Response Time Specs
There are currently two methods which
manufacturers use for measuring and reporting response time for their displays.
This is dictated really by whether the screen is using
Compensation (RTC), otherwise known as 'overdrive' to improve their pixel
For displays where overdrive is not used,
manufacturers will generally quote the more traditional ISO 13406-2 response
time which will represent a measurement for the pixels changing from black >
white > black (0-255-0). This ISO response time is made up of the rise and fall
time as the pixel changes from one state to the other and then back again. You
will see specs quoted where the response time is listed on its own, without a
"G2G" (grey to grey) after it. Typically there are limits for each panel
technology of how far these ISO response times can be pushed. For a TN Film
panel, the fastest ISO response time is usually quoted as 5ms. For VA matrices
it is ~12ms and for IPS is is ~16ms. On these displays without overdrive the ISO
0-255-0 change is the fastest since the highest voltage is applied to
re-orientate the pixels. Manufacturers quote this spec but you need to keep in
mind that the other transitions (between different grey shades for example) will
be higher and are not specified at all. In fact in normal use it would be quite
rare to see a full black > white transition, and changes between different grey
scales (i.e. between different colours) are far more common. Unfortunately the
spec provided will not tell you how fast these are at all. Keep in mind therefore that
the more important grey to grey transitions may be much slower than the quoted
ISO response time, and so overall performance in practice may vary
significantly. This is particularly noticeable when looking at VA or IPS panels
without overdrive, where G2G transitions can be very slow. This ISO response time is only really quoted for screens without
overdrive being used, and in today's market that is fairly rare really.
For displays which do use overdrive, manufacturers
go a different route with their specs. Where overdrive is used it will result in
a marked improvement in actual perceived responsiveness and overall the more
important grey to grey transitions will be greatly sped up. Since the ISO
response time is no longer the fastest (since higher voltages are now being
applied to other transitions), manufacturers instead quote their best grey to
grey (G2G) response time in their spec. While this is still somewhat misleading
since its only the best case, it does give you a better view of how the screen
might perform in practice as we at least know overdrive is being used in screens
where G2G figures are quoted, which should result in an overall faster matrix.
For instance a screen with a quoted 4ms G2G response time should perform much
better in practice than a 5ms ISO response time screen (without overdrive),
despite the spec only suggesting a 1ms difference. The spec being offered is
still not ideal but does give you a better indication of the response times to
Our New Testing Methodology
To help give a more honest view of how a panel
performs in real life we have developed a new testing method which will
feature in our forthcoming reviews. This method has been / is used by other
review sites and provides a better understanding of the real-World pixel
response times. Our method is very similar to the existing oscilloscope +
photosensor methods being used already so hopefully readers will be somewhat
familiar with the principles.
To perform the measurement a custom-made
photosensor device is attached to the screen and used to track changes in the
brightness levels. A special software program is used to simulate the changes
between different shades from the full range of 0 (black) to 255 (white). From there, the
photosensor measures the brightness change and converts it into a voltage
which is passed to an Oscilloscope.
We use an
ETC M526 Digital Storage Oscilloscope supplied to us by
ETC Ltd, a
Slovakian company who specialise in measurement equipment such as this with a
very good price to performance ratio.
The Oscilloscope and accompanying software produces oscillograms such as
that shown above which allow us to observe and measure the response time of
the pixels, and determine how quickly the pixel changes from one state to
another. Depending on the scale used on the oscilloscope, the grid lines can
be used to calculate the response time.
The graph is interpreted as shown above. The
lower flat line is the starting shade and the top line is the final shade
being tested. The vertical parts of the green graph signify the response time
as the pixel changes from one state to another. The upwards curve is the rise
time (i.e. the change from the darker shade to the lighter shade), and the
downwards line is the fall time (i.e. the change from lighter to darker
shade). The speed of these rise and fall times is part of what we will want to
measure and will vary depending on the screen of course, and on the requested
transition between different shades of grey. The above graph shows a fairly
neat response time oscillogram with a pretty straightforward curve.
