Focal-plane image encoder with cascode current mirrors and increased vector quantization bit rate

Focal-plane processing is the target of many studies due to its potential for enhancing the speed of the vision system flow. With focal-plane processing it is possible to perform parallel processing throughout the entire matrix. Usually, in vision systems, analog-to-digital conversion (ADC), transmission and storage represent a bottleneck. In order to alleviate these constraints, analog image compression is implemented at the focal-plane, thereby reducing the amount of data to be transmitted and the bandwidth requirements. The ADC is performed at the focal-plane as well, after the compression operation whose realization is based on differential pulse-code modulation (DPCM), linear transform and vector quantization (VQ) applied on every 4 × 4 pixel block using current-mode circuits. This paper presents experimental results obtained from the second-generation of an image sensor. The main contributions in comparison to the previous realization are: increase of the vector quantizer complexity, number of bits per pixel, pixel matrix size, and the use of cascode current mirrors in the linear transform matrix. The image sensor advanced in this paper was fabricated in a standard 180 nm CMOS process.