WAVEFORM DATA COMPRESSING CIRCUIT AND METHOD

Abstract: A data compression circuit has a compression mode of operation where it determines and stores minimum and maximum values of successive sequences of applied data words and provides an output com­ prising a pair of data words representing the stored minimum and maximum for each sequence, off­ set by a selected amount. The circuit also has a transparent mode of operation where output data sequences match input data sequences.

Description: The present invention relates in general to data compression circuits and in particular to a method and apparatus for reducing sequences of data characterizing sampled waveform magnitudes to data pairs indicating the minimum and maximum data in each sequence. A digital sampling oscilloscope measures the magnitude of a voltage waveform at regular intervals, converts the analog sample voltages to digital data, and then uses the digital data to control a display of the waveform on a screen. Typically such an oscilloscope is capable of displaying a fixed maximum number of sample data points along a portion of the waveform. To increase the portion of the waveform displayed (i.e. "compress" the waveform), the sampling rate is reduced. This results in a loss of waveform display detail since high frequency, high magnitude oscillations between sampling points will be overlooked. What would be useful would be a method and apparatus for compressing a waveform image without decreasing the sampling rate and without decreasing the ability to detect and indicate high frequency, high magnitude voltage swings. Prior art arrangements are disclosed in U.S. Patent No.4183087 (Heulsman), and IBM Technical Disclosure Bulletin, Vol.17, No. 7, December 1974. For example, a conventional waveform data compressor such as that described by Huelsman in U.S. Patent 4183087 generates a sequence of waveform data pairs in response to a succession of waveform data sequences. Each successive waveform data pair indicates a minimum and a maximum data value of a separate one of the input waveform data sequences. Each separate waveform data sequence may, for example, have been obtained by digitizing an input signal during a separate sampling period. When an input signal is monotonically increasing or decreasing through two sampling periods, successive data pairs produced by such conventional compressors do not overlap. Thus, a display produced in accordance with a set of non-overlapping data pairs by the aforementioned vertical raster scan oscilloscope appears discontinuous . Summary of the Invention According to the invention, there is provided in a vertical raster scan display system in which maximum and minimum data pairs representing a waveform data sampling are input for providing successive bars of a waveform display on display means, a method for generating a pair of data words characterizing a sequence of data words, the method including the steps of replacing a first data word (DATA X) in a first register (48) with a next data word of the sequence if the next data word is larger than the first data word and of replacing a second data word (DATA Y) in a second register (54) with the next data word of the sequence if the next data word is smaller than the second data word, wherein the replacing steps are characterized by: replacing the first data word with the next data word of the sequence if the next data word or the first data word is a selected code of a set of codes representing specific predetermined magnitude conditions; and replacing the second data word with the next data word if the next data word or the second data word is a selected one of said codes; whereby after the processing of all the data words in the sequence the first data word is either a maximum data word of the sequence or one of said codes and the second data word is either a minimum data word of the sequence or one of said codes; and generating a selection signal having either a first or a second state, said selection signal determining whether said data word in said first register is a maximum data word when in said first state or a minimum data word when in said second state and determining whether said data word in said second register is a minimum data word when in said first state or said maximum data word when in said first state or a maximum data word when in said second state; changing said state of said selection signal while maintaining last data words in said first and second registers at the end of processing of each sequence of a plurality of sequences; and repeating all said steps for each of said plurality of sequences. Also according to the invention, there is provided in a vertical raster scan display system in which maximum and minimum data pairs representing a waveform data sampling are input for providing successive bars of a waveform display on display means, an apparatus for generating a pair of data words (DATA X, DATA Y) that characterizes a sequence of data words (DATA IN) including first and second storing means (48, 54) for storing a pair of data words and means (44, 46, 52, 58) for replacing the pair of data words with a maximum and a minimum data word from the sequence, wherein the replacing means is characterized by means (64, 52, 58) for replacing the data words in the first and second storing means with a next data word of the sequence if the next data word or the data word is a selected code of a set of codes representing specific predetermined magnitude conditions, such that after the processing of all the data words in