[FFmpeg-user] Record N RTSP frames and average into 1 jpg? In a loop?

David Bernat david.bernat at gmail.com
Sun Jul 9 13:46:38 EEST 2023

On Sat, Jul 8, 2023 at 11:52 PM Carl Zwanzig <cpz at tuunq.com> wrote:

> On 7/8/2023 3:37 PM, Steven Kan wrote:
> > I believe the Wyze cam runs at 20 fps, unadjustable, and I’m guessing the
> > LEDs are flickering at our U.S. AC line frequency of 60 Hz.
> Probably, but that gives three flashes per frame interval.
> > Fortunately they’re very slightly asynchronous, so the black bands
> > “crawl” with a frequency of about 1/32 Hz.
> Which then suggests that one of them is running slow :).
> > But your idea about separating the power is a good one.

Thank you Carl Zwanzig, I had not noticed I had top-commented.
As for EE solutions, all you really need is to pass the AC current through
a full-wave rectifier before the LEDs.
Check the manufacturing specs. Some higher end LEDs have the diodes
necessary for full-wave rectification.
Others do not and need an external one. With it, the current looks like
this (first one) instead of this (second one).
The second one has the LED be dark for half its cycle; which, then, makes
those black on/off bands spatially.
With the first one, you still would get slight dimness bands but nearly
impossible to see without wanting analytic quality photogrammetry.

By the way, it has been a while, but I *think* back of the envelop
calculation allows this:
US AC current: 60 Hz.
Half-Wave Rectification: 30 Hz.
Camera: 20 Hz
Differential: 10 Hz
Beat Frequency: 10Hz

But truth is, feels to me you are happy simply with a bash script. Which is
okay too :)

> If this is in a commercial environment and 3-phase to the building, try
> powering the lights to another phase; that'll change when they flash by
> 1/180th of a second.
> Best solution is to replace the lights with something that doesn't flicker
> at the line rate.
> Please do not top-post on this mailing list, thanks.
> z!
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