What is a CMOS digital camera? For end users, digital cameras with CMOS technology are not much different from traditional cameras. Most photo manipulation methods and the way to position the subject in the viewfinder are exactly the same. So where is the difference? Traditional cameras require a 35 mm film to expose and store images, while digital cameras use CMOS sensors to store photos using built-in flash memory or removable memory cards. But this is not all, the more complicated problem comes from the process.
The CMOS sensor consists of a photodiode array. When a photo is taken, the CMOS sensor is exposed and the image data is read. This information is sent to a coprocessor or backend controller. The coprocessor is responsible for all processing until the final image is compressed and saved in any kind of memory. Although the coprocessor can handle all the problems, we should see that the sensors and lenses are very important in the imaging process. Even if the coprocessor has very complex image processing capabilities, if the image data is severely distorted, it is impossible to completely repair it. Although the imaging process is complex, there are simple solutions.
STMicroelectronics has a wealth of experience in solving these complex problems. The STv0684 coprocessor and VC6700 (2 megapixel) CMOS sensor are one of the solutions that support 2 megapixels or higher resolution. For the user, the tools we provide are not only a set of chipsets, but also a turnkey solution. We will provide schematics, BOM lists, firmware, drivers, production tools, and support and recommendations.
Now let's take a look at the product features and benefits. The VC6700 CMOS sensor is fabricated in a four-transistor (4T) configuration and an H8i (0.18μm) fabrication process. This means that the sensor has 4 transistors per pixel, which is particularly advantageous in low light conditions, and the biggest difficulty with ordinary CMOS sensors is the poor imaging performance in low light environments. With this technology, ST's CMOS sensor can achieve low light performance very close to CCD. It is well known that CMOS has other advantages over CCDs: it uses a single power supply and thus consumes less power; logic can be integrated on the same chip, reducing the number of peripheral passive components. In addition, the VC6700 has been built into 3.3V and 1.8V linear regulators that, when powered by USB or rechargeable batteries, can be used with external power transistors to regulate the power of the remaining circuits.
We also have an audio preamplifier for recording that makes the audio circuit very simple, just connect the microphone. One of the main features of our sensor is a read speed of 48MHz or 48 megapixels per second, making it possible to achieve a video transfer rate of 25 fps (frames per second). This feature allows us to use a mechanical shutter blade without using this solution. At the same time, it requires that the coprocessor must also be able to process data at this rate, which can result in image distortion, such as image fragmentation. Image segmentation occurs when the data read rate is too slow. At this time, it is the object of rapid movement, because the reading order of the sensor is read line by line from top to bottom according to the principle of rolling shutter.
Comparing the STv0684 with other similar components, we found that there are many differences, especially in the video pipeline. We have two separate processors in the same system, which can fundamentally improve overall performance. The VP (Video Processor) consists of a hardware module and a high-speed 8051 like controller. This design is more advantageous than other solutions. The operation of the controller and hardware module remains independent of the system, so the main controller (ST20 in this example) will not have to deal with image quality related functions such as AE (automatic exposure), AWB (automatic white balance), color processing. And images are waved and so on. These high-intensity computational and computational tasks are evenly distributed to various firmware, while speed-related operations are performed by hardware modules.
In order to make image quality more reliable in harsh environments, our video processor adds the ability to recalculate the shading of the lens and reduce the fixed pattern noise (FPN) (used to eliminate unwanted vertical lines) No need for any special settings for fault pixel correction and advanced statistical processors on the production line. The video processor firmware can be changed at any time based on the general firmware of the main core. The main core is a proven ST20 32-bit RISC embedded controller that is also used in many other ST multimedia solutions. The core operates at 48MHz and provides powerful power and flexibility to the system. The ST20 is unique in its embedded real-time operating system, OS20, which further enhances system stability and flexibility. The STv0684 will start from the boot ROM memory first, followed by several possible solutions for code storage. One solution is that the code can be stored in a small package size SPI flash or booted from NAND flash if NAND flash is available. In this case, external NOR flash is no longer needed, saving valuable PCB space for the product. The code is then loaded into SDRAM memory for execution by the processor. For this feature, we have built-in DMA-driven NAND flash, SPI and SDRAM controllers.
