introduction
The Maxim serializer connects and controls the camera IC. These devices include the MAX9257 (with half-duplex UART/I2C control channels), the MAX9259, and the MAX9263 (both with full-duplex synchronous control channels). The MAX9263 also supports Wideband Digital Content Protection (HDCP). This application note describes how to convert the camera's RGB or YUV output to RGB data accepted by a standard monitor.
Camera output data format
Camera chips, such as the OmniVision® OV10630, can be connected via a serializer. The interface pins of the OV10630 include: pixel clock, PCLK, row valid, HREF, frame sync, VSYNC, and parallel data bits D[9:0]. The data bits are stable on the rising edge of the clock.
YUV and raw RGB data format
CMOS camera sensors include millions of light sensitive cells, each of which responds to optical signals of the entire wavelength. Filters are used to make specific sensors respond only to red, green or blue signals. Adjacent photosensitive cells are usually arranged in a color filter of the Bayer structure, and the number of green filters is twice the number of red or blue filters. This method is used to simulate the photographic characteristics of the human eye. The sensor unit output is read from left to right and top to bottom. The original RGB data sequence is blue, green...blue, green (end of the first line), green, red...green, red (end of the second line) ), and so on, as shown in Figure 1.
Figure 1. Raw RGB data arrangement
RGB data of the same density as the sensor unit is generated by interpolation of adjacent cells. In addition, the image can be restored according to a specific rule by using the colors of adjacent cells. One of the rules that make up each pixel RGB data set is to use adjacent cells of the same row, plus the green neighbors of the next row (or the previous row). The interpolated RGB data sequence is..., red (i-1), green (i-1), blue (i-1), red (i), green (i), blue (i), red ( i+1), green (i+1), blue (i+1), ... are shown in Fig. 2. Each pixel requires a set of RGB data to drive the color display and maintain the highest resolution of the camera sensor. The luminance resolution of the interpolated RGB data is close to the resolution of the sensor unit, but the chroma resolution is poor. Since the human eye is more sensitive to the gray level of each pixel than the color component of the pixel, the perceived resolution is substantially the same as the sensor unit resolution.
Figure 2. RGB data arrangement
However, this interpolation algorithm for RGB data increases the data rate by a factor of three. In order to reduce the data rate, especially where image transmission is required, a YUV color space (compressing the analog color television signal to the frequency band simulating the black and white television) can be employed. In the following formula, the luminance is represented by Y, the color difference between blue and luminance is represented by U, and the color difference between red and luminance is represented by V.
In the formula, the typical color weighting is: WR = 0.299, WB = 0.114, WG = 1 - WR - WB = 0.587, and the normalized value is UMAX, VMAX = 0.615.
For a camera sensor employing a Bayer color filter, the U or V data of adjacent pixels is approximately the same, depending on the row index i and the pixel index j (if the rules employed are adjacent colors). Using this guide, you can directly generate YUV data using RGB data according to the following formula.
 | The even row index i and the even pixel index j. |
 | The even row index i and the even pixel index j. |
 | For odd row index i and even pixel index j. |
 | For odd row index i and even pixel index j. |
 | The even row index i and the even pixel index j. |
 | The even row index i and the even pixel index j. |
 | For odd row index i and even pixel index j. |
 | For odd row index i and even pixel index j. |
 | The even row index i and the even pixel index j. |
 | The even row index i and the even pixel index j. |
 | For odd row index i and even pixel index j. |
 | For odd row index i and even pixel index j. |
In order to reduce the data rate, the U data of the even pixel index and the V data of the odd pixel index, and the Y data of the even and odd pixel indexes are utilized. The compressed YUV data is transmitted in the arrangement shown in FIG. 3, that is, Y1, U0, and V1 are data of the pixel 1; Y2, U2, and V1 are data of the pixel 2, and the like.
Figure 3. YUV422 data arrangement
422 represents the sampling ratio of Y:U:V, and the 4:x:x standard is the early color NTSC standard, which is resampled according to 4:1:1 chromaticity, so the color resolution of the image is only four quarters of the luminance resolution. one. Currently, only high-end devices that process uncompressed signals use 4:4:4 color resampling, and the resolution of brightness and color information is exactly the same.
Serializer input format
The parallel interface of the Maxim serializer is designed for 24-bit RGB data, especially the MAX9259, with pixel clock bits (PCLK) and 29 data bits for 24-bit RGB and line sync, field sync, and 3 control bits. In addition to the parallel data interface, the DRS and BWS pins need to be set high or low to select the data rate and bus width, respectively.
Maxim Serializer/Deserializer
The MAX9257 and MAX9258 serializer/deserializer (SerDes) feature 18-bit parallel input/output for YUV data transfer; the MAX9259/MAX9260 chipset features 28-bit parallel input/output for RGB data transfer; MAX9263/MAX9264 SerDes With 28-bit parallel input/output, HDCP is added. In addition, the MAX9265 and MAX9268 28-bit SerDes feature a camera link instead of a parallel I/O interface. All 28-bit Maxim serializers and deserializers have the same parallel/serial data mapping and are used interchangeably. For example, the MAX9259 serializer can be used with the MAX9268 deserializer to transfer RGB data (by means of an FPGA). Data is sent from the CMOS camera through a serial link to the display of the camera link interface.
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