Video playback question

[TheOldMan]

I notice that when watching video of dashcams, the led lights from oncoming cars seem to pulse or flicker. I don't understand how this would be as the led's are DC and are provided a steady DC signal, so why the pulse. The camera ought to see a steady state light, regardless of frames per second, given it's DC voltage, not AC. Anyone have an answer for this?

 

[Nigel]

I notice that when watching video of dashcams, the led lights from oncoming cars seem to pulse or flicker.

Can you post an example?

Generally, I don't see headlamps flickering in dashcam videos, so it is a little hard to comment...

I don't understand how this would be as the led's are DC and are provided a steady DC signal, so why the pulse.

I don't know that all headlamp LEDs are DC? A lot of LEDs are pulsed, brake lamp LEDs often used to flicker, modern ones either don't, or do at a sufficiently high frequency that it is not a problem.

The supply voltage may not be very DC, either due to a dying battery, or a dying alternator.

Some cars have HID headlamps, and when they are dying they can flicker quite badly.

 

[TheOldMan]

I kind of answered my own question...led's do in fact "strobe" on and off, basically a method to lessen the amp load. With a given shutter speed of a camera vs the strobing of the leds, it sets up this effect in the video's.
explained here

 

[Nigel]

I kind of answered my own question...led's do in fact "strobe" on and off, basically a method to lessen the amp load. With a given shutter speed of a camera vs the strobing of the leds, it sets up this effect in the video's.
explained here

I would hope that a headlamp is always used fully on (100% duty cycle), otherwise if you have a scene illuminated by multiple cars, you could get some nasty interactions between the headlamps of the different cars. Other lamps that are only used as indicators rather than to illuminate something, like daytime running lights that can be turned down to become sidelights are more likely to be strobed.

I think the only good reason for the "strobing" is if you need, for some reason, to be able to adjust the brightness of an LED lamp, and that shouldn't really be necessary for any car lamps, so I think the problem is easily sortable by the car manufacturers. The original reason for strobing LED brake lights was to get enough brightness without overheating, but I don't think that is a valid reason anymore.

it sets up this effect in the video's.
explained here

BMW drivers would never neglect to use their turn signals, so it must be an issue with the camera?
Couldn't it be an issue with the BMW car?

 

[Nigel]

The answer to the problem is to get a dashcam that uses a Sony Starvis 2 image sensor. As well as having excellent DOL-HDR for reading license plates at night (if implemented by the dashcam manufacturer), you also get flicker mitigation (if implemented by the dashcam manufacturer), which largely solves the problem of flickering LED lamps in most lighting situations.