The working characteristics of the chips required for digital cameras and camcorders are as follows:
For portable devices, the most important thing is to have advanced power management features to extend battery life. It allows the power of unnecessary hardware modules at all levels of the system to be turned off, and it also reduces the frequency of the CPU when high performance is not required. This design can further reduce power consumption to 6 Ma while the system is operating. Of course, the chip also supports other necessary functions such as standby and USB.
Built-in USB1.1 interface for downloading pictures and PC-camera connection. The camera will support three USB modes: one is MSCD (mass storage class device), it does not require a PC to install the driver, Win98SE presets the support driver we provide. For PC-camera connectivity, we offer two options, one is our proprietary compatible video-like driver, and the drivers supported by our driver do not require drivers.
Built-in TFT controller to directly support controllerless display modules. The controller must be flexible enough to support different vendors, different types of browsers, and support resolutions up to 1024 x 1024.
Built-in PAL or NTSC TV output. It can change the output via software, and any resolution image can be scaled to the correct format.
Built-in audio codec to record and play WAV compressed files. The audio compression ratio can be as high as 4:1.
Built-in interfaces for removable media storage such as CF, SD, MMC, SMC and possibly microdrives.
Finally, another way to save memory space is to compress images and movies, so we have a built-in compression engine. This compression engine (VC) supports a JPEG compression ratio of up to 100:1, while in practice, a 35:1 compression ratio is already acceptable. This is also an important feature of our system over other solutions, because the compression of other solutions is hard to reach than half of the requirements, so customers are forced to purchase more memory. In addition, this compression engine is capable of encoding and decoding real interleaved AVI files, which are a standard for many platforms, including PDAs (Personal Digital Assistants), PVRs (Portable Recorders), PMPs (Personal Media Players) and PCs. .
Of course, we have a lot of opportunities to use this chip, because this chip has much more flexibility than digital camera design. In the consumer electronics market, we will see more and more products integrating multiple functions on a single device, such as a handheld product called PMP that can play MP3s, listen to FM channels, store pictures and movies, exchange data, play movies and Other media. This market presents a boom, so we will take advantage of this chipset to develop solutions. As the resolution of our sensors has increased to 3 million pixels and above, we still believe that the market mainstream will be between 2 million and 4 million pixels, because the photo print shop prints a normal photo of 5" x 7.5" size only 200 CMOS. If we increase the resolution of the sensor, this does not make any sense for image quality. Higher resolution only improves image quality when we print large images or zoom in on a particular area.
Now that people are talking about MPEG4 more and more, we are also planning to integrate this feature in next-generation chips. Other solutions we currently see on the market support standards such as MPEG4 (low profile), but through more detailed analysis, it has been found that it only emphasizes packaging, and in fact, compressed MPEG4 (low profile) files are more compact than compressed standards. The AVI file is bigger. Therefore, we believe that the ST-enhanced compression ratio AVI file is a better film quality solution because it supports a 30fps transfer rate when using the QVGA format. As for patent fees, every device manufacturer must consider whether it makes sense to pay for a compression scheme that is not as valuable as free technology.
Now return to the topic of CMOS technology. CMOS technology is steadily growing, and we believe this technology is promising because CMOS sensors are more flexible, easier to operate, and less expensive. For example, in the mobile imaging market, we see that it already has a certain market share, so we will see the same results in future digital cameras or digital cameras. As the pixel size is further reduced and the quality is getting higher and higher, we believe this is the future trend. But it's worth noting that if we compare CMOS technology products with high-end products, we must use higher quality lenses for CMOS technology, even with auto focus and auto zoom devices. As we are working to solve this problem for our customers, the difference between using CMOS technology and using other technologies will sooner or later be eliminated.
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