Camera module hardware
October 11, 2021
Camera module hardware
Camera module, the full name is Camera Compact Module, abbreviated as CCM.
CCM includes four major parts: lens, sensor, FPC, andDSP.
The important components that determine the quality of a camera are: lens,DSP, and sensor .
Working principle: The light collected by the object through the lens (lens), through the CMOS or CCD integrated circuit, the light signal is converted into an electrical signal, and then the internal image processor (ISP) is converted into a digital image signal and output to the digital signal processor (DSP) ) Processing and conversion into standard GRB, YUV and other format image signals.
2. Hardware composition
Lens is a device that can receive light signals and converge the light signals on the photosensitive device CMOS/CCD.Lens determines the lighting rate of the sensor, and its overall effect is relative to a convex lens.
Generally, the lens structure of a camera is composed of several lenses, which are divided into plastic lens (PLASTIC) and glass lens (GLASS). Generally, the lens structure used for CAMERA is: 1P, 2P, 1G1P, 1G3P, 2G2P, 4G, 8P, etc. The more lenses, the higher the cost;Glass lenses are more expensive than plastic lenses, but the imaging effect of glass lenses is better than that of plastic lenses.At present, the cameras configured for mobile phones on the market are mainly 1G3P (composed of 1 glass lens and 3 plastic lenses), with the purpose of reducing costs.
- Main Indicators of Lens
A. Eliminate as much Flare as possible
- Picture quality clarity
- CRA (Chief Ray Angle) should be matched to reduce shading (Lens cra
- The aperture is as large as possible
- Distortion should be as slight as possible
- Lens main parameters
(1) Focal length: The length of the focal length of the lens determines the size of the image taken, the size of the field of view, the size of the depth of field and the perspective of the picture. Generally speaking, for a single lens, it is the distance from the center of the lens to the focal point, while the camera lens is a combination of multiple lenses, which is much more complicated.The focal length here refers to the distance from the center point of the lens to the clear image formed on the photosensitive component (CCD).
- Field of view: We often use the horizontal field of view to reflect the shooting range of the picture.The larger the focal length f, the smaller the angle of view, and the smaller the image range formed on the photosensitive element;Conversely, the smaller the focal length f, the larger the angle of view, and the larger the image range formed on the photosensitive element.
- F value (aperture ratio): F value refers to the brightness of the lens (that is, the amount of light transmitted by the lens).F=Lens focal length/aperture diameter.With the same F value, the aperture of a long focal length lens is larger than that of a short focal length lens.
- Aperture: The aperture is an adjustable optical-mechanical diaphragm located inside the lens, which can be used to control the amount of light passing through the lens.Iris diaphragm. A mechanical device inside the lens used to control the size of the diaphragm. Or it refers to a device used to open or close the lens diaphragm to adjust the f-stop of the lens.
- Depth of field: When an object is in focus, all scenes from a certain distance in front of the object to a certain distance behind it are also equivalent to being clear. The distance from front to back where the focus is quite clear is called the depth of field.
2.1 VCM (Voice Coil Motor)
The full name is Voice Coil Montor, the voice coil motor in electronics, which is a type of motor. Because the principle is similar to that of a speaker, it is called a voice coil motor, which has the characteristics of high frequency response and high precision. The main principle is to control the stretched position of the spring leaf by changing the DC current of the coil in the motor in a permanent magnetic field, thereby driving up and down movement. Mobile phone cameras widely use VCM to achieve auto-focusing. Through VCM, the position of the lens can be adjusted to present clear images.
- VCM performance indicators
The performance of VCM mainly depends on the ratio of current to travel distance. Starting from the starting current, the current rise should be proportional to the stroke distance that can be driven. The smaller the rising current required, the higher the accuracy. At the same time, it also depends on the maximum power consumption, maximum power, and size.
- VCM classification
From the structure, it can be roughly divided into three categories: (1) Shrapnel structure; (2) Ball type structure; (3) Friction type structure.
From the function, it can be roughly divided into five categories: (1) Open loop open motor; (2) Close loop closed loop motor; (3) Alternate central motor; Memory metal formula, etc.); (5) OIS+Close loop six-axis motor.
- AF principle
After entering the auto focus mode, the Driver moves from 0 to the maximum value, which makes the lens move from the original position to the maximum displacement. At this time, the sensor imaging surface automatically takes pictures and saves them in the DSP. The DSP uses these pictures to calculate the value of each picture. MTF (Modulation transfer function) value, so as to find the maximum value in this MTF curve, and through the algorithm, get the current size corresponding to this point, and once again instruct the Driver to provide the current to the voice coil, so that the lens is stabilized on the imaging surface , So that automatic zoom is achieved.