the sequence the data word in the first storing means is either the maximum data word of the sequence or one of said codes and the data word in the second storing means is either the minimum data word of the sequence or one of the codes, and further characterized by: means (42) for generating a selection signal (MSEL) having one of two states, said selection signal determining whether the data words in the first and second storing means are the maximum data word or the minimum data word, respectively, or the minimum data word or the maximum data word, respectively, according to the state of the selection signal and means (16, 42) for changing the state of the selection signal at the end of processing of each of a plurality of sequences while maintaining as the first and second data words in the first and second storing means for each sequence the data words in the first and second storing means at the end of the prior sequence. It is therefore an object of the present invention to provide a new and improved method and apparatus mapping a series of a selected number of acquired data words into a pair of data words representing the minimum and maximum data words of the series. The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. Drawings FIG. 1 is a block diagram of a sampling oscilloscope system employing a waveform compressor circuit according to the present invention; FIG. 2 is an illustration of a typical waveform to be sampled by the oscilloscope system of FIG. 1; FIG. 3A is an illustration of an image of the waveform of FIG. 2 as displayed by the oscilloscope system of FIG. 1; FIG. 3B is an illustration of an non-vectored image of the waveform of FIG. 2; FIG. 3C is an illustration of a vectored image of the waveform of FIG. 2; FIG. 4 is a block diagram of the waveform compressor circuit of FIG. 1; FIG. 5 is a block diagram of the minimum/maximum detection circuit of FIG. 4; FIG. 6 is a block diagram of the state machine of FIGS. 5 and 7; and FIG. 7 is a block diagram of the offset circuit of FIG. 4. Detailed Description Referring to FIG. 1, there is depicted in block diagram form, an oscilloscope system 10 employing a data compressor circuit 11 according to the present invention adapted to permit digitized waveforms to be displayed on screen 20 of a vertically scanned cathode ray tube (CRT) 22. A digitizer 14 samples and digitizes an analog voltage waveform at regular intervals and transmits digitized waveform data obtained to a memory management unit 15 which stores the sample data in a random access waveform memory 18. A typical sampled waveform is depicted in FIG. 2. The voltage of the waveform is scaled along the vertical axis while time is scaled along the horizontal axis. Vertical grid lines bound sampling intervals while horizontal grid lines represent measurable voltages. For purposes of illustration, the digitizer 14 is assumed to have a measurement resolution of 1 mV and a sampling interval of 1 mS although other resolutions and sampling intervals may be used. The waveform may be sampled several times during each sampling interval. In the instant case, the digitizer output data at, for instance the beginning of sampling interval 1, indicates that the magnitude of the waveform is 11 mV, while the digitizer output data at the beginning of sampling interval 2 would be 13 mV. An image of the sampled waveform of FIG. 2 is displayed as being displayed on a cathode ray tube (CRT) 22 screen 20 of FIG. 1. A portion of the screen is illustrated in FIG. 3A. The screen is divided into rows and columns of pixels, each pixel column corresponding to one sampling interval and each higher pixel row corresponding to a higher waveform magnitude. The CRT is of the vertical raster scan type wherein an electron beam is rapidly scanned in the vertical direction (to provide the raster scan) and more slowly scanned in the horizontal direction (the field scan), so that pixels are scanned one column at a time. When the beam is at the bottom of a column, it is off and does not illuminate a currently scanned pixel. As the beam scans towards the top of a column it reaches a pixel row corresponding to the smaller of two data words characterizing the waveform magnitude range during the sample interval corresponding to the vertical raster line. At this point the beam is set to illuminate scanned pixels. When the beam reaches the pixel row corresponding to the higher of the two data words, the beam turns the pixels off. When this procedure is repeated for every vertical scan, the resulting image appears as a series of vertical lines as in FIG. 3A. Referring again to FIG.1 , the scanning rate and resolution of the digitizer 14 is controlled by an executive processor 16 in response to input from a user. Waveform memory 18 stores the sample data output of the digitizer 14. The memory management unit 15 then transfers the data to a compressor circuit 11 of the present invention which determines the minimum and maximum values of the data during any sampling period and then transmits the minimum/maximum data pairs to a display controller 24. The display controller 24 stores successive minimum/maximum sample data pairs corresponding to successive sampling intervals in a vertical raster scan (VRS) memory 26 prior to a screen 20 refresh operation. During a refresh operation, controller 24 causes the random access VRS memory 26 to transmit a minimum/maximum data pair to a VRS controller 12 prior to each vertical scan. VRS controller 12 generates a VIDEO OUTPUT control signal to a CRT driver 30 which controls the