color depth).

Each method has advantages and shortcomings. The intellectual drivers are much more expensive than simple IC drivers; the rise in clock rate leads to higher power consumption (consequently requires additional measures for heat transfer to avoid overheating); low number of brightness levels negatively affects the image quality.

Conclusion: Refresh on LED video screens

LED video screen manufacturers frequently use refresh rate as a marketing tool when boasting excellent screen quality. The presupposition is that the higher the refresh rate the better is image quality. However, often the numbers serve only to confuse potential customers. For instance, refresh rate of several kHz means that either the modified PWM generation method is used (when refresh rate is in fact different for different brightness levels) or that the color depth is unacceptably low.

We should remember that high refresh rate and high color depth values may only occur at high brightness levels which in itself are a misconception, since a LED video screen should not always operate at 100% capacity.

For the case of interlaced scanning the refresh rate value will only correspond to one PWM cycle for one LED group, while the actual refresh rate for the screen (which affects our perception) will be several times lower.

It is more informative and honest to mention color depth and clock rate for PWM and approximate range of refresh rate for the screen (for example, 200 -1000 Hz) in case of modified PWM screen function. If a LED video screen is based on the time division principle (for example, time division = 1:1 – absence of time division, time division = 1:2 – PWM only operates on half of the screen etc.).

The above parameter is not essential for our perception. Human eye does not register any difference in image quality at frequencies above 100 Hz. Consequently, one should decide if high refresh rate is really necessary and if it is worth while paying extra for it.

Refresh rate and uniformity of recorded screen image are only important in cases where a LED screen frequently becomes an object for video recording (stadiums and concert halls). Therefore, it is better to first conduct some trial recording prior to signing the purchase contract.

 
Previous page: LED video screen and brightness control Next page: Flexible LED screens are hitting the street