Advanced Digital Signal Processing Techniques For Image Processing In Remote Sensing
Amal G. Punchihewa
Seniro Lecturer
Department of Electronics & Telecommunication Engineering
University of Moratuwa
Director
Sri Lanka Television Training Institute
Abstract
This paper describes the evolution of the microprocessor architecture from Von Neumann architecture to advance processor architectures. Intensive signal processing for remotely sensed data can be performed quite speedily with special microprocessor architectures such as Harvard architecture. Until the Pentium processor come up with the features of Harvard architecture, all general purpose processors were based on Von Neumann architecture. Certain bottle necks in the Von Neumann architecture are overcome through the Pentium having two cache memories; one for codes and other the for data.
Introduction
In remote sensing, the data captured by the satellite have to be processed before they transform into a useable from. These were mainly done by software programmes that were written to process the data in a computer. Due to the high quantum of data and the extensive processing required, it takes quite a long time to output data in a useable from. Since data mainly contain on images, signal processing that is done is called image processing. This paper tries to explain the potential of computers that are based on general purpose processors which have high processing power.
Since the invention of Von Neumann architecture, we have been using this architecture for general purpose microprocessors for nearly three decades. During the last two decades special architectures were developed for very specific applications such as digital signal processing in the audio and video industry. However these were restricted only to those applications and not incorporated into general processors until the Pentium adopted certain of their features. These processors can be used to process image data from satellite.
1. Image Processing
To develop a map using remote sensed row data have to be processed using algorithms. These algorithms are software programmes which may be a filter, an edge detection or image enhancement. Due to large quantum of data per iamge will take a long time to process an image.
In image processing simple operations such as addition subtraction division by 2 etc. are carried out repeatedly. Therefore in signal processing smaller set of instructions are repeatedly used. However in a general purpose processor we find only one processing element in the microprocessor. This is a bottle neck in signal processing with processors of fourth generation and below.
2. Conventional Microprocessor Architecture
Conventional microprocessors use the Von Neumann architecture. This is the first and most popular architecture of microprocessors in use for the past three decades. It consists of three phases or cycles where "stored programmes" and data are retrieved, interpreted and executed. This architecture has so many bottle necks and most fast enough for I today's applications.
2.1 Phases or three cycles of the Von Neumann Architecture
A microprocessor based on the Von Neumann architecture has to go through three stags viz.
- Fetch
- Decode
- Execution
In the etch stage an instruction or data is taken into a register from the memory, which may be outside or inside the CPU. If is an instruction, then it is decoded to perform the required function Then the function is performed in the execution process.
2.2 Shortcomings in the Conventional & Von Neumann Architecture
The basic Von Neumann architecture has one serious disadvantage: everything happens consecutively. Before any one operations completed, many steps (often very many) have to be completed. There are many shortcomings or bottle necks in the Conventional Architecture. They are,
- Only one process (fetch, decode or execution) takes place at a clock cycle.
- Only one processing element (one ALU)
- Limited data paths
These will limit the processing power of the processor. The performance of a micro-processor depends on word length (number of bits in a data) clock speed (frequency), internal bus architecture and the interfaces to peripherals. Maximum clock speed depends on the fabrication technology. Execution time is used to assess the performance of a microprocessor. Microprocessor has an execution time,
Execution time = (Number of instructions x (Average number of cycles/instruction)x (cycle time)