vertical and horizontal scanning of the beam in CRT 22. The VIDEO OUTPUT control signal from the VRS controller 12 tells the CRT driver when to turn on and off the beam during each vertical scan, according to the minimum and maximum values of the data pair associated with the scan. During a data compression and transfer operation, compressor circuit 11 transmits an MSEND signal to the memory management unit (MMU) 15 each time it is ready to receive data from the MMU 15. Once the MMU has transmitted the data it generates an MLATCH signal to the compressor circuit 11, causing it to store the new data and to begin comparing it to previously transmitted data. If the magnitude of the new data exceeds the magnitude of any previous data during a current sampling interval, the new data is stored in an internal register as the new maximum data for the sample period. If the magnitude of the new data is less than the magnitude of any previous data obtained during a current sample interval, the new data is stored in another register as the new minimum data for the sampling interval. Compressor circuit 11 includes means to count the number of data points received during a sampling interval. When the number of digitized data points received equals a "compression factor" (CF) limit, set by data previously transmitted to compressor circuit 11 from the executive processor 16 according to user input, the compressor circuit 11 stops requesting more data points from the MMU 15 and retains the current sample period minimum and maximum data in its internal registers until display controller 24 transmits a DSEND signal to the compressor circuit 11, indicating that it is ready to receive a new data pair. The compressor circuit 11 then transmits the data stored in the internal registers as the minimum/maximum data pair for the sampling interval, asserting a DLATCH output signal to the display controller 24 when the data pair has been sent. The compressor circuit 11 then may reset its internal registers and generate an MSEND signal to the MMU 15 to acquire the first data point of the next sampling period. When the internal registers are reset after each sampling period, the display on screen 20 may have vertical gaps between successive vertical raster lines in a waveform image if the minimum sample data of one sampling period exceeds the maximum sample data acquired during the preceding sampling period. A gap will also appear in the display if the maximum sample data obtained during a sampling period is less than the minimum sample data obtained during the preceding sampling period. In FIG. 3A a high sampling rate is used and these gaps do not appear. FIG. 3B is an example of how the image of the waveform of FIG. 2A would appear on screen 20 if the digitizer sampled the waveform only once during each interval at the beginning of the interval. The waveform compressor circuit 11 is therefore adapted to eliminate these gaps when a VECTOR signal from the executive processor is asserted. Receipt of a VECTOR signal causes the compressor circuit 11 to retain the maximum value obtained during one sampling period as the first minimum value for the next sampling period and to retain the minimum value obtained during one sampling period as the first maximum value for the next sampling period. This ensures that minimum/maximum data pairs associated with successive intervals will overlap. FIG. 3C illustrates how the gaps in the image of FIG. 3B would be filled in when compressor circuit 11 is operating in a "vector" mode. The compressor circuit 11 is also adapted to increase or decrease the magnitudes of the minimum and maximum data by a selected amount prior to transmitting the pair to display controller 24. The amount of the offset is controlled by OFFSET data transmitted to the compressor/offset from the executive controller 16 and stored therein prior to data acquisition. This offset permits a convenient means for controlling the relative vertical position of the waveform image on screen 20. It may happen that the waveform magnitude is outside the digitizing range of the digitizer 14. If the waveform magnitude is too large in a positive sense, the digitizer generates a special overrange code instead of the digitized waveform data. If the waveform is too large in a negative sense, the digitizer generates a special underrange code. The compressor circuit 11 checks incoming data for underrange and overrange codes and stores these codes as minimums and maximums. If, at the end of a sampling interval, the minimum or maximum sample data stored by the compressor circuit is an overrange or an underrange code, the compressor circuit 11 transmits the overrange or underrange code to the display controller 24 without increasing or decreasing it by the offset amount. The display controller 24 can recognize the overrange and underrange code and generate inhibit bits to be stored in VRS memory 26 which cause VRS controller 28 to appropriately clip the waveform image display in the portions corresponding to the overrange and underrange sections of the waveform. At the beginning of each waveform data acquisition a unique "null" code is stored in each waveform memory 18 location by the executive controller. As sampling occurs, the stored null codes are replaced by digitizer 14 output data. During a data compression operation, the compressor circuit 11 checks incoming data to determine if it is the null and transfers the null to the display controller 24 as a minimum or a maximum under appropriate conditions. The VRS controller 12 may then