Are LED backlight monitors always quiet?

Sake · Post by **Sake** » Sat Jun 06, 2009 3:10 am

I was thinking after looking in the "Quit/Noisy Monitor Survey" topic viewtopic.php?t=50879&start=90

are LED backlight monitors always quiet compared to TTL backlight monitors?

Post by **NeilBlanchard** » Sat Jun 06, 2009 4:15 am

Hi,

Well they are steady state light sources, right? The fluorescent back lights have to flicker at some frequency, but LED's are on all the time. So that cause of (possible) noise is eliminated. But, I can say that there won't be a something in the power supply that will make a little noise.

mond · Post by **mond** » Sat Jun 06, 2009 5:03 am

From what I understand (please correct me), the highly anoying whine or hum that a lot of monitors (Samsung!!!) exhibit when brightness is reduced happens because the adjustment done by lowering the backlight frequency.

Post by **CA_Steve** » Sat Jun 06, 2009 12:27 pm

It's the cold cathode fluorescent backlight drive circuit components that whine. LED backlight doesn't have this PWM/inverter circuitry...so it leads to presume they don't whine.

utc_pyro · Post by **utc_pyro** » Thu Jun 11, 2009 8:39 am

In general, they wont. But that being said, if the power supply inside the monitor has been poorly designed, then there is a chance to still get some noise, but at a much lower pitch.

I've personally not run into any monitors that have any noise come the PSU portion, but Iâ€™ve had many other computer-related electronics from lesser manufacturers exhibit that issue.

lm · Post by lm » Thu Jun 11, 2009 1:25 pm

How is the LED backlight brightness controlled then?

In the regular LCDs, it's the power circuitry that whines, not the backlight itself, if I am not mistaken.

jessekopelman · Post by **jessekopelman** » Thu Jun 11, 2009 2:30 pm

lm wrote:How is the LED backlight brightness controlled then?

In the regular LCDs, it's the power circuitry that whines, not the backlight itself, if I am not mistaken.

There are two potential methods for controlling LED backlight brightness: PWM could be used to reduce the apparent emission of the LEDs or you could take advantage of the fact that the backlight is a large array of LEDs and turn some of them off (CCFL is usually just a handful of light bulbs). I'm thinking that CCFL can really only be dimmed by voltage control, since using PWM to achieve a large degree of dimming would be bad for the bulb (LED would not have this worry).

The circuitry to run a CCFL is kind of weird. You actually need to provide AC to the bulb, not DC, but a very high frequency AC (usually around 20kHz). The thing that does this is called an electronic ballast. In the standard fixture CCFL bulbs you buy for home use, the ballast is built into the bulb. They often whine for a while until they warm up. In the case of CCFL backlights I think the ballast is still the culprit. Feeding it lower voltage may put it in a similar state as the not-yet warmed up ballast inside those standard fixture bulbs. Ballast issues are usually what cause the flickering and humming of fluorescent office fixtures, as well. LED don't need these complicated ballasts and should be free of ballast-related noises.

subsonik · Post by **subsonik** » Sat Jun 13, 2009 12:54 pm

CCFL backlights are always dimmed using PWM circuitry. It's the cheapest and most reliable way to dim a CCFL. They could be dimmed using current control, but this would increase the complexity of the circuit quite a lot.

With a CCFL backlight, it's usually the inverters that are humming. Each inverter has a small coil in the power supply line, which unfortunately happen to hum loudly when PWM'ed.

With a LED backlight, current control is easier to implement for dimming. However, PWM still is the cheapest and most reliable method. Luckily the switching frequency of a LED constant current source is much higher than the switching frequency of a CCFL inverter - meaning way smaller filter coils in the power supply lines. Thus, less coil noise.

jessekopelman · Post by **jessekopelman** » Sun Jun 14, 2009 5:42 pm

subsonik wrote:CCFL backlights are always dimmed using PWM circuitry. It's the cheapest and most reliable way to dim a CCFL. They could be dimmed using current control, but this would increase the complexity of the circuit quite a lot.

With a CCFL backlight, it's usually the inverters that are humming. Each inverter has a small coil in the power supply line, which unfortunately happen to hum loudly when PWM'ed.

With a LED backlight, current control is easier to implement for dimming. However, PWM still is the cheapest and most reliable method. Luckily the switching frequency of a LED constant current source is much higher than the switching frequency of a CCFL inverter - meaning way smaller filter coils in the power supply lines. Thus, less coil noise.

I agree with you that the inverter is the likely culprit. However; you say that both CCFL and LED use PWM, but that is a bit misleading because of what stage the PWM happens in. CCFL is powered by AC while LED is powered by DC. An LED-backlight monitor just has a rectifier to to change your household AC into DC to power all components of the monitor. A CCFL-backlight monitor has a rectifier to change household AC into DC for the LCD and it has an inverter to change that DC into AC for the CCFL-backlight. The PWM happens between the rectifier and the inverter.

Arvo · Post by **Arvo** » Mon Jun 15, 2009 7:42 am

subsonik wrote:With a LED backlight, current control is easier to implement for dimming.

Yes, but unfortunately LED emission spectra depends on current and thereby PWM is used. (Same for CCFL - their current can't be regulated in wide range, they'll change their characteristics or become unstable.)
Even for regulating current (if it would the case) hi-frequency circuit is used anyway.

Actually I don't think that LED backlighted displays are much quieter (although probably somewhat are). Yes, you can avoid CCFL inverter noise, but displays need to have power supply unit, which is of course switching type, which does whine or hum, if produced cheaply. They need to have some internal voltage transformers too, which again produce whine, if produced cheaply.

Most electronic is produced cheaply anyway.