JVC DLA-RS640 Custom 4K HDR Gamma Curves
JVC DLA-RS640 D-ILA Projector
Introduction
HDR or High Dynamic Range needs lots of luminance to do it full justice, no matter how black your blacks are. So while many modern flat panel displays can provide a peak brightness that can really provide a stunningly bright image to support those bright white specular highlights. No cost effective home theater projector can match that performance. Remember, a projector has to illuminate an entire screen from a single point light source. Flat panel displays are composed on either thousands of individual light sources or banks of rear panel lights. These flat screens are also much smaller than the average projection screen that will often have a diagonal of more than 120″. So what options are left to the home theater enthusiast to create a really impressive HDR image? Or is HDR all a futile attempt, leaving us all in the “dark”?
Practicalities
The current HDR specification maxes out at a brightness of 10,000Nits, or 2919 fL (1fL =3.426nits) . This however is NOT the images average peak level (APL). If it was you would need to wear sunglasses and sunblock 1000! It relates to the brightness available to illuminate relatively small areas of your screen like stars, lights, torches etc, not the whole image. In short, for specular parts of the image. These brightness levels are akin to what we see in real life, but do we want, or can we afford, real life in our theaters? Well clearly flat panels are starting to provide high brightness values of well over a thousand lumens but getting these levels from projectors will, for the immediate future, be challenging.
Genarlly flat panel displays are used in environments that have some degree of background lighting and really require to be very bright in order to create the required high dynamic range, as the rooms backround light is well above the ideal perfect black. Unfortunately even if you have a projector room that is totally black, with no refelections at all, while your dynamic range maybe high easily hitting 100,000:1, without a high peak brightness, images will not “pop” of the screen, (there are other eysight issues that have to be considered to). The question here is just how much brightness is required to give the projector lover the HDR expereience without having the capacity to generate huge values of lumens. As of todays date I have seen no reseaerch or numbers that provide any concrete answer to this question. Clearly a projector, or even flat panel display, that can create these huge brightness levels may not be cost effective or even worth while.
So for projectors it is certainly going to be a compromise for the immediate future. That compromise is how to map the brighness of an HDR movie to a lamp/laser source that cannot output the peak levels that the movie was produced to match. Fortunately most movies are not currently produced to levels much above about 1200nits with a few going as high as 4000nits. Even with this difference it challenges projectors to be able to show both peak brightness levels without having the specular highlites either too dim or compressed/washed out.
Note: Flat panel displays can achieve nits values of 2000 or higher but only over very small areas of the screen at a given instant. This is achieved by local control of the backlighting system. Attempts to illuminate the entire screen at maximum brightness can drop this level down to as low as several hundred nits.
The ST. 2084 OETF/EOTF Picture Quantization (PQ) Curve
The gamma curve used for SDR was not created by any scientific research it just happened that it was the natural response of the cahode ray tube (CRT). So in order to get a linear light output that followed the response of the CRT, cameras simply created the CRT displays inverse response to light. This curve was “standardized” with a power value of between 2.2 and 2.5. However, production can change this value in order to obtain a particular “look” to a scene. An overall linear response was quite acceptable for SDR (SD and HD) as the peak display light levels (typically 30fL or 100nits) were low and the system was analog so there were no discrete digital steps in brightness caused by using digital codes. Even 8 bit digital SDR displays and projectors still had 256 equal steps for the brightness, each step barely being observable to the naked eye.
Typical Camera and CRT Gamma Curves
HDR is designed to handle up to a hundred times the old brightness levels of SDR and support black levels as low as 0.0005 fL, a dynamic range of 100’s of thousands to one. In the new world of digital signal processing, converting this dynamic range in a linear process like SDR would result in a huge number of levels of digital data rarely being used. Why? Firstly, cost effective digital video processing is currently limited to 12 bits or 4096 discrete levels. Secondly your eyes are very sensitive to small changes in light intensity at low brightness levels and quite insensitive to small changes in light intensity at high brightness levels. Assigning the same luminace change to every digital word in a linear fashion would be very wastefull. At high levels it would take several digital words to see any change and at low levels a single word change would cause a sudden jump in percieved brightnes level. This reserach of the eyes response resulted in the devlopement of the Perceptual Quantization (PQ) curve. It was developed by Dolby and standardized in SMPTE ST.2084.
Image Credit: Cannon. Comparison of Gamma Curves
ST2084 Perceptual Quantizer Curve
The OETF curve on the top shows how the luminance is converted into an electrical signal by a camera. Low brightness level changes causing a large signal voltage change. High brightness level changes causing only small signal voltage change. The inverse process is created in your display system. This is the OETF/EOTF process. The actual decode process is shown in the bottom graph. It is designed to allocate digital bits as efficiently as possible with respect to how the human vision perceives changes in light. It is important to note that for a 1000 nit peak, 72% of all the digital signal codes are used up and at 4000 nits 85% of all available digital codes have been used. Due to the sensitivity of the eye at lower light levels most digital codes are assigned to the luminance values up to 1000 nit, leaving only 28% of the levels for the remaining 9000 nits.