suppress display of the raster line associated with a display period if either the minimum or maximum data words is the null code. A C/T signal is applied to compressor circuit 11 from the executive processor 16. If the C/T signal is not asserted, if the offset data is set to zero, and if the CF data is set to one, the compressor circuit 11 will operate in a "transparent mode" wherein it will pass data from the MMU 15 to the display controller 24 without modification. This feature is particularly useful when the compressor circuit 11 is used in conjunction with an oscilloscope system 10 as in Fig. 1, wherein graphics such as a grid may also be displayed on screen 20 superimposed on the waveform image. The graphical display is bit-mapped by data stored in a conventional bit map circuit 31. The bit map data is transmitted to the VRS controller 12 as the screen is scanned and the VRS controller uses the bit map graphics data in conjunction with waveform data from VRS memory 26 in controlling the VIDEO OUTPUT signal to create the superimposed image. Bit map data is generated by executive processor 16 in response to user input and then transmitted through MMU 15 and compressor circuit 11 (in the transparent mode) to display controller 24. Display controller 24 then transmits the data for storage in bit map 31 at the appropriate address locations. The compressor circuit 11 of FIG. 1, shown in more detail in FIG. 4 in block diagram form, comprises a minimum/maximum (M/M) circuit 32, an offset circuit 34 and a clock pulse generator 36 for generating a clock signal to synchronize the operation of the M/M and offset circuits. The incoming data DATA IN from the MMU 15 is compressed by the circuit 32 to a maximum/minimum data pair, DATA X and DATA Y, and applied to the offset circuit 34. The M/M circuit 32 then asserts an ADDLCH signal to the offset circuit 34 to latch the data pair into registers therein. The offset circuit 34 adds the appropriate OFFSET value to each member of the data pair and then, on receipt of a DSEND signal from the display controller 24 indicating the controller is ready to accept data, transmits the data sequentially over a DATA OUT bus to the display controller, asserting a DLATCH signal to the display controller to indicate that data has been transmitted and an adder available (ADDAV) signal to the M/M circuit 32 to indicate that it is ready to receive another data pair. The M/M circuit 32 also generates NULL, OVER, UNDER and C/T bits associated with each word of the minimum/maximum data pair and transmits the indicating bits to the offset circuit 34 with the data pair as four bit FLAG X and FLAG Y words. If the NULL bit is set, indicating that the stored minimum or maximum data is the NULL code, the offset circuit 34 transmits the NULL code to the display controller 24 as the minimum or maximum regardless of the offset value of the data word. If either the OVER or UNDER indicating bit is set, indicating that the associated stored minimum or maximum data comprises over or under range codes, the offset circuit 34 transmits the over or under range code as the minimum or maximum data, regardless of offset value of the data. If the C/T bit is set, the unmodified DATA X word is sent to the display controller, regardless of the state of the other flag bits, while the DATA Y word is ignored. The M/M circuit 32 is depicted in more detailed block diagram form in FIG. 5. The input data is latched into an input register 40 by assertion of the MLATCH signal from the MMU 15. The MLATCH signal is also applied as an input to a compressor state machine 42 which controls the sequencing of the compression circuit 32. The data stored in input register 40 is applied to an X comparator circuit 44 and to a Y comparator circuit 46. If the new data in register 40 exceeds the value of a LAST X data word, previously stored in an X1 register 48, the X comparator 44 asserts an output signal on line 50 connected to an input of an X latch decoder 52. Similarly, if the new data word stored in input register 40 exceeds the value of a LAST Y data word stored in a Y register 54, the Y comparator 46 asserts an output signal on line 56 applied to an input of a Y latch decoder 58. The X and Y latch decoders 52 and 58 each have two outputs, MIN and MAX. The two outputs of the X latch decoder 52 are applied to two inputs of an X multiplexer 60 while the two outputs of the Y latch decoder are applied to two inputs of a Y multiplexer 62. The output of the X multiplexer 60 is applied to a load control input of X1 register 48 while the output of the Y multiplexer is applied to a load control input of the Y1 register 54. The switching states of the X and Y multiplexers are controlled by an applied MSEL signal from the compressor state machine 42. The sample data stored in input register 40 is transmitted to the input terminals of both the X1 and Y1 registers 48 and 54 and is stored therein whenever the output signal from the associated X or Y multiplexer 60 or 62 is asserted. If, during current data compression cycle, the MIN output of the X latch decoder 52 controls loading of data into the register X1, then the X register will store the current input data from input register 40 if the input data is less than the LAST X data stored in the X1 register 48. Conversely, if the MAX output of X latch decoder 52 controls the X1 register loading, then the new sample data in register 40 will be stored in the X1 register if it is larger than the currently stored LAST X data. Thus the data stored in X1 register 48 may