Abstract:
A method for generating a variable delay of a signal, including: providing a clock indicating a sequence of sample times at regular intervals and receiving a sequence of input samples representing input values of the signal at respective sample times indicated by the clock. The method further includes determining the delay with a temporal resolution substantially finer than the clock interval to be applied to the signal at each of the respective sample times. For each of the sample times, responsive to the respectively-determined delay, one or more of the input samples are processed so as to generate a corresponding output sample representing a delayed output value of the signal at the sample time.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to signal processing systems, and specifically to devices for generating signal delays of controlled, variable duration. 
     II. Related Art 
     The sophistication of communication systems is rapidly increasing. For example, a communication system may include an array of satellites which serve as relay stations. In developing an apparatus which uses the array of satellites, it is desirable to test the apparatus during development without placing the satellites in orbit. In such a case it is desirable to have a simulator which simulates the operation of the array of satellites. A major component of such a simulator is a delay generator which accurately delays signals for calculated periods. Due to simulated motion of the satellites, the delay varies dynamically over time. 
     A standard delay generator receives as input samples of an input signal and provides the samples as an output after their respective delays. Since the delay may be different for each sample, the output samples are not synchronized with each other or with the input. The lack of synchronization imposes constraints on apparatus receiving the output samples for further processing, for example, on a digital-to-analog converter or other circuits operating on a fixed, synchronous clock. 
     U.S. Pat. No. 4,907,247, to Miyake et al., which is incorporated herein by reference, describes a satellite simulation system in which multiple digital satellite communication terminals are interconnectable over multiple channels. The equipment includes a single satellite delay simulator intervening between transmit communication terminals and receive communication terminals. The delay simulator is implemented by a satellite delay circuit accommodating multiple channels and clock matching circuits. The communication terminals are individually connectable to the delay circuit via the clock matching circuits. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide a delay generator which provides a synchronized output. 
     It is another object of some aspects of the present invention to provide methods and apparatus for delaying signals for a predetermined period and providing the delayed signals synchronously. 
     It is still another object of some aspects of the present invention to provide a delay generator whose output is synchronized with its input. 
     In preferred embodiments of the present invention, a delay generator receives samples of an input signal which have been sampled at a constant clock rate and calculates for each sample the required time it is to be delayed. Rather than delaying each sample by the required delay time, however, the samples are actually delayed so as to remain in synchronization with the clock rate. The values of the delayed samples are corrected to compensate for the difference between the actual delay time and the required delay time, which difference is evaluated to a resolution substantially greater than the clock rate, typically by orders of magnitude. Adjusting the delayed samples to be in synchronization with the input samples, and using the difference to evaluate the delayed sample, significantly simplifies construction and operation of other processing circuitry associated with the delay generator. 
     In some preferred embodiments of the present invention, the required time by which each sample is to be delayed is calculated by integration. At the beginning of a simulation session an actual delay, and a first differential of the actual delay, are calculated and are loaded into respective registers of an integrator. Preferably, new precalculated values of the differential are loaded into its register at predetermined times during the simulation. During the simulation, the integrator iteratively calculates new values of the actual delay, and updates the appropriate register. 
     In some preferred embodiments of the present invention, the delay generator comprises a first-in first-out (FIFO) unit which provides the actual delay, and an interpolation filter which corrects the values of the samples. The FIFO unit preferably provides the delay to a sample by writing the sample into a memory and reading the sample from the memory after an integral number of cycles of the clock driving the FIFO unit. The period of the clock driving the FIFO unit is referred to herein as a delay step. Preferably, the compensated value for a particular input sample is calculated by interpolation between a number of input samples, so as to correct for the difference between the precise, required delay time and the actual delay time defined in clock cycle steps. Most preferably, the value of the corrected sample is interpolated based on four neighboring samples. Further preferably, the filter handles I and Q samples separately due to their phase difference. 
     In some preferred embodiments of the present invention, the required delay is a continuous function which changes slowly relative to the delay step. Preferably, a sequence of samples are written into the memory at consecutive addresses and are read from consecutive addresses in the memory. Further preferably, the samples are read from the memory at a constant rate while the samples are written into the memory at a rate which depends on changes in the number of delay steps required from the FIFO unit. Preferably, when a simulation period is started, beginning read and write addresses are assigned to the memory and they are thereafter updated consecutively with each read and write operation, respectively. When the required delay remains substantially constant, read and write operations are performed at the same rate. However, when the required delay increases by an amount greater than the size of the delay step, two write operations are performed in the time a single read is performed. Conversely, when the required delay decreases by an amount greater than the size of the delay step, a cycle without a write operation is performed. 
     In a preferred embodiment of the present invention, the delay generator is used in order to simulate the transmission of the samples from a base station, via a satellite, to a receiver. Preferably, the delay generator forms part of a multi-channel simulation system as described in a U.S. Patent application Ser. No. 09/531,981 entitled “Satellite Motion Simulator,” filed on even date, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. 
     There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for generating a variable delay of a signal, including: providing a clock indicating a sequence of sample times at regular intervals; receiving a sequence of input samples representing input values of the signal at respective sample times indicated by the clock; determining the delay with a temporal resolution substantially finer than the clock interval to be applied to the signal at each of the respective sample times; and for each of the sample times, responsive to the respectively-determined delay, processing one or more of the input samples so as to generate a corresponding output sample representing a delayed output value of the signal at the sample time. 
     Preferably, the method includes outputting the samples at the regular intervals indicated by the clock. 
