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I. Definition
CMS frame rate refers to the total number of valid images completely displayed by the vehicle Camera Monitor System (CMS) per unit of time, encompassing image capture, processing, transmission, and display. The unit is fps (frames per second).

This figure shows continuous image frames (I₁, I₂, I₃…) in a video data stream. The display frequency per unit of time is the CMS frame rate.

A frame rate that is too low can cause stuttering, jumping, or severe motion blur, preventing the driver from perceiving vehicles approaching rapidly from behind in real time, thereby increasing the risk of accidents. Therefore, CMS regulations and standards such as ISO 16505 and GB 15084-2022 have explicit requirements for the CMS frame rate.

II. Influencing Factors
The frame rate is limited by the performance of the entire pipeline from capture to display. The core factors include:
1. Image Sensor Exposure and Readout Timing:
In low-light environments, the sensor extends the exposure time to ensure an adequate signal-to-noise ratio. When the sum of the exposure time and data readout time approaches the frame period (e.g., 33.3 ms for 30 fps), the system will forcibly reduce the frame rate (frame dropping strategy) to maintain image quality.
2. ISP and SoC Real-time Processing Load:
The ISP must execute high-computing algorithms in real time, such as HDR fusion, LFM (LED Flicker Mitigation), dynamic denoising, and distortion correction. If the algorithm complexity exceeds the processing limits of the SoC's hardware accelerators or DSP, it will lead to increased processing latency, active frame dropping, or frame rate fluctuations.
3. Hard Upper Limit of Data Transmission Link Bandwidth:
When video streams are transmitted via GMSL, FPD-Link, or automotive Ethernet, they are constrained by the physical layer bandwidth and the payload limits of the SerDes (serializer/deserializer). When transmitting high-resolution or high-bit-depth images, insufficient bandwidth will force the system to reduce the frame rate or enable compression protocols to maintain link stability.
4. Display-side Frame Synchronization and Refresh Matching:
The monitor's physical refresh rate (e.g., 60 Hz) and the front-end output frame rate (e.g., 30 fps) must maintain clock synchronization (Genlock/VSync). If the front-end and back-end timing are out of sync or buffer management is improper, even if the average frame rate meets the standard, screen tearing or perceptible micro-stuttering will occur, affecting visual fluency.

III. How to Test?
Regulatory Requirements (4.3.2.14)
Frame rate >= 30 Hz under normal lighting conditions
Frame rate >= 15 Hz under low-light conditions or when the vehicle is traveling at low speeds

Test Conditions
Ambient Illuminance
Camera side:
a. Illuminance at the camera side: 800 lux (normal environment)
b. Illuminance at the camera side: 2 lux (low-light environment);
Monitor side:
Dark environment of 0-10 lx.

Test Equipment
Automated Chart Switching Bracket Test System, Multi-CCT LED Fill Light Source (Visible and NIR), 18% Neutral Gray Card, Temporal Frame Rate Tester, 2D Imaging Luminance Meter, Monitor and Luminance Meter Mobile Bracket

Operation Steps:
Camera side:
1. Switch the chart to the gray card and turn on the fill light source;
2. Place the frame rate tester on the light support bracket in front of the chart, and aim the camera at the running light.
Monitor side:
Observe the screen and gradually increase the frame rate on the frame rate tester from low to high until the image appears stationary. Record the frame rate displayed in the upper right corner at this time.