represent either the maximum or the minimum sample data obtained during the current compression cycle depending on the switching state of multiplexer 60. Latch decoder 58 operates in a similar fashion so the LAST Y data stored in Y register 54 may also represent either the maximum or the minimum sample sample data obtained during the current compression cycle, depending on the switching state of Y multiplexer 62. The states of X multiplexer 60 and Y multiplexer 62 are oppositely switched so that if LAST X is the minimum, then LAST Y is the maximum, and vice versa. The state machine switches the state of the MSEL control signal after each data compression cycle, corresponding to one sampling interval. Since the MSEL signal changes state after each data compression period, the states of the X and Y multiplexers also change so that during one compression cycle the MIN output of the X latch decoder controls the loading of data into the register X1 while the MAX output of the Y latch decoder controls the loading of data into the register Y1, and during a next data compression cycle, the MAX output of the X latch decoder controls the loading of data into the register X1 while the MIN output of the Y latch decoder controls the loading of data into the register Y1. When the VECTOR signal is not asserted, state machine 42 asserts an MMUND signal, applied to inputs of decoders 52 and 58, when the first data of a sampling interval is stored in register 40. The asserted MMUND signal causes both decoders to output the appropriate minimum or maximum signal so that the associated X1 and Y1 registers store the first data of the sequence regardless of the value of the last data stored therein. The compressor circuit thus operates in the "non-vector" mode. If the VECTOR signal is not asserted, state machine 42 does not assert the MMUND signal at the start of each compression cycle and the first data word is not automatically stored in the X1 and Y1 registers. Thus during each compression cycle, the new data must be less than the maximum of the previous conpression cycle in order to be stored as a new minimum, and must be larger than the previous minimum to be stored as the new maximum, thereby eliminating the possibility of gaps between successive minimum/maximum data pairs by ensuring one point of overlap. The number of sample data points to be included in a compression cycle is determined by the compression factor CF data from the executive processor 16 applied to an input of a counter 66 of FIG. 5. Counter 66 stores the CF data on receipt of a CDATA signal generated by the compressor state machine 42 prior to the start of a compression cycle. When the M/M circuit 32 is ready to receive another data word from the MMU 15, the compressor state machine 42 transmits the MSEND signal to the MMU and also to a count input of the counter 66 so that the counter 66 counts MSEND signals. When the number of MSEND signals, and therefore the number of input data words received, equals the CF count limit, the counter 66 transmits an end of count (EOC) signal to the state machine 42, indicating the end of the compression cycle. The new sample data stored in input register 40 is also applied to a flag decoder 64 which determines if the new data matches the overrange, underrange, or null codes and generates a FLAG data word comprising the appropriately set OVER, UNDER and NULL bits. The FLAG data word is transmitted to other inputs of the X1 and Y1 registers 48 and 54 where it is stored as a FLAG X or FLAG word Y if the associated new input data word is also stored as the DATA X or DATA Y minimum or maximum word. The FLAG data word from the flag decoder 64 and the current FLAG X data word stored in X1 register 48 are both applied to additional inputs to X latch decoder 52 along with a LATCH signal, an ALLNUL signal, and the MMUND signal from the compressor state machine 42. The EOC signal from the counter 66 is also applied to the decoder. Similarly, the FLAG data word from the flag decoder 64 and the current FLAG Y data word stored in the Y1 register 54 are both applied to additional inputs of Y latch decoder 58 along with the LATCH, ALLNUL, and MMUND signals from the compressor state machine 42, and the EOC signal from the counter 66. The state machine 42 generates the LATCH signal after receiving the MLATCH signal from the MMU indicating new data is stored in input register 40. The MIN output of the X latch decoder 52 is asserted when the X latch decoder receives the LATCH signal from the compressor state machine 42, but only under the following input conditions: 1) the NULL bit of the FLAG word is false, and the OVER bit of the FLAG X word is true; 2) the NULL bit of th FLAG X word is true; 3) the UNDER bit of the FLAG word is true; 4) the UNDER bit of the FLAG X word and the NULL and OVER bits of the FLAG word are false and the output of the X comparator is true; 5) the EOC bit and ALLNUL bits are true; or 6) the MMUND signal from the state machine is true. In case (1) the new data word is stored in X1 register 48 as the new minimum if it is not null and the currently stored minimum word is overrange. It does not matter whether the new data word is overrange, underrange, larger or smaller than the currently stored minimum word in the X1 register. In case (2) the new data word is stored as the new minimum if the currently stored minimum word is null. In case (3) the new data word is stored as the minimum if it is underrange. It does not matter if it is less than or greater than the currently stored minimum. In case (4) the