Since its broadcast camera introduction in 1987, CCD technology has been used in nearly all of the typical broadcast applications. Most of today’s camera systems that are used for these applications are still based on three 2/3in CCD imagers. This imager size, in combination with the RGB prism beam-splitter technology, has been the de facto standard for more than 25 years. Even when the SD cameras were replaced by HD cameras, which offer more than four times the pixel count, this standard has not changed.
There are good reasons for this: Some are economical, while others are based on technology and physical factors. For instance, a 1920 x 1080 2/3in HD imager has a pixel size of 5µm x 5µm, which is approximately four times smaller when compared to an SD imager. That is why the first generation of HD cameras were approximately 1 to 1.5 f-stops less sensitive than the SD cameras, while also providing an approximate 6dB to 9dB lower S/N ratio. This gap has been closed thanks to improvements in imaging technology, combined with the implementation of digital noise reduction systems.
The latest HD cameras now offer sensitivity and S/N ratios that compare closely to the last generation of SD cameras from more than 10 years ago. However, this is only true of the 1080i and 720p formats. Changing to any of the 1080p formats will lower the sensitivity of the CCD camera by 1 f-stop, or 6dB, again. Why is that so? CCD imagers have always had the advantage that the signal charges from two adjacent pixels could be added to each other in the vertical shift register. That means a CCD imager that reads out an interlaced format has an improved sensitivity of 6dB, compared to a full progressive read out.
Today’s interlaced formats are now only used for broadcast applications, and they will soon be replaced by progressive formats. The demand for 1080p, 4K and even 8K production is increasing. They are all progressive formats, and the improvements in the interlaced formats of the CCD imagers will no longer work.
In CMOS imagers, the signal charges are converted inside the pixel into a signal voltage. Therefore, they must always work in a progressive mode as they cannot add signal charges from two adjacent pixels to one another. If needed, the interlace formats can be generated from inside the camera signal processing using the full progressive signals from the imager. A camera with a CMOS imager will have an identical sensitivity in the interlaced and progressive formats. This is one of the reasons why CMOS technology has fully replaced CCD technology in all applications other than broadcast.
In the past, CMOS imaging technology was not accepted for broadcast applications because of its performance when reacting to fast movements and light flashes — the so-called “rolling shutter” effect. This was caused by each pixel taking a slightly different start and end in terms of exposure time. This problem has been solved in the latest CMOS imagers by adding a storage node inside every pixel. They now react to fast movements and light flashes identically to CCD imagers with their “global shutters.”