strobing monitor/viewfinder?

[J. P. Gilliver]

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?

(P. S.: my spell checker doesn't know "backlight", and suggests
"jacklight" - a word I've never heard of! Anyone heard of it?)

[NY]

Post by J. P. Gilliver
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?

That strobing is really obnoxious. Reminds me of 1970s news reports from
NTSC-land where no 30-to-25 Hz standards converter was available at
short notice so they pointed a 25 Hz camera (either video or 16 mm film)
at a TV showing original 30 Hz US footage. Usually preceded by Reginald
Bosanquet or Sandy Gall apologising for "the poor picture quality in the
next report".

If I view my computer or laptop monitor (60 Hz or 120 Hz refresh) with a
PAL video camera (CCD rather than tube sensor) I can see a faint
rippling. Likewise for a UK 50 Hz TV viewed through a GoPro that can
only work at 30 Hz. But nothing like as bad as the camera that recorded
the Will Vernon footage.

Do camera viewfinders/monitors tend to display each frame for almost the
full 1/25 or 1/30 second, or is there a significant period of unlit
screen between each frame? I've noticed that some car brake lights
flicker very badly when seen by a dashcam, and I wonder whether that is
more noticeable if the LEDs in the brake lights have a short mark:space
ratio (to achieve the required brightness) as well as the pulsing
frequency being mismatched with the camera frame rate.

Post by J. P. Gilliver
(P. S.: my spell checker doesn't know "backlight", and suggests
"jacklight" - a word I've never heard of! Anyone heard of it?)

The only words following "jack" that spring to mind are hammer, rabbit,
off and shit ;-) I've never heard of jacklight.

[Liz Tuddenham]

.. I've noticed that some car brake lights
flicker very badly when seen by a dashcam,...

The flicker frequency of some car lights is stupidly low and is very
distracting in peripheral vision. I suspect this is to draw the
attention of other motorists: "Look! I've got these new fantastic LED
lights on my car!". Fine when they were the latest thing, but very
irritating now that everyone has them.

[MB]

Post by NY
The only words following "jack" that spring to mind are hammer, rabbit,
off and shit 😉 I've never heard of jacklight.

Not a new word


OED

1841 Ladies' Cabinet Sept. 160 A stranger made his appearance
through the murky shade, and paddling his old shattered boat alongside
of Jaac's skiff, presented in the glare of the jack-light an object of
fear and admiration.

[J. P. Gilliver]

Post by NY

Post by J. P. Gilliver
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?

That strobing is really obnoxious. Reminds me of 1970s news reports
from NTSC-land where no 30-to-25 Hz standards converter was available
at short notice so they pointed a 25 Hz camera (either video or 16 mm
film) at a TV showing original 30 Hz US footage. Usually preceded by
Reginald Bosanquet or Sandy Gall apologising for "the poor picture
quality in the next report".

(That's what I meant by "the relays of NASA headquarters" - room full of
computer monitors, strobing horribly. I used to think - before I was old
enough to realise what was happening - "how do they stand using
those?".)

Post by NY
If I view my computer or laptop monitor (60 Hz or 120 Hz refresh) with
a PAL video camera (CCD rather than tube sensor) I can see a faint
rippling. Likewise for a UK 50 Hz TV viewed through a GoPro that can
only work at 30 Hz. But nothing like as bad as the camera that recorded
the Will Vernon footage.

Yes, because these days the light source in your monitor, or TV, is
_not_ the scanning electron beam, but a steady backlight - the slight
rippling is from the interaction with the update, see below. If you were
to point your GoPro or even video camera at a CRT display, strobing
would be back.

The "Will Vernon footage" was looking over the shoulder of someone
filming the wonderful Steve Rosenberg, playing things to do with the
2023 Eurovision, so would have been in the last few weeks, which is what
prompted my post: the equipment would presumably all have been modern,
including the monitor that was strobing (it didn't _look_ to be a CRT
one - too thin, though quite bulky so not _ultra_-modern); even if the
camera _taking_ the "footage" was an old tube one, I wouldn't have
expected such strobing. I presume it wasn't visible live, otherwise the
camera operator - the guy with headphones - would surely have complained
(especially as it was more or less in his peripheral vision, sort of
above him). So I was puzzled why the strobing we see is so bad. Any
thoughts?

Post by NY
Do camera viewfinders/monitors tend to display each frame for almost
the full 1/25 or 1/30 second, or is there a significant period of unlit
screen between each frame? I've noticed that some car brake lights

I can't see any reason to turn the backlight off at all, and as it would
involve extra circuitry (and involving power semiconductors), I doubt it
is done at all. I don't think, with modern displays, "between each
frame" has a meaning, for the whole image: I think the pixels are
refreshed at frame (or field) rate, but sequentially, so there isn't a
time when the whole image doesn't have some part changing, so there
isn't a between-frame gap. A lot of modern resolution/frame-rate
standards don't even have the blanking period - line or field.

(If you've got an old camera with a CRT viewfinder, that's different.)

Post by NY
flicker very badly when seen by a dashcam, and I wonder whether that is
more noticeable if the LEDs in the brake lights have a short mark:space
ratio (to achieve the required brightness) as well as the pulsing
frequency being mismatched with the camera frame rate.

Certainly the latter. I think in quite a lot of modern cars, they don't
_have_ separate tail, brake, and rear fog lights, but just change the
mark:space ratio to provide the three functions.

Post by NY

Post by J. P. Gilliver
(P. S.: my spell checker doesn't know "backlight", and suggests
"jacklight" - a word I've never heard of! Anyone heard of it?)

The only words following "jack" that spring to mind are hammer, rabbit,
off and shit ;-) I've never heard of jacklight.

