Research and design of digital telephoto camera based on DSP

In recent years, with the development of semiconductor manufacturing technology and the improvement of computer architecture, digital signal processing technology has been rapidly developed and applied, the functions of DSP chips are becoming more and more powerful, and digital signal processing has become the mainstream of signal processing technology. Combining the trend of optical instruments towards the integration of light, machinery, electricity, and computing and intelligent modern optical instruments, a synchronous adjustable binocular telephoto digital camera based on high-performance DSP chips is designed.

1 Basic ideas and principles of design

Separate lighting, according to the principle of matching the entrance pupil diameter of the telescope objective lens with the digital camera lens, the digital lens and the telescope are designed and calculated to obtain the defocus corresponding curve of the imaging of the digital camera system and the telescope system for 3 m to infinity targets. The telescope and the digital lens are driven by the rotation of the center adjustment hand wheel to achieve synchronous focusing, so that the imaging of the same scene target in the distance through the telescope objective lens and the digital lens is synchronous and clear at the same time, making the telescope truly become the optical viewfinder of the digital camera, and then the acquisition, storage, compression of the observation target image information and the conversion, display and transmission of the digital image are realized through the CMOS image sensor at the image plane position of the digital lens.

2 Design and research of digital imaging system

According to the requirements, digital image signal processing technology based on high-performance DSP chips is used to achieve the acquisition, storage, conversion, transmission and display of real-time image information. The high-performance multimedia processing chip TMS320DM642 from Texas Instruments (TI) is selected as the main processor; the SDRAM is Micron's T48LC4M32B-6; the video acquisition chip is Micron's 3-megapixel CMOS image sensor MT9T001; an efficient, stable and reliable embedded computing platform is used, and the digital camera system structure diagram is shown in Figure 1.

Since CMOS APS image sensors are superior to CCD image sensors in terms of price, performance, power consumption, etc., and integrate many image processing functions, the CMOS APS image sensor chip MT9T001 produced by Micron was selected in the design of the video acquisition module of this system.

MT9T001 is a CMOS digital image sensor with OxGA format (effective pixels are 2 048×1 536). The chip integrates analog and digital automatic gain adjustment, level offset adjustment, window size switching, row and column adjustment and flash mode functions, all of which can be programmed and controlled through the I2C bus interface. The sensor can work in default mode or user mode set by register programming. The default mode will output QxGA format images at a speed of 12 frames/s. The APC converter on the chip provides a 10 bit data stream for each pixel, accompanied by line and field synchronization signal outputs.

The connection between DM642 and cMOS image sensor is shown in Figure 2. In order to receive video data, the video port of DM642 must be configured in raw data acquisition mode. In this mode, DM642 does not perform any selection or interpolation processing on the received data. This operation mode is suitable for receiving data in special formats such as CMOS image sensors. Since raw data is transmitted, the connection between DM642 and MT9T001 is relatively simple and does not require horizontal and vertical synchronization signals. When the CAPENA signal is enabled, the VPID data bus will start to receive data; the acquisition rate is determined by the PIXCLK clock of the CMOS sensor. DM642 controls the operating mode of the CMOS image sensor through the I2C bus CSCL and SDA.

3 Prototype testing and inspection

The prototype was tested for performance and digital lens discrimination, and the resolution value was read after taking a photo of the ISO Resolution Chart for Electronic Still Cameras at a distance of 3 m. The vertical resolution of the product reached 8 groups, and the horizontal resolution reached 9 groups, and the resolution met the design requirements. A 4-megapixel Canon camera and the prototype were used to shoot the same scene (the arrow indicates the shooting target) at the same location and time, and the results are shown in Figures 3 and 4. This design realizes the digital telephoto function.

4 Conclusion

This research is based on traditional binoculars and uses advanced digital imaging technology to creatively solve the problem of observing and photographing the same object through accurate synchronous transmission of the structure, making the telescope truly a viewfinder for a digital camera, and realizing the true vision of what is photographed. The specially designed telephoto lens eliminates the imaging distortion of the telescope system and increases the depth of field effect of the image. The telescope system and the telephoto system achieve synchronous focusing, ensuring the consistency of the change in imaging clarity on the telescope image plane and the photosensitive chip of the digital camera system. The proportion of the imaging of distant scenes in the telescope system is the same as that in the photo. At present, this research has been applied to some electronic telescope equipment.