new data is stored in the X1 register as a minimum if it is less than the currently stored minimum, and is not null or overrange, as long as the current minimum is not underrange. The NULL bit of the FLAG word is applied to another input of the compressor state machine 42. As long as all of the data words received during a compression cycle are null, the ALLNUL output signal of the state machine remains high. Therefore, in case (5) the new data is stored in the register X1 as a minimum if it is the last data received during a compression cycle (i.e. EOC is true) and all of the other data words received during the compression cycle were also null (i.e. ALLNUL is true). This ensures that the FLAG X word contains a true NULL bit at the end of the compression cycle. The state machine 42 continues to assert the MMUND signal until the first data for the first compression cycle has been stored in the X1 and Y1 registers. An asserted MMUND signal at the start of the first compression cycle indicates that the contents of the X1 and Y1 registers are undefined and should be replaced. This feature is necessary in the preferred embodiment because there is no provisions to reset the registers. As indicated hereinabove, when operating in the nonvector mode (i.e. VECTOR is false), the state machine 42 asserts the MMUND signal at the start of every compression cycle in order to prevent automatic data overlap between successive data pairs. Therefore, in case (6) the X1 register stores the new data word as the current minimum if it is the first word of the first compression cycle, regardless of operation mode, and stores the new data word if it is the first data word of any compression cycle if the system is operating in the nonvector mode, thereby insuring that the previously stored data is overwritten. If the X1 register is storing maximum data, the MAX output of the X latch decoder 52 is asserted, causing the X1 register to store the current data when the X latch decoder receives a LATCH signal from the compressor state machine 42, but only under the following input conditions: 7) the NULL bit of the FLAG word is false, and the UNDER bit of the FLAG X word is true; 8) the NULL bit of the FLAG X word is true; 9) the OVER bit of the FLAG word is true; 10) the OVER bit of the FLAG X word and the NULL and UNDER bits of the FLAG word are false and the output of the X comparator is false; 11) the EOC bit and ALLNUL bits are true; or 12) the MMUND signal from the state machine is true. In case (7) the new data word is stored as a maximum if it is not null and the current maximum word is underrange. In case (8) the new data word is stored as the new maximum if the currently stored maximum word is null. In case (9) the new data word is stored as the current maximum for the compression cycle if it is underrange. In case (10) the new data is stored in the X1 register as the maximum if it is greater than the currently stored maximum and is not null or underrange, provided that the currently stored maximum is not overrange. In case (11) the new data is stored in the X1 register as the maximum if it is the last data received during a compression cycle and the all of the other data words of the cycle were null. Finally, in case (12) the new data word is stored in the X1 register if it is the first data received during the first compression cycle of a sampling operation, or when the compressor 11 is operating in a nonvector mode, if the new data word is the first data received during any compression cycle. The MIN and MAX outputs of the Y latch decoder 58 behave in a similar fashion to the MIN and MAX outputs of the X latch decoder 52 as described hereinabove with the substitution of the FLAG Y indicator bits and the Y comparator output for the FLAG X indicator bits and X comparator output. In the preferred embodiment of the present invention, each X and Y latch decoder is suitably implemented using a read only memory addressed by the decoder inputs, the output MIN and MAX signals being controlled by two data bits stored at each ROM address. The stored ROM data is selected to achieve the appropriate input/output characteristics when read enabled by the LATCH signal. State machine 42, detailed in block diagram form in FIG. 6, comprises a read only memory 70 addressed in part by the input data, and a register 72 for storing currently addressed read only memory data when a clock input of the register is strobed. The data stored in the register 72 is reapplied to the ROM 70 address input terminals as the remaining portion of the ROM 70 address. Each bit of the data stored in register 72 comprises one of the state machine output signals. The data stored at the various addresses in ROM 70 is such that the current ROM 70 output data is appropriate for a given combination of current input data and current state data. The offset circuit 34 of FIG. 4, depicted in block diagram form in FIG. 7, comprises an X2 register 90 for storing the FLAG X and DATA X words and a Y2 register 92 for storing the FLAG Y and DATA Y words from the M/M circuit 32 of FIG. 4. The ADDLCH signal from M/M circuit 32 is applied to the storage control terminals of both registers and also to an input of an offset circuit state machine 94 similar in topology to the state machine 42 of M/M circuit 32 as shown in FIG. 6. The DATA X data stored in register X2 is applied to one input of a multiplexer 96 while the DATA Y data stored in the Y2 register is applied to another input of the multiplexer. The output of multiplexer 96 is connected to one input of an