A Typical Screen and Projector
If we take my 115″ diagonal 2.35:1 unity gain screen it has an approximate screen area of 33 sqft. My personnal preference for SDR material is a peak brightness of about 30fL (100 nits). That means with a unity gain screen I need a projector with calibrated minimum output of (33sqftx30fL) 990 lumens (3392 nits). So far so good, my JVC DLA-RS640 can provide almost twice that level of brightness at a native contrast level of about 35,000:1. Clearly, even at double this value of 200nits, the required luminance specified by the PQ curve is not remotely attainable, even to reach 1000nits screen luminance, just one tenth of the peak desired brightness. In order to hit 1000 nits my projectors single point light source would need to provide 9,990 lumens AND still give me the required black levels with a contrast ratio of at least 35,000:1. That kind of projector bulb/LED power and contrast ratio is not technically doable in a home environment. Even if it was, you are creating all this light and power consumption just for the occassional use, and in many images virtually no use at all. Its benefit to the average peak level (APL) of brightness is very small, so the average image brightness barely increases when compared to SDR. Remember that most of this brightness (luminance) is only for the specular highlights.
Note: This example is for a relatively small screen. Some HT enthusiasts have screens that are almost twice the area of this example and the light output of current projectors is often reduced when producing 4K HDR WCG.
So is all lost? Do projectors need all this luminance to provide a satisfactory HDR experience? Not really. With modern day projector native contrast ratios of up to 200,000: 1 and luminance levels now hitting 3500, fairly high dynamic range with exceptional blacks and very bright highlights can now be achieved. Is it true HDR? No, BUT I am damn sure that I don’t want a projector that can create 10,000 nits on a large screen in a totally black room. Projection rooms would become the order of the day, together with sun glasses and sun screen!
HDR Luminance Mapping
If a projector or display cannot create these high values of brightness, how is it to handle an HDR signal that is telling it to create them? It is done by a process of mapping. The incoming signal levels are mapped to those that the projector can actually handle, trying not to impact the image brightness too much. It is essentially a look up table (LUT) that takes the incoming brightness value and converts it to that level assigned to it from the LUT. This mapping is generally not a linear process for a projector or even a flat panel display. Projector and display manufacturers optimize this mapping process in order to obtain a display characteristic that matches the capabilities of their particular projector or display. This process of mapping is true for BOTH the luminance of an HDR signal and its color gamut. Wide color gamut (WCG) HDR signals still exceed the capabilities of all HT projectors and flat panel displays that generally struggle to reach DCI P3, therefore requiring gamut mapping also. This is a whole other topic and will not be covered here. – Professional mastering displays can handle color gamuts that exceed DCI P3 and flat panel displays can provide partial screen area luminance values up to 10,000nits. (Sony X1 Ultimate Full-Spec HDR 8K Display CES 2018)
Most 4K HDR projectors provide some degree of control over the EOTF/PQ/gamma curve used for 4K to map the HDR luminance to the luminance capabilities of the projector. 4K players also provide the ability to map HDR to SDR if required. However, this process is not always optimal as I discovered when I tried to use it to convert HDR to SDR for my old SDR BenQ W10000 DLP projector. However, it worked very well when driving my RS640 projector. Almost as good as the projectors own HDR mapping!!
Theoretically your projector should be creating the inverse of the OETF curve that would result in a perfect straight line from black to 10,000nits….only in your dreams! Projectors are a long way from perfect and have very limited brightness so the projectors EOTF curve has to be modified to provide an image that looks “right’.
A Typical EOTF Calibration Curve
The above calibration curve shows that the display device follows the inverse of the PQ curve up to some defined luminance value, after which its output is maxed out. This flat line represents brightness levels that are all the same or clipped. Resulting in loss of detail and severe brightness compression. The inverse can also apply to the very dark areas of the screen.
The RS640 comes with its own ST2084 PQ curve with several stated assumptions and is far from optimal, generally being far too dark. Fortunately, the gamma menu provides three controls that allow for this mapping to be adjusted to match your screen brightness, environment and personnel preference:
- Picture Tone – Area C
- Dark Level – Area B
- Bright Level – Area A
Image Source: JVC Operation Manual
By adjusting these three controls in conjunction with the standard brightness and contrast controls it is possible to create a 4K gamma curve that will map the HDR range to that available from the projector/screen. These adjustments are rather rudimentary and even using the JVC projectors external gamma adjustment software (only 12 points) is not optimal to customize this curve. In order to make these adjustments correctly you will need both a suitable colorimeter and associated software. Doing this process by eye is not the ideal option. In my case I ended up with the following settings that provided quite an acceptable 4K image:
- Brightness 8 to 15
- Contrast -3
- Picture Tone +3
- Dark Level -7
- Bright Level +5
- Lamp Lo (or Hi)
- HDR/4K
- ST2084
- Iris Auto 1 (manual -6)
- MPC Enhancement +2
- All other settings at zero.
