Patent claim: A method as in any of claims 8 to 12 in which a location and size of the region of the bitmap is specified by a user by a test and measurement instrument or by a standard.

Related passage: Accordingly embodiments of the present invention provide a test and measurement instrument that converts digital data that represents an input signal into a plurality of bitmaps and then subtracts one of the bitmaps from another one of the bitmaps to produce a difference bitmap.The difference bitmap does not contain density values that are common to the two bitmaps but instead only contains the density differences between the two thereby revealing very small density variations in the presence of large density values.In some embodiments the difference bitmap is displayed on a display device.In other embodiments the difference bitmap is used to generate a trigger signal.Further aspects of the invention relate to the following points1.A test and measurement instrument comprisingan analogtodigital converter for receiving an input signal and producing a digital signal a trigger detector for processing digital data from the digital signal by converting it into a plurality of bitmaps and subtracting one of the bitmaps referred to as a reference bitmap from another one of the bitmaps to produce a difference bitmap and for generating a trigger signal when the difference bitmap satisfies a userspecified trigger criterion and a memory for storing digital data from the digital signal in response to the trigger signal.2.A test and measurement instrument as in point 1 wherein the trigger detector converts the digital data into a series of bitmaps in realtime and subtracts the reference bitmap from each newly created bitmap.3.A test and measurement instrument as in point 2 wherein the trigger detector updates the reference bitmap.4.A test and measurement instrument as in point 3 wherein the trigger detector updates the reference bitmap in response to a user command.5.A test and measurement instrument as in point 3 wherein the processor updates the reference bitmap in response to a signal that is generated internally or externally to the test and measurement instrument.6.A test and measurement instrument as in point 3 wherein the trigger detector updates thereference bitmap periodically.7.A test and measurement instrument as in point 5 wherein the trigger detector updates the reference bitmap after each bitmap is created.8.A test and measurement instrument as in point 5 wherein the trigger detector updates the reference bitmap after every N bitmaps is created where N is a userspecified integer that is greater than one.9.A test and measurement instrument as in point 1 wherein the difference bitmap is displayed on a display device.10.A test and measurement instrument as in point 1 wherein the trigger detector uses the difference bitmap to perform a measurement.11.A test and measurement instrument as in point 1 wherein the trigger detector indicates when a signal appears or disappears within the difference bitmap.12.A test and measurement instrument as in point 1 wherein the test and measurement instrument is a test and measurement instrument selected from the group consisting of a realtime spectrum analyzer a swept spectrum analyzer a vector signal analyzer a logic analyzer and an oscilloscope.13.A test and measurement instrument as in point 1 wherein the difference bitmap satisfies the userspecified trigger criterion when a density difference within the difference bitmapexceeds a userspecified density threshold.