- Zoom and focus
A: Use zoom motor (ZOOM) to achieve optical zoom
By moving the lens inside the lens to change the position of the focus, change the length of the lens focal length, and change the angle of view of the lens, so as to achieve the effect of zooming in and out.
B: Achieve auto focus using focus motor (AF)
Move the position of the entire lens (rather than the lens inside the lens) too close to control the length of the lens focal length to achieve a clear image. A commonly used method in mobile phones.
Optical focus and optical zoom are different concepts:
Optical zoom is to change the position of the focus by moving the relative position of the lens inside the lens, change the length of the lens focal length, and change the angle of view of the lens, so as to realize the image enlargement and reduction;
Optical focus is actually to adjust the position of the entire lens (not the position of the lens in the lens) to control the image distance, so that the image is the clearest.
There are various wavelengths of light in nature. The human eye recognizes the light in the wavelength range of 320nm-760nm, and the light beyond 320nm-760nm cannot be seen by the human eye; while the imaging components of the camera CCD or CMOS can see the great Part of the wavelength of light.Due to the participation of various lights, the color restored by the camera differs from the color seen by the naked eye.For example, green plants turn gray, red pictures turn light red, black turns purple, etc. At night, due to the filtering effect of the bimodal filter, the CCD cannot make full use of all the light, does not produce snowflake noise, and its low-light performance is unsatisfactory. In order to solve this problem, the IR-CUT dual filter is used.
The IR-CUT dual filter refers to a set of filters built into the camera lens group. When the infrared sensor outside the lens detects the intensity of the light, the built-in IR-CUT automatically switches the filter according to the external light. The strength of the will automatically switch, so that the image achieves the best effect.In other words, during the day or night, the dual filter can automatically switch the filter, so the best imaging effect can be obtained regardless of whether it is in the day or night.
- IR-CUT composition and principle
The IR CUT dual filter switcher consists of an infrared cut-off low-pass filter (an infrared cut-off or absorption filter), a full-spectrum optical glass (a full-transmission spectrum filter), and a power mechanism (which can be electromagnetic, It consists of a motor or other power source) and a housing, which is switched and positioned through a circuit control board.When the daytime light is sufficient, the circuit control board drives the switcher to switch and position the infrared cut-off filter to work, CCD or CMOS restores the true color; when the visible light at night is insufficient, the infrared cut-off filter automatically moves away, full-spectrum optics The glass starts to work. At this time, it can sense the infrared light of the infrared lamp, so that the CCD or CMOS can make full use of all the light, which greatly improves the night vision performance of the infrared camera, and the whole picture is clear and natural.
- IR-CUT indicators
a. The infrared cut-off degree of the filter, the light transmittance, and the light shaping effect, etc.
- Power drive part
- Control circuit
2.4.1 Sensor structure
4.Filter: generally use IR Coating or blue glass to filter out infrared light.
Image sensor is a semiconductor chip with millions to tens of millions of photodiodes on its surface. The photodiodes will generate electric charges when exposed to light, which converts light into electrical signals.
Photosensitive original: CCD, CMOS (PPS and APS)
Different processes: front-illuminated FSI, back-illuminated BSI, stacked
There are many photosensitive units on the sensor, which can convert light into electric charges to form an electronic image corresponding to the scene. In the sensor, each photosensitive unit corresponds to a pixel (Pixels). The more pixels, it means that it can sense more object details, so the image is clearer, and the higher the pixels, the clearer the imaging effect. The product of the camera resolution is the pixel value, for example: 1280×960=1228800
2.Target surface size
The size of the photosensitive part of the image sensor is generally expressed in inches. Like a TV, this data usually refers to the diagonal length of the image sensor, such as 1/3 inches in common. The larger the target surface, the better the light flux, and the smaller the target surface, the easier it is to obtain. Greater depth of field.
That is, the intensity of incident light is sensed through CCD or CMOS and related electronic circuits. The higher the sensitivity, the stronger the sensitivity of the photosensitive surface to light, and the higher the shutter speed. This is especially important when shooting moving vehicles and monitoring at night.
