120hz versus 240hz
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From: William Sommerwerck on 1 Mar 2010 07:13 > Lots of tricks have been used over the years to take advantage > of the limitations of the "equipment" and the process. For example, > anything faster than 24 frames a second is not perceived as being > discrete images, but one smooth image. Actually, it's 16 frames a second. However, that rate is not fast enough to prevent flicker -- which is why silent films were sometimes called "flickers". This is one of the reasons the frame rate was increased to 24 with the introduction of sound. > The 50 and 60 fields per second (a field being half an interlaced frame) > were chosen not because they needed to be that fast (48 would have > done), but to eliminate interefence effects from electrical lights. That's new to me. > Color is another issue. The NTSC (and later adopted by the BBC for PAL) > determined that a 4:1 color system was good enough, i.e. color information > only needed to be changed (and recorded) at 1/4 the speed of the light level. NTSC is actually 4.2/1.5, or roughly 2.8 to 1. PAL is closer to 5:1. > That's the aim of LED backlit TV screens (besides less power consumption, > heat, etc). They only are lit when the crystals are "open", so there is no time > where you see partially lit "pixels". I hate to spoil things, Geoff, but liquid crystals are quite capable of taking intermediate positions -- that is, forming a continuous gray scale.
From: Sylvia Else on 1 Mar 2010 07:29 On 1/03/2010 11:13 PM, William Sommerwerck wrote: >> Lots of tricks have been used over the years to take advantage >> of the limitations of the "equipment" and the process. For example, >> anything faster than 24 frames a second is not perceived as being >> discrete images, but one smooth image. > > Actually, it's 16 frames a second. However, that rate is not fast enough to > prevent flicker -- which is why silent films were sometimes called > "flickers". This is one of the reasons the frame rate was increased to 24 > with the introduction of sound. > > >> The 50 and 60 fields per second (a field being half an interlaced frame) >> were chosen not because they needed to be that fast (48 would have >> done), but to eliminate interefence effects from electrical lights. > > That's new to me. Well, the story I heard way back when is that it was to synchronise the picture's vertical frequency with the mains frequency, so that inadequacies in power smoothing produced static distortions in the picture rather than much more noticable rolling distortions. Sylvia.
From: William Sommerwerck on 1 Mar 2010 07:39 >>> The 50 and 60 fields per second (a field being half an interlaced frame) >>> were chosen not because they needed to be that fast (48 would have >>> done), but to eliminate interefence effects from electrical lights. >> That's new to me. > Well, the story I heard way back when is that it was to synchronise > the picture's vertical frequency with the mains frequency, so that > inadequacies in power smoothing produced static distortions in the > picture rather than much more noticable rolling distortions. That's what I heard, too. But that's not "interefence effects from electrical lights".
From: Geoffrey S. Mendelson on 1 Mar 2010 08:29 William Sommerwerck wrote: >> Well, the story I heard way back when is that it was to synchronise >> the picture's vertical frequency with the mains frequency, so that >> inadequacies in power smoothing produced static distortions in the >> picture rather than much more noticable rolling distortions. > > That's what I heard, too. But that's not "interefence effects from > electrical lights". You are assuming that all interference would be on the screen itself and none would be visual. Since flourescent and to some extent incandescent lights blink (what is the persistance of an incadescent light?) at 60 Hz, there is a strobing effect if there are lights on in the room with the TV. While some peope (me) like to watch TV in the dark, many people watch TV's with lights on. Some manufacturers went as far as to include light sensors in their TV sets automaticly adjusting the brightness to compensate for room lighting as it changes. Since some people live in places where only flourescent lights are allowed, they have no choice if there is interference, either turn off the lights entirely, or live with it. I guess that could be a new tourism slogan for this summer, "Visit Israel, and bring home real light bulbs." :-) Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel gsm(a)mendelson.com N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia.
From: William Sommerwerck on 1 Mar 2010 08:43
>> That's what I heard, too. But that's not "interefence effects from >> electrical lights". > You are assuming that all interference would be on the screen > itself and none would be visual. Since flourescent and to some > extent incandescent lights blink (what is the persistance of an > incadescent light?) at 60 Hz, there is a strobing effect if there > are lights on in the room with the TV. Incandescent lights have almost no flicker, due to the thermal inertia of the filament. Fluorescent lighting was not common in living rooms at the time the standards were set. |