A CMOS image sensor with analog two-dimensional DCT-based compression circuits for one-chip cameras

This paper presents a CMOS image sensor with on-chip compression using an analog two-dimensional discrete cosine transform (2-D DCT) processor and a variable quantization level analog-to-digital converter (ADC). The analog 2-D DCT processor is essentially suitable for the on-sensor image compression, since the analog image sensor signal can be directly processed. The small and low-power nature of the analog design allows us to achieve low-power, low-cost, one-chip digital video cameras. The 8×8-point analog 2-D DCT processor is designed with fully differential switched-capacitor circuits to obtain sufficient precision for video compression purposes. An imager array has a dedicated eight-channel parallel readout scheme for direct encoding with the analog 2-D DCT processor. The variable level quantization after the 2-D DCT can be performed by the ADC at the same time. A prototype CMOS image sensor integrating these core circuits for compression is implemented based on triple-metal double-polysilicon 0.35-μm CMOS technology. Image encoding using the implemented analog 2-D DCT processor to the image captured by the sensor is successfully performed. The maximum peak signal-to-noise ratio (PSNR) is 36.7 dB