Perhaps you should try some spelling variations:
"Digital signal and image processing devices" » "Digital signal and image processing service", "Digital signal and image processing design"
"digital signal and image processing series" » "digital signal and image processing services", "digital signal and image processing queries", "digital signal and image processing species"

Try widening your search to All Fields.

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• If you are trying to use Boolean operators, they must be ALL CAPS: (("digital signal and image processing series") OR ("Digital signal and image processing devices")).
• Removing quotes may allow a broader search: (( digital signal and image processing series ) or ( Digital signal and image processing devices )).
• Adding a wildcard symbol may retrieve word variants: (("digital signal and image processing series") or ("Digital signal and image processing devices"))*.