Keiang's electronics hobby side

This website deals with the topics of electronics, digital signal processing and embedded programming.

 

 FAQ (Frequently Asked Questions)


Category: Main -> SCIMO (TV Ambient-Light)


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I just received the question if maybe higher LED pitch (60 LEDs per meter) gives better Ambient-Light feeling. So spent some hours and make direct compare between 30 LEDs per meter and 60 LEDs per meter.
For this test i used in both cases same LED stripe (WS2812) and same configuration of SCIMO, which means that defined video zones are identical and used same signal source (PS3, HDMI), distance from TV to wall is round 15cm. The used LED stripe is equiped with 60 LEDs per meter, so i deactivated every second LED (in firmware) for test with 30 LED per meter.
I tried to figure out some visible difference but i have to save that it looks identical to me and higher LED pitch won't give better Ambient-Light feeling or higher visible resolution of LEDs colors. The only difference that was visible for me is that 60 LEDs per meter looks a bit brighter. The reason why it looks like just a bit brighter and not twice (as maybe expected) is caused from human brightness feeling that is not linear even more a exponential function.

Below you will find from both tests (30 LEDs/meter and 60 LEDs/meter) a video, so you can compare on your own.

 

On the other hand doubeling LEDs will give some technical difficulties like:

  • required current consumption (ampere) are doubled -> bigger power supply unit is needed (not easy to get PSU over 8 ampere with protection of hazardous voltage)
  • emitted heat from LEDs are doubled -> some TVs are sensitve to heat which maybe results in "clouding"
  • wiring between SCIMO and LED stripes becomes more difficult because bigger wires are needed which is difficult to handle
  • voltage drop along LED stripe are doubled -> additional power injection to LED stripe maybe needed
  • doubeling power costs

 

As a conclusion i have to say that it is a good idea to avoid high currents (more than 8 ampere) for suppling LEDs by limiting used LEDs to 174 LEDs maximum. This can be done by using LED stripes equipped with 30 LEDs per meter for TV sizes over 40".

 

30 LEDs per meter   60 LEDs per meter 
 

 

 


The main reason for that is more caused by law then on technical restrictions.
If a device uses HDMI it needs to be HDMI certified, especially if its want to process HCDP (copyright protected content) otherwise it becomes in conflict with law. But such a HDMI / HDCP certification for a device is a very very expensive way... Because of that i decided not to use HDMI as direct input source, instead i use S-Video or Composite signal which also can be connected to a HDMI source by using external HDMI splitter and HDMI to S-Video converter.
Another very importend reason is that normally a TV has not an HDMI output. So if for example the internal tuner from TV (DVB-C, DVB-S, DVB-T) is used then video signal from TV can only be taken from SCART connector and not HDMI !

Right here are some connection examples shown: SCIMO Connection Examples


This is because Rasberry-PI based solutions are having some technical restrictions which i want to avoid. Main reason is that Raspberry-PI is using an USB stick video grabber to get videodata. But those USB video grabber are delivering only 25 / 30 FPS (frames per second) and not 50 / 60 FPS like SCIMO is using. Additional USB video grabber mostly supporting Composite videosignal only (RCA plug) and do not support S-Video signal which delivers better video quality and thus gives better light effects. Also it is very difficult to control asynchronous "WS2812B" based LED stripes with Rasberry-PI because of timing problems with results that synchronous LED stripes must be used (additional clock wire) but those LED stripes are more expensive than asynchronous LED stripes "WS2812B".
To get rid of those problems i decided to develop a specialized hardware where i take care of those limitations and was able to add some special hardware feature like: switched power supply for LED stripes , integrated remote control , low power standby mode, automatic power depending on video signal.


 

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