5.2 Device selection
Image sensor selection
Whilst deciding to use a Blackfin DSP, a suitable image sensor was also considered. CCD image sensors and CMOS sensors were considered. CCD sensors traditionally have better noise performance compared with CMOS sensors  but require an extra stage to convert their analog image data to digital data so that it can be used by the board’s processor. Due to this CMOS sensors were favoured over CCDs.
ptina Imaging (formerly Micron ) produce a range of sensors that were of interest. The
MT9V022 sensor  is one of these and is a 752 x 480 Wide VG
log Devices publish an application note  that discusses interfacing this sensor to their
Blackfin chips and this was an important factor in the decision to use this sensor.
importance is the relatively high frame rate of 60 fps.
t the time of selection many chips
provided 15 or 30 fps output but fewer output at 60 fps or higher.
greater frame rate reduces the processing requirements for the tracking algorithms. This because the marker images cannot move as far between frames and so smaller ROIs can
be used to produce the same tracking performance.
For example, an increase in frame
rate of 2× means that a marker can only move half as far per frame.
This means a ROI
of half the width of the original ROI can be used.
lthough this results in twice as many
ROIs to process over time, the number of pixels to analyse is lower. This is because the number of pixels in a ROI is equal to the square of the width of the ROI and the width of a ROI required to give equivalent performance decreases proportionally with the frame rate.
nother important feature of the sensor is that it implements a global shutter. Essentially, a global shutter means that all of the data in an image frame is available to the processor at a given time. In a rolling shutter system, data from the current frame can be mixed with data from a previous frame leading to a tearing or wobbling effect during high motion. These artefacts could corrupt data from the tracking system and, therefore, it is important that a
global shutter is used.
t the time of selection many chips implemented a rolling shutter.
The MT9V022 sensor is available without a Bayer colour filter thereby allowing gray scale images to be captured with complete intensity data for each pixel. This is important in this application as a colour filter could warp the data presented to the centroid algorithm and colour data is not required. Many other models were not available in monochrome.
later model of the image sensor, the MT9V023 , is also available.
Both sensors are
similar but initially
ptina marketing material indicated that the later model would re-
quire approximately half as much power as the older model (this claim was later revoked).