Video Parameters You Must Understand

1. Resolution

Video resolution refers to the size or dimensions of the image produced by a video imaging device. It is expressed as "horizontal pixels * vertical pixels." Common image resolutions include QCIF (176×144), CIF (352×288), D1 (704×576), 720P (1280×720), and 1080P (1920×1080). The maximum resolution of a camera is determined by the CCD or CMOS sensor. Some cameras now support resolution adjustment, which is achieved by cropping the original image using the camera's built-in software.

Note: Difference between CMOS and CCD

1. Imaging Process

The photoelectric conversion principle of CCD and CMOS image sensors is the same; the main difference lies in the signal readout process. Since CCD has only one (or a few) output nodes for unified readout, its signal output consistency is very good. In contrast, each pixel in a CMOS chip has its own signal amplifier, performing charge-to-voltage conversion individually, resulting in poorer signal output consistency. However, to read the entire image signal, CCD requires a wide bandwidth for the output amplifier, while in CMOS chips, the bandwidth requirement for each pixel's amplifier is lower, significantly reducing chip power consumption. This is the main reason why CMOS chips consume less power than CCD. Despite lower power consumption, the inconsistency of millions of amplifiers introduces higher fixed pattern noise, which is an inherent disadvantage of CMOS compared to CCD.

From a manufacturing process perspective, CCD circuits and devices are integrated on semiconductor single-crystal materials, making the process complex. Only a few manufacturers worldwide can produce CCD wafers, such as DALSA, SONY, and Panasonic. CCD can only output analog electrical signals, requiring subsequent address decoders, analog-to-digital converters, and image signal processors, as well as three sets of different voltage power supplies and synchronous clock control circuits, resulting in very low integration. In contrast, CMOS is integrated on a semiconductor material called metal oxide, using the same process as producing millions of computer chips and storage devices, making CMOS production much cheaper than CCD. Additionally, CMOS chips can integrate image signal amplifiers, signal readout circuits, A/D converters, image signal processors, and controllers onto a single chip, enabling all basic camera functions with just one chip. This high integration gave rise to the concept of chip-level cameras. With the continuous development of CMOS imaging technology, more and more companies can provide high-quality CMOS imaging chips, including Micron, CMOSIS, and Cypress.

3. Speed

CCD outputs signals one photosite at a time, following a fixed program, resulting in slower speed. CMOS has multiple charge-to-voltage converters and row/column switch controls, allowing much faster readout. Most high-speed cameras above 500fps are CMOS cameras. Additionally, CMOS address selection switches enable random sampling and sub-window output, achieving higher speeds when outputting only a sub-window image.

4. Noise

CCD technology developed earlier and is more mature, using PN junctions or silicon dioxide (SiO₂) isolation layers to isolate noise, giving it an advantage in image quality over CMOS photoelectric sensors. Due to the high integration of CMOS image sensors, components and circuits are very close together, causing significant interference and noise that greatly affects image quality. With the continuous development of CMOS noise reduction technology, favorable conditions have been created for producing high-density, high-quality CMOS image sensors.

2. Frame Rate

A frame is a single still image, and continuous frames form animation, such as in movies. The frame rate we commonly refer to is the number of frames transmitted per second, usually expressed as fps (Frames Per Second). Each frame is a still image, and rapidly displaying consecutive frames creates the illusion of motion, reproducing the object's state at that time. A higher frame rate results in smoother and more realistic animation. The more frames per second (fps), the smoother the displayed motion. Generally, setting the image frame rate to 25fps or 30fps is sufficient.

3. Data Rate (Bitrate)

Data rate refers to the data flow per unit time after video images are encoded and compressed, also called bitstream. It is the most important factor in video encoding for quality control. At the same resolution, the smaller the compression ratio, the higher the video image's data rate, and the better the image quality.