1. Technical Field
This disclosure relates to image capturing, and more specifically, to the compression of image data in an image sensor chip, prior to transmission to a digital signal processor (DSP).
2. Description of the Related Arts
The advancement of digital video and image encoding has led to increasingly sophisticated image capture techniques at increasingly high resolutions and frame rates. For instance, common media formats, such as Blu-Ray discs, internet video, and cable/satellite television, are able to display content at a 1080P resolution (1920×1080 pixels progressively scanned) at 60 frames per second (“fps”). Certain displays are able to display resolutions of 2560×2048 pixels or 3260×1830 pixels or higher at frame rates of 120 frames per second or higher. As encoding and display technology advances, frame rates and resolutions will increase accordingly.
The capture of digital images by an image capture device (hereinafter “camera”) is performed by an image sensor. Many types of image sensors are commonly used in cameras and other image-capturing devices, such as charge-coupled devices (CCDs) and complementary metal-oxide-semiconductors (CMOSs). Image sensors convert light, such as light entering the aperture of a camera through a camera lens, into image information. In this way, a camera can “capture” objects before it by converting the light reflected from the objects and passing through the camera lens into an image.
Image data from image sensors must often be processed into a particular image format prior to the image being rendered by a display. Image data is typically processed by a DSP, which is often located off-chip from the image sensor. Image sensors and DSPs are typically connected by buses coupled to the image sensors' pins and the DSPs' pins. As image resolution and frame rate increase, the amount of image data produced by the image sensor increases, along with the amount of power required to accommodate the transfer of image data from the image sensor to the DSP. In some circumstances, the amount of power required to accommodate such a transfer of image data rapidly depletes the battery life of a camera, or exceeds the amount of power available to the camera. In addition, increasing the power consumption in transferring image data correlatively increases the noise (such as the electromagnetic noise) impacting the image data.
The limited ability to transfer image data between an image sensor and a DSP thus creates a bottleneck in the image capture process. One solution is to increase the number of pins on the image sensor and DSP to increase the bus bandwidth between the image sensor and DSP. Such a solution, however, may not be physically possible due to the limited real estate of many image sensor chips and DSPs. Alternatively, images or video may be captured at a lower resolution and at a lower frame rate to reduce the power-limiting bandwidth requirements between the image sensor and the DSP. However, this solution results in a lower quality image or video. Thus, camera makers are increasingly required to balance the competing requirements of image and video quality with power consumption and chip real estate.