What might be more typical though is an
oscillogram such as that shown above. You may note that the rise time actually
shoots a little above the required level before dropping back down. The fall
time does the same thing, dropping a little too far before it levels out at
the desired shade. This is a classic case of overshoot caused by a RTC /
overdrive impulse. This may vary significantly from one screen to another as
well, depending on the level of overdrive impulse being used, how aggressively
it is applied and how well the internal electronics are controlling the
The oscilloscope software allows us to define
the lower and upper levels as shown above using the horizontal red and blue
lines. These represent the two shades we are switching between.
You can then use the vertical grid lines within
the software to mark the interception points where the response time curve
crosses the horizontal lines. The software tells you the distance horizontally
between these two vertical lines, which is the response time. This will be
dependent on the scale of the graph you are working with at the time.
The scale of the graph can be easily changed to
allow for more accurate adjustment of those intersection points and allows you
to take the response time measurements at a higher level of accuracy. The same process can be
followed for the fall time as well of course.
For our tests we will take a 10% allowance at
either end of the scale which is the same process used by all panel
manufacturers, and also incorporated at other sites using similar measurement
techniques. So we will measure start point when the brightness has changed
more than 10% and measure the end point when it reaches 90% of it's required
brightness. Thankfully the oscilloscope software allows
us to accurately mark these 10 and 90% positions, and we can then simply
measure the response time from there.
Using the horizontal red and blue lines we can
also work out the vertical frequency of the oscillogram, and this allows us to
measure the overshoot as a percentage of the overall change in terms of how
far over the desired tone the pixel reaches. We can of course also measure the
time in which the pixel is in this unwanted state in milliseconds. In that way we
will also be able to articulate the level of overshoot for any given pixel
transition and determine whether it is of a level which could prove
distracting or troublesome to the user. For our tests we measure the value of
the overshoot both as a percentage of how far it has gone beyond the desired
shade, and how long it is in the unwanted state. For example, if the pixel shade should have
changed from 0 to 100, but was actually increased to 150 (a very extreme
example) due to the aggressive overdrive impulse, then returned to 100, the
value of the miss is 50%. We will also calculate the average value of the
overshoot errors for the range of transitions.
Measurements are taken for 20 different
transitions across the entire range from 0 - 255. They are plotted into a table
as shown below. The starting point is shown in the rows and the columns signify
the end point of the transition. Those measurements in the upper right portion
signify rise times, i.e. changes from a darker to a lighter shade. Those in the
bottom left portion are fall times, changes from a lighter to a darker shade.
We have then colour coded the measurements based
on the scale shown to give you an easy visual representation of the pixel
response times and whether they are good or bad. A response time of less than 5
m G2G can be considered very fast today. A response time of around 10 ms G2G is
pretty fast, but a response of over 15 ms G2G is rather slow.
From here we can also easily calculate the
fastest, slowest and average grey to grey response time across all transitions,
as well as the average rise and average fall times. For reference we also identify the traditional ISO (0-255-0) response time.
The average G2G response time is perhaps the most important measure and can be
used to help compare performance between different monitors. Moving forward we
will provide graphical comparisons of the monitors we test using the average G2G
response time measurement as a reference.
We then provide a 3D histogram plotting these G2G
response times across the range. This is animated in order to display the values
We also provide a table showing the RTC
(overdrive) overshoot as a percentage for each transition measured. This is
based on how far past the desired state it shoots. This can have a profound
impact on the image and if the overshoot is high, it can lead to unwanted pale
and dark trails in moving images. This is often easy to see in practice as well,
and is something we regularly see in heavily overdrive displays. If the
below 5%, you are unlikely to notice any RTC artefacts in practice. If it is
within 5-10%, there are artefacts, but nothing too severe or noticeable. If they
are above 10%, the artefacts are likely to be visible to the eye. The table is
colour coordinated using the scale shown as well for easy reference.
RTC Overshoot %
We again provide the results on an animated 3D
histogram for a useful visual representation of the RTC overshoot % for each
We hope that these new testing methods and results
are useful to our readers and we will begin to incorporate them into our future
reviews. We will continue to measure and compare the perceived response times as
well in our current way using PixPerAn for completeness as well, and so as not
to completely abandon that method. Response times and the impact of overshoot
are best interpreted based on a combination of these oscilloscope tests, and the
visual real-life motion experience.