To mirror or not to mirror: how camera monitoring systems are expanding the driver’s perspective


Objects in the mirror are closer than they appear--this message is the tried and true safety warning that has reminded drivers for decades that their rearview mirrors are reflecting a slightly-distorted view of reality. Despite their limitations, mirrors are vital equipment on the car, helping drivers reverse or change lanes. But today, advanced driver assistance systems (ADAS) are going beyond a mirror’s reflection to give drivers an expanded view from the driver’s seat through the use of cameras.

Camera monitoring systems (CMS), also known as e-mirrors or smart mirrors, are designed to provide the experience of mirrors but with cameras and displays. Imagine looking into a rearview mirror display and seeing a panoramic view behind your vehicle. When you look to your side mirror, you see a high-resolution display showing the vehicles to your side. These scenarios are becoming reality, as are other features such as blind-spot detection and park assist.

It’s important to understand the current transition from mirrors to CMS. It’s no surprise that systems in today’s vehicles are already leveraging ADAS features for mirrors. Most new vehicles in the past decade have added a camera to the back of the vehicle or attached a camera to the existing side mirror, with a display inside the vehicle to give drivers a different perspective of what’s behind or at the side of the vehicle.

Figure 1 shows the routing of this rearview camera and display system. The backup display is embedded in the rearview mirror and a cable routes to the rear of the vehicle.

Figure 1: Rearview mirror display and rearview camera for panoramic or backup views

The side mirror is different because the camera is located on the mirror. The side mirror still exists for viewing, but typically its camera works when the driver activates a turn signal or shifts in reverse. During a turn or a lane change, the camera outputs a video feed to the infotainment display in the dashboard and may show a slightly different angle than the side mirror itself, as shown in Figure 2.

Figure 2: Side mirror with camera viewed on an infotainment display

Now that I’ve reviewed current CMS configurations that incorporate a mirror with a camera and display, it’s worth noting it’s possible to achieve a CMS rearview mirror replacement through the addition of one or two cameras installed on the rear of the vehicle.

From the rear of vehicle, video data from an imager is input to TI’s DS90UB933 parallel interface serializer or DS90UB953 serializer with Camera Serial Interface (CSI)-2. This data is then serialized over a flat panel display (FPD)-Link III coax cable to a DS90UB934 or DS90UB954 deserializer, and then output to an application processor for video processing, such as JacintoTM TDAx processors, and then shown on a rearview mirror display. If the display is located far from the Jacinto applications processor, you will need a display interface serializer and deserializer to route the data over a coax cable again. You could use the DS90UB921 and DS90UB922 red-green-blue (RGB) format serializer and deserializer, respectively, or, if you’re implementing higher-resolution displays, the DS90UB947 and DS90UB948 Open Low-Voltage Differential Signaling Display Interface (LDI) devices.

Figure 3 shows the connections between these devices when using a display onboard with an applications processor.


Figure 3: Rearview mirror system block diagram

The second CMS is the side mirror replacement portion. The camera must be located in the same location where the mirror used to be. This camera’s video data displays a view of what the driver would see in the mirror. To achieve this, the camera data is serialized and sent over an FPD-Link III coax cable to a remote display located in the upper part of the door panel or included in the rearview mirror display. With a camera and display, now the side view can be in more direct line-of-sight locations for the driver. For example, if both the displays for side view and rear view are included in the rearview mirror, the driver only needs to look in one location.

Another option available in a side mirror replacement is to add a second co-located camera with the first, but at a different viewing angle. The benefit of this setup versus a standard mirror is that with two differently angled cameras, one camera can be used for the view that a side mirror would have provided and the second camera can provide a wider field of view for blind-spot detection and collision warning features. Figure 4 shows a two-camera side mirror replacement system.


Figure 4: Side mirror replacement system block diagram

The question you may be asking now is why drivers need cameras and displays if they can achieve most of the same functionality with a mirror. The answer lies in the features that cameras can provide over mirrors alone. If only a side mirror exists, side collision avoidance is solely up to the driver. With a camera, the detection of a potential collision before a lane change could activate vehicle warning alerts that prevent drivers from making an unwise action. Panoramic rear views with wide field-of-view (FOV) rear cameras or a separate narrowly focused backup camera can provide a driver with different line of sights and reduces or eliminates blind spots that would not be possible with mirrors alone.

This is just the beginning, though, because in order for vehicles to move from driver assistance systems to autonomous systems, a CMS can be incorporated into sensor fusion systems. CMS has the opportunity to incorporate ultrasonics and possibly even radar. The fusion of rear and side cameras with ultrasonics adds the capability to assist drivers in parking or can even park the vehicle for them. Radars fused with side mirrors will add an extra measure of protection for changing lanes and even side collision avoidance.

To learn more about how to implement sensor fusion, check out the follow-up blog posts on park assist sensor fusion using CMS and ultrasonics or front sensor fusion with front camera and radar for lane departure warning, pedestrian detection and even assisted braking.

Additional resources