- Electronic shutter
is a term referring to the mechanical shutter function of a camera. It controls the light-sensing time of the image sensor. Since the light-sensing value of the image sensor is the accumulation of signal charge, the longer the light-sensing, the longer the signal charge accumulation time, and the larger the amplitude of the output signal current. The faster the electronic shutter, the lower the sensitivity, which is suitable for shooting under strong light.
- Frame rate
It refers to the number of pictures recorded or played per unit time. Continuously playing a series of pictures will produce animation effects. According to the human visual system, when the picture playback speed is greater than 15 frames per second (that is, 15 frames), the human eye can hardly see the jump of the picture; when it reaches 24 frames /s——The flicker phenomenon is almost imperceptible when it is between 30 frames/s (that is, between 24 frames and 30 frames).
The number of frames per second (fps) or frame rate indicates the number of times the graphics sensor can update per second while processing the field. A high frame rate can get a smoother and more realistic visual experience.
- Signal-to-noise ratio
is the ratio of signal voltage to noise voltage, and the unit of signal-to-noise ratio is expressed in dB. Generally, the signal-to-noise ratio value given by the camera is the value when the AGC (automatic gain control) is turned off, because when the AGC is turned on, the small signal will be increased, so that the noise level will also be increased accordingly.
The typical value of the signal-to-noise ratio is 45-55dB. If it is 50dB, the image has a small amount of noise, but the image quality is good; if it is 60dB, the image quality is good and no noise occurs. The larger the signal-to-noise ratio indicates the control of noise The better. This parameter is related to the number of noise points in the image. The higher the signal-to-noise ratio, the cleaner the picture, and the less point-like noise in the night vision picture.
Digital signal processor DSP (DIGITAL SIGNAL PROCESSING) function: mainly through a series of complex mathematical algorithms to optimize the digital image signal parameters, and transfer the processed signals to PCs and other devices through USB and other interfaces
2.5.1 The difference between DSP and ISP
ISP is the abbreviation of Image Signal Processor, that is, image signal processor.
DSP is the abbreviation of Digital Signal Processor, that is, digital signal processor.
ISP is generally used to process the output data of the Image Sensor, such as AEC (automatic exposure control), AGC (automatic gain control), AWB (automatic white balance), color correction, Lens Shading, Gamma correction, and removal of dead pixels , Auto Black Level, Auto White Level and other functions.
DSP functions are more, it can do some photos and echo (JPEG encoding and decoding), recording and playback (Video encoding and decoding), H.264 encoding and decoding, and many other aspects of processing, in short, processing Digital signal too.
3. Classification and related concepts
- According to the interface
UVC, the full name of USB Video Class, is a set of standards customized by USB-IF. All USB interface cameras that follow this standard can be used almost directly under Windows, Linux and other systems to achieve a similar drive-free effect.
Of course, it’s not that you don’t really need a driver, but as long as a camera with a USB interface complies with the UVC standard, it can be driven by a set of universal drivers in Windows and Linux systems, without you needing to install it. Other drivers. For example, there is a set of universal drivers for UVC devices in the Linux kernel: drivers/media/usb/uvc. As long as this set of drivers is turned on, almost all UVC cameras can be used directly.
MIPI (Mobile Industry Processor Interface) is the abbreviation of Mobile Industry Processor Interface. MIPI is an open standard for mobile application processors initiated by the MIPI Alliance.
The MIPI organization is committed to standardizing the internal interfaces of mobile communication devices to reduce compatibility issues and simplify design. For example, camera interface CSI, display interface DSI, radio frequency interface DigRF, microphone/speaker interface SLIMbus, etc.
DVP is a parallel port transmission, the speed is slower, the transmission bandwidth is low, the use requires PCLKsensor output clock, MCLK (XCLK)external clock input, VSYNCfield synchronization, HSYNCline synchronization, D[0:11]parallel port Data-can be 8/10/12bit data bit size.
LVDS interface: LVDS (Low Voltage Differential Signaling) is low voltage differential signal transmission
Network port: IPC network camera
SDI interface: professional video interface, using coaxial cable, digital signal, now there are products with 18GHz bandwidth, which can meet 8K video transmission.
RGB interface: analog video interface, using coaxial cable, analog signal, 720x576 standard definition signal or less.
eDP interface: a relatively new specification, which will be widely used in the notebook industry to replace LVDS, support ultra-high resolution, and support above 1080P
3.2 Classification by lens
3.3 According to imaging points
Black and white camera
3.4 According to the photosensitive original
3.5 According to whether the focal length is adjustable
Fixed focus camera
3.6 Classified by sensor package