Re: Please Explain Refresh Rate and Response Time
- From: Jan B <nospam@xxxxxxxxx>
- Date: Sat, 17 Nov 2007 08:57:49 +0100
On Fri, 16 Nov 2007 18:16:34 -0500, Sam <sammy@xxxxxxxxxxxx> wrote:
On Fri, 16 Nov 2007 17:48:24 GMT, Wes Newell
<w.newell@xxxxxxxxxxxxxxxxxx> wrote:
On Fri, 16 Nov 2007 09:13:54 -0500, Sam wrote:
I notice the first uses units like Hz and the second ns. What do they
measure? TIA.
http://en.wikipedia.org/wiki/Refresh_rate
http://en.wikipedia.org/wiki/Response_time
And the second is in ms (milliseconds) not ns (nanoseconds). Care to guess
where you can find definitions for those?
http://en.wikipedia.org/wiki/Hz
http://en.wikipedia.org/wiki/Nanosecond
http://en.wikipedia.org/wiki/Millisecond
I didn't find your references helpful. For example, the first has the
following:
********************************************************************************
LCD displays
Much of the discussion of refresh rate does not apply to the liquid
crystal portion of an LCD monitor. This is because while a CRT monitor
uses the same mechanism for both illumination and imaging, LCDs employ
a separate backlight to illuminate the image being portrayed by the
LCD's liquid crystal shutters. The shutters themselves do not have a
"refresh rate" as such due to the fact that they always stay at
whatever opacity they were last instructed to continuously, and do not
become more or less transparent until instructed to produce a
different opacity.
The closest thing liquid crystal shutters have to a refresh rate is
their response time, while nearly all LCD backlights (most notably
fluorescent cathodes, which commonly operate at ~200Hz) have a
separate figure known as flicker, which describes how many times a
second the backlight pulses on and off.
******************************************************************************************
I have read reviews saying that the state of the art in refresh rates
for LCDs is 120Hz while for response times, it is 4ms. What is the
connection?
Ok, I'll try to expand a bit on the above.
First, just to be complete:
There are certain models of LCD panel type that also blink (or scan)
the backlight. (Not including the mentioned fluorescent frequency that
in practice is referred to as "constant" backlight.)
If we consider the "constant" backlight type, the picture stays lit
and in the same position between each Frame update.
This means that with a still picture, there is no need to update the
Frame content until the input frame data has changed (i.e. input frame
rate.) A blinking display like CRT is annoying also with a still
picture if the Refresh rate is too low (like 24/25 Hz). Compare with a
computer monitor where most people needs say 85Hz refresh rate on a
CRT to 'stop' the blinking.
With motion, things are different. The Response Time is (has been) a
difficult parameter for LCD panels. This is the reaction time for each
sub-pixel to change the brightness. With motion, this happens every
time the Frame content is updated. Also with slow motion, each pixel
has to change the level. When an object moved, say 5 pixels, a slow
response and in particular if the response from high to low and low to
high is asymmetric, there will be blurring of the edges or even a
trail after the object.
It should be noted that the exposure time in a camera also produce
blur. Compare with photography where you need to use a short exposure
time if you want to "freeze" the motion and get a sharp photo of
motion.
The drawback of showing a series of such sharp photos at a rather slow
change rate (like 24 or 25Hz) like film material, is that these
changes are seen as stutter/judder.
Some producers of film knows this and therefore use longer exposure
time to achieve a smoother motion. There is a trade-off between
sharpness and smoothness.
Back to the "constant" lit LCD panel.
In the case of film/video material with short exposure time or
computer rendered overlays (like scrolling overlaid text) which can be
very sharp, there is anyway a blurring in our eyes caused by showing
the object in the same position on the screen for a fairly long time
before it "jumps" to the next position. The blurring is created when
you try to follow the motion with your eyes (that in real world would
have been smooth). If you don't follow the motion and the distance is
over a certain threshold, you will be able to see several objects (or
edges) at the same time. This is true also if the response time is
very short (0ms). If the picture stays the same for 16/20/40 ms, the
blurring is not dominated by a 4/6/8 ms response time.
This is the reason to calculate intermediate frame content to update
the LCD panel at a higher Frame rate. The objective is to reduce the
time a moving object is shown at the same position. There are two
benefits from this; One is that if you increase the frame frequency to
the double, the blur width reduces (in theory) to the half. The other
is that increased frame frequency (with calculated intermediate
contents) provides a smoother motion.
It shall also be noted that calculation of intermediate frame content
is difficult and different implementations have different kinds of
limitations and artefacts. They have improved over the years but in
general, the more aggressive smoothing the more visible artefacts.
The different implementations are also differently optimised regarding
film or TV (really interlaced) material.
They have different threshold limits in motion speed, that is how many
pixels an object can move between input frames (or fields), before the
motion estimation gives up.
So bottom line is "frame rate upsampling using motion estimation" can,
on an LCD panel, help reproduce 50/60 Hz material with reduced motion
blur (higher clarity) and may reduce the motion stutter on 24/25 Hz
content.
/Jan
.
- References:
- Please Explain Refresh Rate and Response Time
- From: Sam
- Re: Please Explain Refresh Rate and Response Time
- From: Wes Newell
- Re: Please Explain Refresh Rate and Response Time
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