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aus+uk / uk.tech.broadcast / Re: strobing monitor/viewfinder?

Re: strobing monitor/viewfinder?

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From: theom+news@chiark.greenend.org.uk (Theo)
Newsgroups: uk.tech.broadcast
Subject: Re: strobing monitor/viewfinder?
Date: 21 May 2023 12:34:58 +0100 (BST)
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by: Theo - Sun, 21 May 2023 11:34 UTC

J. P. Gilliver <G6JPG@255soft.uk> wrote:
> I thought, now the light source is displays is not the actual raster on
> a CRT but a steady backlight, strobing was a thing of the past - or at
> least heavy strobing. But in
> https://twitter.com/BBCWillVernon/status/1656226547634774016 (once I
> click the play button), I see - I'm not sure if it is a monitor or
> viewfinder - with strobing so strong it reminds me of the relays of NASA
> headquarters in the moonshot days.
>
> Anybody?

LCD backlights are often dimmed using pulse width modulation (PWM). That
means the backlight is flashed too fast for most people to see, with the
duty cycle achieving the dimming.

The exact frequency varies, but might be say 180-240Hz. That can beat awkwardly
with cameras recording at 30-60Hz.

Some OLED displays can also use PWM for dimming individual pixels. Some
LCDs don't use PWM, they use analogue DC dimming, which doesn't flicker.

PWM can also be used for dimmable LED room lighting (eg smart bulbs), which
would cause the whole room to flicker.

Theo

Re: strobing monitor/viewfinder?

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From: G6JPG@255soft.uk (J. P. Gilliver)
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Subject: Re: strobing monitor/viewfinder?
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by: J. P. Gilliver - Sun, 21 May 2023 14:21 UTC

In message <Q+A*4bOgz@news.chiark.greenend.org.uk> at Sun, 21 May 2023
12:34:58, Theo <theom+news@chiark.greenend.org.uk> writes
>J. P. Gilliver <G6JPG@255soft.uk> wrote:
>> I thought, now the light source is displays is not the actual raster on
>> a CRT but a steady backlight, strobing was a thing of the past - or at
>> least heavy strobing. But in
>> https://twitter.com/BBCWillVernon/status/1656226547634774016 (once I
>> click the play button), I see - I'm not sure if it is a monitor or
>> viewfinder - with strobing so strong it reminds me of the relays of NASA
>> headquarters in the moonshot days.
>>
>> Anybody?
>
>LCD backlights are often dimmed using pulse width modulation (PWM). That
>means the backlight is flashed too fast for most people to see, with the
>duty cycle achieving the dimming.
[]
Ah, that probably explains it - that little monitor/viewfinder/whatever
was using PWM brightness control, at a frequency (or factor/multiple)
close enough to the frame rate of the camera we were seeing it through.

I'm still surprised though - though it could be that the person shooting
what we were seeing wasn't doing so in any official capacity.
--
J. P. Gilliver. UMRA: 1960/<1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

Don't play "stupid" with me... I'm better at it.

Subject	Author
Re: strobing monitor/viewfinder?	Theo
Re: strobing monitor/viewfinder?	J. P. Gilliver