120hz versus 240hz
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From: Geoffrey S. Mendelson on 26 Feb 2010 08:09 Sylvia Else wrote: > CRT TVs refresh at 50Hz or 60Hz (near enough) depending on region. FYI, Computer CRT screens refresh at 60 to 85 Hz. The main difference between CRT's and LED's or LCD's is persistance. The CRT's have long persistance phosphors, when they are illuminated, they stay lit for a relatively long time. That's why the interlacing system works, the odd lines are still lit when the even ones are illuminated. 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: Sylvia Else on 26 Feb 2010 08:42 On 27/02/2010 12:09 AM, Geoffrey S. Mendelson wrote: > Sylvia Else wrote: > >> CRT TVs refresh at 50Hz or 60Hz (near enough) depending on region. > > FYI, Computer CRT screens refresh at 60 to 85 Hz. > > The main difference between CRT's and LED's or LCD's is persistance. > The CRT's have long persistance phosphors, when they are illuminated, they > stay lit for a relatively long time. That's why the interlacing system > works, the odd lines are still lit when the even ones are illuminated. It's not that long, which is why photographs of television pictures look so awful. Interlacing is used to avoid flicker without having to transmit 50 or 60 full frames per second. LCDs don't flicker anyway, regardless of their framerate. The frame rate issue relates to addressing the judder you get as a result of the image consisting of a sequence of discrete images, rather than one that continously varies. It doesn't help that much TV material that was recorded on film is transmitted with with odd and even interlaced frames that are scans of the same underlying image (or some variation thereon), so that the effective refresh rate considerably lower that the interlaced rate. Sylvia.
From: William Sommerwerck on 26 Feb 2010 09:22 > LCDs don't flicker anyway, regardless of their framerate. The frame > rate issue relates to addressing the judder you get as a result of > the image consisting of a sequence of discrete images, rather than > one that continously varies. Not quite, otherwise the issue would occur with plasma displays. Indeed, it would with any moving-image recording system. The problem is that LCDs don't respond "instantaneously". They take a finite time to go from opaque to the desired transmission level, and then back again. The result is that the image can lag and "smear". (25 years ago, the first pocket LCD color TVs from Casio had terrible smear, which added an oddly "artistic" quality to sports.) For reasons not clear to me, adding interpolated images reduces the smear. This makes absolutely no sense whatever, as the LCD now has /less/ time to switch. I've never gotten an answer on this. > It doesn't help that much TV material that was recorded on film is > transmitted with with odd and even interlaced frames that are scans of > the same underlying image (or some variation thereon), so that the > effective refresh rate considerably lower that the interlaced rate. Interlaced images can be de-interlaced. Note that most product reviews test displays for how well they do this.
From: bob urz on 26 Feb 2010 10:20 > > CRT TVs refresh at 50Hz or 60Hz (near enough) depending on region. > > Since a TV program will only contain images (interlaced) at that rate - > or frequently less - a TV that purports to offer a higher refresh rate > will have to create the extra images by some kind of interpolation. If > it does a bad job, then the result will be unwatchable regardless of how > high the refresh rate is. > > Sylvia. It can get more complicated than that. Dolby has a new thing out HDR LCD that on the fly modulates the LED backlights for brightness in groups. that was not possible with CFL LCD backlights. http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/dolby-hdr-video-technical-overview.pdf bob
From: William Sommerwerck on 26 Feb 2010 10:19
> Dolby has a new thing -- HDR LCD that > modulates the LED backlights on-the-fly. This is neither new, nor was it invented by Dolby. |