adder 98 while the OFFSET data from the executive processor 16 of FIG. 1 is applied to another input of the adder. The adder 98 sums the output of multiplexer 96 and the OFFSET data and transmits the result to an input of another multiplexer 100. After the M/M circuit 32 sends an ADDLCH signal to the offset circuit state machine 94, the state machine transmits a SEL output signal to the switch control input of multiplexer 96 switching the multiplexer to a state wherein the DATA X word stored in X2 register 90 is passed to the adder 98. Adder 98 then adds the OFFSET data to the DATA X word and passes the result to multiplexer 100. If the DATA X word is not null, overrange, or underrange, multiplexer 100 passes the offset DATA X word to a buffer 102. The DSEND signal from the display controller 24 is applied to another input of offset state machine 94 and when the signal is asserted, indicating that the display controller is prepared to receive more data, the offset circuit state machine 94 generates a DATAOE signal to output enable the buffer 102. Buffer 102 then transmits the offset DATA X word to the display controller over the DATA OUT bus. Offset state machine 94 then transmits the DLATCH signal to the display controller to indicate data transmission has occurred. After the offset DATA X word is sent, the state machine 90 changes the state of the SEL output signal to change the switching state of multiplexer 96 so that the multiplexer transmits the DATA Y word from Y2 register 92 to the adder 98. The adder 98 then adds the OFFSET data to the DATA Y word. The sum, if the DATA Y word is not null, overrange or underrange, is then transmitted through multiplexer 100 and buffer 102 to the display processor 24 following the offset state machine's receipt of another DSEND signal from the display processor. The FLAG X and FLAG Y data stored in the X2 and Y2 registers are applied to separate inputs of another multiplexer 104 and the output of multiplexer 104 is applied to an input of an output select decoder 106. The most significant bit (bit 15) of the adder 98 output, of the OFFSET data, and of the multiplexer 96 sum output are also applied to output select decoder 106 inputs along with a CARRY signal from the adder 98. The CARRY signal indicates when the adder output sum has exceeded the adder capacity. A set of data codes NULL, OVER, and UNDER, are applied to three other inputs of multiplexer 100 and a two bit output of output select decoder 106 controls the switching state of the multiplexer. The output select encoder 106 controls the switching state of multiplexer 100 as follows: The NULL, OVER, and UNDER codes indicate to the display controller 24 that the minimum or maximum data word of the current pair is null, overrange or underrange. When the display processor 26 of FIG. 1 receives data from the compressor circuit 11, it can check to see if it indicates that the minimum or maximum data word is overrange, underrange or null and can add appropriate indicating bits to the word prior to storing it in VRS memory 26. VRS controller 12 may then use these indicating bits to determine whether or not to suppress or clip the vertical raster line corresponding to the data. Referring again to FIG. 5, the C/T signal from the executive processor 16 is applied as another input to flag decoder 64. When the C/T signal is asserted, the compression circuit operates to compress data 65 described hereinabove. However when the C/T signal is not asserted, the X and Y latch decoders 52 and 58 transmit the appropriate MIN or MAX signal to the X and Y multiplexers 60 and 62 to cause the X1 and Y1 registers both to store every data word sent to input register 40 regardless of its value. If the CF data defining the count limit is preset to one, the data stored in X1 and Y1 registers 48 and 54 is passed on to X2 and Y2 registers 90 and 92 of FIG. 7 without compression. The C/T signal is also applied to state machine 94 of FIG. 7 and when the C/T signal is not asserted, the SEL signal does not change state after each compression cycle so that the magnitude data in X2 register 90 is always passed through MUX 96 and ADDER 98 to MUX 100 and the flag data in X2 register 90 is always passed through MUX 104 to decoder 106, the data in Y2 register 92 being ignored. The flag data applied to decoder 106 includes a bit indicating that C/T is low and causes decoder 106 to maintain multiplexer 100 in a switching state wherein the sum data from adder 98 is passed to buffer 102 regardless of the states of the other FLAG bits. Thus if the OFFSET data is set to zero, the CF data is set to one, and the C/T signal is not asserted, the compressor circuit operates on the transparent mode and graphics or other data from the MMU 15 will pass through the compressor circuit 11 without compression or offset. It is therefore seen that the waveform compressor circuit 11 of the present invention provides rapid sample data compression for a vertical raster scan display system by determining the minimum and maximum values of sequences of applied data and sequentially outputting a selectively offset minimum and maximum pair to the display system, according to either the vector or non-vector mode of operation. The sequence length is adjustable. Further, if the minimum or maximum is overrange, underrange, or null the corresponding word of the output data pair comprises a special underrange, overrange, or null indicating code. The compressor circuit 11 may also be placed in the transparent mode where data is passed through without modification.

