Bandwidth

Several years ago, I worked on an imaging application. This system was to

capture data using a charge coupled device (CCD) image sensor. We were

capturing 1024 pixels per scan. We had to capture items moving 150 inches

per second at a resolution of 200 pixels per inch. Each pixel was converted with

an 8-bit ADC, resulting in 1 byte per pixel. The data rate was therefore

150 _ 1024 _ 200, or 30,720,000 bytes per second.

We planned to use the VME bus as the basis for the system. Each scan from the

CCD had to be read, normalized, filtered, and then converted to 1-bit-per-pixel

monochrome. During the meetings that were held to establish the system architecture,

one of the engineers insisted that we pass all the data through the VME

bus. In those days, the VME bus had a maximum bandwidth specification of 40

megabytes per second, and very few systems could achieve the maximum theoretical

bandwidth. The bandwidth we needed looked like this:

Read data from camera into system: 30.72Mbytes/sec

Pass data to normalizer: 30.72 Mbytes/sec

Pass data to filter: 30.72 Mbytes/sec

Pass data to monochrome converter: 30.72Mbytes/sec

Pass monochrome data to output: 3.84 Mbytes/sec

If you add all this up, you get 126.72 Mbytes/sec, well beyond even the theoretical

capability of the VME bus back then. More recently, I worked on a similar

imaging application that was implemented with digital signal processors (DSPs)

and multiple PCI buses, and one of the PCI buses was near its maximum

capability when all the features were added. The point is, know how much data

you have to push around and what buses or data paths you are going to use. If

you are using a standard interface such as Ethernet or Firewire, be sure it will

support the total bandwidth required