Thanks to the person who found an OED example! I'm often
puzzled/surprised by spellcheckers. I suppose it's _possible_ that at
some point I mistyped it with the J, and then when it queried it I hit
"add to dictionary". (Most spellcheckers make doing that by mistake only
too easy - and then it's tedious to correct - you have to find where the
dictionary _is_ for a start, then figure out how to edit it.) The most
surprising one I came across was when I used the word blazon - which
means to describe in heraldic terms (e. g. a coat of arms) - and the
'checker _did_ know the word.

[NY]

Post by J. P. Gilliver

Post by NY
That strobing is really obnoxious. Reminds me of 1970s news reports
from NTSC-land where no 30-to-25 Hz standards converter was available
at short notice so they pointed a 25 Hz camera (either video or 16 mm
film) at a TV showing original 30 Hz US footage. Usually preceded by
Reginald Bosanquet or Sandy Gall apologising for "the poor picture
quality in the next report".

(That's what I meant by "the relays of NASA headquarters" - room full of
computer monitors, strobing horribly. I used to think - before I was old
enough to realise what was happening - "how do they stand using those?".)

Post by NY
If I view my computer or laptop monitor (60 Hz or 120 Hz refresh) with
a PAL video camera (CCD rather than tube sensor) I can see a faint
rippling. Likewise for a UK 50 Hz TV viewed through a GoPro that can
only work at 30 Hz. But nothing like as bad as the camera that
recorded the Will Vernon footage.

Yes, because these days the light source in your monitor, or TV, is
_not_ the scanning electron beam, but a steady backlight - the slight
rippling is from the interaction with the update, see below. If you were
to point your GoPro or even video camera at a CRT display, strobing
would be back.
The "Will Vernon footage" was looking over the shoulder of someone
filming the wonderful Steve Rosenberg, playing things to do with the
2023 Eurovision, so would have been in the last few weeks, which is what
prompted my post: the equipment would presumably all have been modern,
including the monitor that was strobing (it didn't _look_ to be a CRT
one - too thin, though quite bulky so not _ultra_-modern); even if the
camera _taking_ the "footage" was an old tube one, I wouldn't have
expected such strobing. I presume it wasn't visible live, otherwise the
camera operator - the guy with headphones - would surely have complained
(especially as it was more or less in his peripheral vision, sort of
above him). So I was puzzled why the strobing we see is so bad. Any
thoughts?

Post by NY
Do camera viewfinders/monitors tend to display each frame for almost
the full 1/25 or 1/30 second, or is there a significant period of
unlit screen between each frame? I've noticed that some car brake lights

I can't see any reason to turn the backlight off at all, and as it would
involve extra circuitry (and involving power semiconductors), I doubt it
is done at all. I don't think, with modern displays, "between each
frame" has a meaning, for the whole image: I think the pixels are
refreshed at frame (or field) rate, but sequentially, so there isn't a
time when the whole image doesn't have some part changing, so there
isn't a between-frame gap. A lot of modern resolution/frame-rate
standards don't even have the blanking period - line or field.
(If you've got an old camera with a CRT viewfinder, that's different.)

Post by NY
flicker very badly when seen by a dashcam, and I wonder whether that
is more noticeable if the LEDs in the brake lights have a short
mark:space ratio (to achieve the required brightness) as well as the
pulsing frequency being mismatched with the camera frame rate.

Certainly the latter. I think in quite a lot of modern cars, they don't
_have_ separate tail, brake, and rear fog lights, but just change the
mark:space ratio to provide the three functions.

Post by NY

Post by J. P. Gilliver
(P. S.: my spell checker doesn't know "backlight", and suggests
"jacklight" - a word I've never heard of! Anyone heard of it?)

The only words following "jack" that spring to mind are hammer,
rabbit, off and shit ;-)  I've never heard of jacklight.

Thanks to the person who found an OED example! I'm often
puzzled/surprised by spellcheckers. I suppose it's _possible_ that at
some point I mistyped it with the J, and then when it queried it I hit
"add to dictionary". (Most spellcheckers make doing that by mistake only
too easy - and then it's tedious to correct - you have to find where the
dictionary _is_ for a start, then figure out how to edit it.) The most
surprising one I came across was when I used the word blazon - which
means to describe in heraldic terms (e. g. a coat of arms) - and the
'checker _did_ know the word.

I was intrigued to see a British news report in the aftermath of Trump's
inauguration as president. They were interviewing Members of the Public
for their reaction. It was in a bar in America, with a TV screen seen in
the background. And there was no flicker whatsoever.

Either the British reporter was using a 30 fps camera and this was being
standards-converted to 25 fps for the target British audience, or else
he was using a 25 fps camera but the TV (presumably LCD/LED rather than
CRT) was not causing strobing. Very impressive, either way.


The worst standards-conversion I ever saw was at the time of Princess
Diana's funeral in 1997. I was able to receive the European version of
CNN (a perk of Bracknell's cable TV feed to all houses in the town) and
during an idle moment I channel-hopped from BBC to ITV to CNN. CNN were
taking their feed from a mixture of BBC and ITN but converting it to 30
fps for the US market. This 30 fps programme was then converted back to
25 for the European version of the channel. As soon as there was any
movement, the picture went loopy, with moving objects (eg the hearse)
repeatedly moving three steps forwards and two steps back several times
a second: 3/2-pulldown followed by inverse-3/2-pulldown is not pretty
:-( (OK, it's not quite 3/2, is it? That's for mapping 24 fps to 30
fps; they do something slightly different for 25 <-> 30 fps mapping.)

[Theo]

Post by J. P. Gilliver
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

[J. P. Gilliver]

Post by Theo

Post by J. P. Gilliver
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.