While these setting produced a nice bright image with well saturated colors, and minimal highlight compression for 1200Nits, blacks were still not ideal for HDR, even if I used the iris in either manual and/or auto.
4K Gamma Calibration
The following graph shows two typical mapping curves, The yellow line follows the PQ curve up to one maximum brightness level the other orange curve following the lower PQ curve but deviating from it up to a lower maximum peak output level. It is these curves that will define what an image looks like, on the assumption that the display devices have similar color gamuts, calibration and performance.
Practical Typical Calibration PQ Curves
The yellow line below represents the result of the summation of the original PQ curve and its inverse in the projector to decode the HDR signal. In an ideal world the encode and decode processes would be equal and opposite resulting a perfect straight line as shown below. Unfortunately the world is not perfect nor are our eyes or projectors, and this straight line decode process is only applicable up to the maximum output of the projector. This restriction means that the projectors output has to become non linear and limited at some level in order to display the entire HDR signal by compressing the bright highlights as shown by the flat portion of the resulting curve. It may also be necessary to modify the straight part of the graph in order to better suite the characteristics of both the projector and the viewers preferences.
That is what I like about standards, we all have one!
ST2084 PQ Calibration Curve – Yellow Line. Calibrated RS640 – Grey Line
Fortunately the JVC projector supports a custom application, Arves Tool, that allows the user to create sophisticated customized gamma curves having up to 256 data points, and then load these curves into the projector and edit them in real time to see the effects of your changes. This custom curve is used to modify the projectors own gamma curve. It DOES NOT replace it.
A Typical Remapped HDR Gamma Curve for 4000 NITS Using Arves Tool. Screen brightness on the vertical axis (0-100nits) and HDR disc brightness on the horizontal axis (0-10,000 nits).
So for now we all need to map the peak HDR brightness to the peak brightness that your projector can create. If your peak screen brightness is 100 nits (30fL) then that value has to roughly equate to either 1000 nits, 1200 nits or 4000 nits depending upon what peak value you have selected to work to. Mapping to 10,000 nits is not currently necessary as no movies are produced to that peak value and would waste available luminance. The shape of this curve is what will determine how you perceive HDR. In particular the highlights that have to be dealt with as your projector runs out of lumens to reproduce them. The resulting curve is a compromise between getting a great black levels and detail, a good mid tone and not compressing the highlights too much, quite a juggling act!
Fortunately there are a group of enthusiasts on the AVS Forum who have created a number of gamma curves with this tool, and the JVC calibration software, that will set the user onto the correct path. Ideally you first need to know your screens peak brightness and calibrate the projectors gamma (I didn’t initially). From there on in, you can tailor the curve to meet your projectors output, together with setting the optimal black levels.
So go get Arves Tool, break out your Windows laptop, and load a few of Javs curves to start you off. You should also buy a copy of DVS UHD|HDR-10 Video Calibration Disc or the file download, its worth the money, will help you set/confirm your black levels and provides an excellent array of 4K HDR test signals and workflows.
Loading the curves is very simple, particularly if both your projector and PC are connected to a hub or switch and you set the projector to DHCP; noting the resulting projectors IP address. You will need it for Arves tool or the JVC software. To be honest, the setup and connection time is significantly longer than the few seconds it takes to load the curve data.
Javs 1200nitv3 and 4000nitv3 curves that I loaded looked very good and needed no adjustments for either black or white levels. To be honest I was no too sure whether I preferred my ST2084 gamma settings for HDR or Jav’s curves. My ST2084 curve is a little brighter than Javs but his black level is better than I can create with the JVC controls. Regardless, there is a large range of gamma curves available for both Arves Tool and the JVC software (Javs and Dominic Chan) that will allow the newcomer a good place to start. You should be pleasantly surprised by the resulting improvements in your 4K HDR images over what you can achieve using the projectors ST2084 curve and associated projector controls.
NOTE: For HDR you should not set black level (if you have to) using the brightness control, you need to use bbo after loading a curve. Why? Black brightness out (bbo) sets your black levels above level 64, without affecting the black floor level of 64. The projectors brightness control will raise this floor level (64), impacting the projectors ultimate black level performance.
Don’t forget that you should also have your projector calibrated in order to set the gamma, determine your screens peak brightness and set the correct color balance for both SDR REC 709 and HDR REC2020 (DCIP3).
See here for the AVS Forum Group supporting this and other JVC D-ILA projectors.
See here for the JVC D-ILA range of software for different projectors.
See my JVC DLA-RS640 review here.
See my introduction to selecting a screen here.
See the review of my Da-Lite Electrol dual masking screen here.
See here for a great introduction to HDR, Dolby Vision and HDR10.
