Grass Valley Sees Value in 8K+ Capture
At the recent NAB Show, Grass Valley announced a new broadcast camera in its LDX line, the LDX 180. A slight surprise to us was that the camera was described as having a 10K sensor although the rest of the camera was specified as supporting up to 4K output only. Grass Valley highlighted in its press release that the use of a higher resolution sensor enables better 4K to be produced when the captured content has been downscaled.
Oversampling is an Advantage
This advantage in capturing at more than the delivery resolution is a topic that the 8K Association has been talking about for some time. We contacted the firm and interviewed Klaus Weber, Director, Product Marketing, and a seasoned specialist in cameras with decades of experience. Grass Valley is well known for its wide range of high quality products aimed at professional TV content production. We often start our articles with a brief history of companies that we interview, but in this case, the company history is long and complex, involving the original Grass Valley company, Tektronix, Thomson, Technicolor, Snell & Wilcox, Canopus, Belden and Miranda, among others! Wikipedia has a history if you’d like to know more.
Weber explained that the LDX 180 camera was developed in response to a desire from the firm’s live broadcast clients to get a more ‘cinematic’ look than is typically provided by the existing cameras in the LDX line. The other cameras use a three chip capture system based on 2/3” sensors. Each of the sensors captures one of the RGB primary colors. Although some aspects of the ‘cinema look’, such as frame rate and colorimetry are possible to create using the existing cameras, one of the key aspects of cinema cameras is the limited depth of field. Cinematographers use this limited depth of field to separate the foreground and background and to help to direct the viewers’ attention. The shallow depth of field is a function of the larger image sensors used (a major reason that ‘full frame’ cameras are popular with video and photography enthusiasts). It is also influenced by the lens used.
Grass Valley Has its Own Global Shutter Sensors
Grass Valley has been developing sensors of its own for nearly 40 years with CMOS devices arriving in 2006 and its first global shutter sensor developed in 2012. In 2021, it developed its own 4K UltraHD global shutter sensor which has a pixel pitch of around 2.5 x 2.5 microns. That makes an UltraHD (3840 pixels wide) sensor of 8.8 x 6.6mm (2/3″ format in the image sensor size convention). Global shutter operation, where the whole image is captured at the same time) is critical for live action such as sports where there is a lot of unpredictable and rapid motion. Sensors that use a line by line readout approach can suffer from an artefact called “rolling shutter”, where lines that should be straight are curved as the camera or subject moves during the readout.
Further, Weber told us, in sports, the LED displays around the ground that carry advertising often carry more than one set of adverts, with the displays separated by time. When the cameras are synchronised to the different adverts, using global shutter sensors, they can be displayed accurately. That’s not possible for cameras with rolling shutters (which can be perfectly fine for cinematic capture).
Dynamic Range
The 4K sensor can capture up to 15 stops of dynamic range, which is more than enough for live TV. Cinema cameras, such as those from Arri and Red may be able to capture more dynamic range as the aim is to capture as much content as possible, with the material being optimised in post-production. Typically, to capture this wider range, pixel sizes are bigger to improve the signal to noise ratio. Sport and other live TV programming does not allow the time for post-processing. HLG is the key HDR technology for live sports capture and that has a range of around 13 F stops, Weber clarified, so 15 stops is plenty for this application.
The ability to capture slow motion is important in live TV and the UltraHD sensor can also output 1080 at up to 6X frame rate or three times frame rate for UltraHD (together with a regular UHD feed which is also important in live capture). Weber told us that the sensor itself can output much more than this, but at the time of development, FPGA limits meant that 3X was the top speed achievable.
Lens Choice Affected by Sensor Type
Three chip cameras use beam splitter optics in the back of the lens and in a B4 lens mount, that takes around 5cm of space. Grass Valley wanted to develop a sensor of Super35 format (around 18.6 by 24.9mm) to allow the shallow depth of field and that would have meant up to 10cm of space needed for the beam splitting optics. There also needs to be space for neutral density (ND) and other filters. Further, the slight difference in focal length between the different colors would also have needed attention. All these factors would have meant the need to develop a new lens mount – and that is a major commitment.
Instead, the firm took the 2.5 x 2.5 micron sensor pixel that it had already designed and scaled up the size to the Super35 format (24.89 x 18.66). The resulting resolution is then 10K with over 50 million pixels. The sensor is fitted with a Bayer filter to allow its use in a single chip configuration and that meant that the new camera can use a ‘PL’ mount. The adoption of the PL mount means that a wide variety of zoom and prime lenses is immediately available.
Weber is a keen proponent of the over sampling of video and he told us that many programs these days that are delivered in 1080P FullHD are captured in UltraHD and sampled down to improve the visual quality. With 10K sampling, there can be a big improvement even at UltraHD. However, the 10K sensor also opens up a possibility for the future.
Today the Processing is the Limit
At the moment, the limits on the number of frames that can be output from the sensor are set by the FPGAs that perform the processing in the camera. In the future, as processors improve, there is a possibility that the sensor could also be used to output high frame rate 8K, downsampled from the 10K. Grass Valley does not have a current plan to do this, but the technical possibility is there as the 8K ecosystem continues to evolve.
Weber wrote an article for the SMPTE Motion Imaging Journal about the issues in getting a cinematic look from live content and Grass Valley has made the content available in a paper on its website. The content provides a deeper dive into the technical aspects than we can cover and is worth a read if the topic is interesting to you.
The camera itself was demonstrated at NAB in a working form, although the prototype on show had not got the final firmware with support for all the different options. That work is concluding now and the firm hopes to have a small number of pre-production cameras in the hands of its customers over the summer months. Once it has feedback from practical use, it will finalize the product which should ship in Q4 2025. Weber told us that there is already a strong demand for the features that the new camera offers. Watch out for a final version at the IBC show in September!
- Correction. A mistake in our first version of this article said that the camera had an L mount. This was an error and it should have read ‘PL’ mount.
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