1394 cameras: Simple designs with high bandwidth, low latency, scalability

Automotive camera systems are progressing from a single backup camera and simple sensors to detect objects to smart driver assist systems such as collision avoidance and sign recognition. These systems require high bandwidth, low latency, multi-camera synchronization, and scalability, and they must meet general automotive considerations like low cost, low maintenance and reduced weight, achievable using a flexible wire harness.

These system requirements are interrelated. For example, there are many high bandwidth solutions, but none can meet the low latency, multi-camera synchronization requirements while maintaining low costs.

Typical system

The typical driver assist system consists of a camera controller designed to interface to multiple image sensors (cameras). The controller communicates with the cameras to provide configuration information, and the cameras communicate with the controller to provide status and image stream data. The image stream data from each camera is received by the controller and processed to determine items like distance from objects, relative speed and lane information. To compute multiple interrelated streams in real-time places special requirements on the network, requirements the 1394 Automotive Camera System was designed to meet.

High bandwidth

There are several factors that drive the need for high bandwidth: items like uncompressed or lightly compressed video, video resolution, color depth and frame rate. For driver assist applications the video is typically uncompressed or lightly compressed video. This is required for two primary reasons: the delay from image sensor frame capture to start of frame processing must be small and predictable; few, if any, compression artifacts can be tolerated.

Depending on the system, the video resolution may range from 640 x 480 to 2048 x 1536 and color depth may range from 8 to 24-bit and the frame rate may range from 15 fps to 60 fps. 1394 Automotive is based on IEEE-1394-2008 which supports 98.304 Mb/s to 3.932 Gb/s (S3200) with 983.04Mb/s (S800) chips shipping in high volume now. Gaining higher speeds is easy, since 8B10B based technologies are common now and S1600 and S3200 devices have been demonstrated successfully.

At S800 data rates, six uncompressed 640 x 480 at 30 fps and 8-bit monochrome cameras plus one uncompressed 640 x 480, 30 fps, 16-bit color camera for driver viewing is possible with nearly 200 Mb/s bandwidth remaining for the back channel or other network devices.

At S3200 data rates, four uncompressed 2048 x 1536 at 30 fps and 8-bit monochrome cameras is possible for a dedicated camera network with 125 Mb/s bandwidth left for the back channel or other network devices.

Low latency

Adequate bandwidth brings the opportunity for low latency. However, adequate bandwidth does not always guarantee low latency. The 1394 standard provides enough bandwidth to send uncompressed video from multiple cameras. which significantly reduces latency. Also, 1394's isochronous capability allows each camera's image data to reach the camera controller predictably, with a guaranteed maximum latency of 250 microseconds.

If light compression is required, it must meet the latency requirements of driver assist systems. Safety critical applications can tolerate very little latency from sensor frame capture to start of frame processing (from encode to decode). For safety critical applications, which are the most stringent, a maximum of 5ms can be tolerated. A 1394 Automotive Camera System, with 250 microseconds, coupled with Fujitsu's SmartCodec, provides 4x compression with 5 ms of encode-to-decode latency.

This means at S3200 1394 can support two maximum resolution, frame rate and color depth cameras (2048 x 1526 at 60 fps and 24-bit color) with enough bandwidth to support six compressed 1024 x 768 at 30 fps and 16-bit color cameras while maintaining a maximum latency of 5ms for all cameras.

Multi-camera synchronization

Images from the cameras are streamed real-time 8,000 times a second. Before each 125-microsecond interval a 40-nanosecond resolution timestamp packet is broadcast to all devices, which resynchronizes all of them. This timestamp is generated by hardware and isn't affected by non-1394 system loading which allows for highly accurate triangulation calculations. The 40 nsec resolution timestamp coupled with the VersaPHY Remote Sensor Profile enables highly accurate "look ahead" camera triggers for pixel and line accurate synchronization with latency far below the 10 microseconds typically required.