Claims: In a vertical raster scan display system in which maximum and minimum data pairs representing a waveform data sampling are input for providing successive bars of a waveform display on display means, a method for generating a pair of data words characterizing a sequence of data words, the method including the steps of replacing a first data word (DATA X) in a first register (48) with a next data word of the sequence if the next data word is larger than the first data word and of replacing a second data word (DATA Y) in a second register (54) with the next data word of the sequence if the next data word is smaller than the second data word, wherein the replacing steps are characterized by; replacing the first data word with the next data word of the sequence if the next data word or the first data word is a selected code of a set of codes representing specific predetermined magnitude conditions; and replacing the second data word with the next data word if the next data word or the second data word is a selected one of said codes, whereby after the processing of all the data words in the sequence the first data word is either a maximum data word of the sequence or one of said codes and the second data word is either a minimum data word of the sequence or one of said codes; and generating a selection signal having either a first or a second state said selection signal determining whether said data word in said first register is a maximum data word when in said first state or a minimum data word when in said second state and determining whether said data word in said second register is a minimum data word when in said first state or a maximum data word when in said second state; changing said state of said selection signal while maintaining last data words in said first and second registers at the end of processing of each sequence of a plurality of sequences; and repeating all said steps for each of said plurality of sequences. A method as recited in claim 1 where the set of codes include an overrange code, an underrange code and a null code wherein the replacing steps comprise the steps of: (b) replacing the data word stored in said first register with said next data word under any one of the following conditions: b1. said next data word is not said null code and said data word stored in said first register is said underrange code: b2. said data word stored in said first register is said null code: b3. said next data word is said overrange code: b4. said next data word is larger than said data word stored in said first register, said next data word is not said null or underrange code, and said data word stored in said first register is not said overrange code; or b5. said next data word is a last data word of said sequence and each data word of said sequence was said null code; and (c) replacing the data word stored in said second register with said next data word under any one of said following conditions: c1. said next data word is not said null code and said data word stored in said second register is said overrange code; c2. said data word stored in said second register is said null code; c3. said next data word is said underrange code; c4. said next data word is less than said data word stored in said second register, said next data word is not said null or overrrange code, and said data word stored in said second register is not said underrange code; or c5. said next data word is said last data word of said sequence and each data word of said sequence was said null code. A method as in claim 2 wherein said data word stored in said first register is also replaced with said next data word in step (b) under said following condition: b6. an applied transparent mode indicating signal is not asserted, said mode indicating signal being asserted when said data word pair is to be generated and being not asserted when each data word of said sequence is to be successively stored in said first register irrespective of its value and irrespective of said data word stored in said first register. In a vertical raster scan display system in which maximum and minimum data pairs representing a waveform data sampling are input for providing successive bars of a waveform display on display means, an apparatus for generating a pair of data words (DATA X, DATA Y) that characterizes a sequence of data words (DATA IN) including first and second storing means (48, 54) for storing a pair of data words and means (44, 46, 52, 58) for replacing the pair of data words with a maximum and a minimum data word from the sequence, wherein the replacing means is characterized by means (64, 52, 58) for replacing the data words in the first and second storing means with a next data word of the sequence if the next data word or the data word is a selected code of a set of codes representing specific predetermined magnitude conditions, such that after the processing of all the data words in the sequence the data word in the first storing means is either the maximum data word of the sequence or one of said codes and the data word in the second storing means is either the minimum data word of the sequence or one of said codes, and further characterized by: means (42) for generating a selection signal (MSEL) having one of two states, said selection signal determining whether the data words in the first and second storing means are the maximum data word or the minimum data word, respectively, or the minimum data word or the maximum data word, respectively, according to the state of the selection signal and means (16, 42) for changing the state of the selection signal at the end of processing of each of a plurality of sequences while maintaining as the first and second data words in the first and second storing means for each sequence the data words in the first and second storing means at the end of the prior sequence. An apparatus as recited in claim 5 wherein the alternate replacing means is characterized by: means (64) for generating an underrange indicating signal if the next data word is an underrange code representing a magnitude less than a lower limit, and overrange code representing a magnitude value greater than an upper limit, and a null indicating signal if the next data word is a null code representing no magnitude value; and means (52, 58) for decoding the code from the generating means together with the pair of data words to generate a storage indicating signal to store the next data word in either the first or second storing means on occurrence of a selected combination of overrange, underrange and null indicating signals and of magnitudes of the pair of data words, and a maximum indicating signal from said third comparing means; and said fourth replacing means comprising second decoding means (58) for generating a second indicating signal to store said next data word in a said second means on occurrence of a selected combination of overrange, underrange and null indicating signals, and a minimum indicating signal from said fourth comparing means. An apparatus as recited in claim 6 further comprising means for outputting (34) said data word pair from said first and second storing means, a first data word of said data word pair being either said overrange, said underrange or said null code if said data stored in said first storing means is said overrange, said underrange or said null code, otherwise being said maximum data word plus a selected offset value, and a second data word of said data word pair being either said overrange, said underrange or said null code, if said data stored in said second storing means is said overrange, said underrange or said null code, otherwise being said minimum data word plus said selected offset value.

Inventors: BATSON, BRIAN E., FLADSTOL, GARY R.
Applicants: TEKTRONIX, INC.
Application year: 1986
Main classification: G06F7
IPC codes: H03M7