     Preferably, determining the delay includes calculating a delay responsive to a parameter that varies over time, most preferably by calculating a delay that varies from one sample to the next. 
     Preferably, determining the delay includes determining a delay induced by transmission of the signal through a communications link. 
     Preferably, determining the delay includes utilizing a process of integration to determine the delay, most preferably utilizing a precalculated differential of the delay based on a model of motion of an object with which the delay is associated. 
     Preferably, processing the input samples includes dividing the delay into a coarse and a fine component. 
     Preferably, the coarse component includes the largest number of clock intervals by which the determined delay can be divided, and the fine component includes the remainder of the division. Preferably, processing the input samples includes interpolating between the samples to compute the output sample responsive to the fine component. Most preferably, interpolating between the samples includes assigning respective predetermined coefficients to the one or more input samples, wherein the predetermined coefficients are selected responsive to a magnitude of the fine delay component relative to the clock interval. 
     Preferably, processing the input samples includes writing the samples to a write address in a memory and reading the samples from a read address therein, wherein the read and write addresses are separated by a difference responsive to a magnitude of the coarse component of the delay. 
     Preferably, writing the samples includes writing a sample twice in the time of a single read operation when the delay increases. Preferably, writing the samples includes not writing a sample in the time of a read operation when the delay decreases. 
     There is further provided, in accordance with a preferred embodiment of the present invention, a variable delay generator, which receives as input a sequence of samples of a signal at sample times indicated by a sample clock having a predetermined clock period, and which includes: a delay controller, which determines a variable delay having a temporal resolution substantially finer than the clock interval to be applied to the signal at each of the respective sample times; and a delay line which receives the input sequence of samples and outputs a synchronous stream of output samples at the sample times, each output sample representing a respective value of the signal following the delay determined by the delay controller. 
     Preferably, the delay controller divides the delay into a coarse and a fine component. 
     Preferably, the delay line includes an interpolation filter which interpolates among the input samples to generate a value of the output sample dependent on the fine component. Most preferably, the interpolation filter selects interpolation coefficients from among a plurality of predetermined coefficients, which are respectively assigned to the sequence of input samples. 
     Preferably, the delay unit includes a coarse delay unit which delays the output samples by the coarse component. Preferably, the coarse delay unit includes a first-in first-out memory device, wherein the samples are written to a write address therein and wherein the samples are read from a read address therein, wherein the read and write addresses are separated by a difference responsive to the delay. 
     Preferably, the delay controller iteratively calculates the delay. Most preferably, the delay controller includes an integrator to determine the variable delay for each of the samples based on a precalculated differential of the delay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: 
     FIG. 1 is a schematic block diagram of a satellite communication system and a corresponding delay generator, in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a schematic block diagram of a delay generator, in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a schematic block diagram of a delay controller, in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a timing diagram that schematically illustrates operation of an interpolation filter, in accordance with a preferred embodiment of the present invention; 
     FIG. 5 is a schematic block diagram of the interpolation filter referred to in FIG. 4, in accordance with a preferred embodiment of the present invention; 
     FIG. 6 is a timing diagram that schematically illustrates calculation of values in a coefficient look up table, in accordance with a preferred embodiment of the present invention; 
     FIG. 7 is a schematic block diagram of a delay unit, in accordance with a preferred embodiment of the present invention; 
     FIG. 8 is a schematic block diagram of a FIFO control circuit in the delay unit of FIG. 7, in accordance with a preferred embodiment of the present invention; and 
     FIG. 9 is a schematic timing diagram of the delay unit of FIG. 7, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic block diagram of a satellite communication system  20  and a corresponding simulation system  27 , in accordance with a preferred embodiment of the present invention. A handset  22  communicates with a base station  24  via a satellite  26  which, for example, belongs to a low-earth orbit (LEO) satellite array, such as the Globalstar system. 
     In order to test the operation of base station  24  and handset  22  without going through satellite  26 , simulation system  27  is connected between RF ports  30  of base station  24  and handset  22 . System  27  processes the signals passing between base station  24  and handset  22  as if the signals were passing along a path  31  via satellite  26 , wherein the signals undergo varying delays. System  27  preferably comprises a delay generator  28  which delays the signals from base station  24  according to the calculated position of satellite  26 . Other elements, such as a noise generator and a Doppler shift generator (not shown) are preferably also included in system  27 . These elements may be implemented entirely separately from delay generator  28 , or partly combined with the delay generator. 
     Preferably, system  27  further comprises an A/D converter  32  which samples and digitizes the RF signals from base station  24 . The delayed sampled signals are preferably converted back to analog form by a D/A converter  34 . Preferably, generator  28  and converters  32  and  34 , as well as the other elements of system  27 , operate at the same clock rate using synchronous clock signals so as to simplify the operation of the system. In the following description, differing clock rates are referred to as being based on a basic “chip” rate and multiples thereof. For example, a chip×4 rate corresponds to a rate four times faster than the basic chip rate. In system  27 , a single period of the chip rate clock is preferably of the order of 800 ns. The delay imposed by delay generator  28  is typically on the order of delay times for signals transmitted via satellite  26 , i.e., in the range of 9-23 ms. It will be understood, however, that delay generator  28 , which is described further hereinbelow, may be used over a wide range of different delay times and in various applications not limited to the simulation application described herein. 
     FIG. 2 is a schematic block diagram of delay generator  28 , in accordance with a preferred embodiment of the present invention. Preferably, generator  28  receives an intermediate frequency (IF) signal on a “data in” line  41  at a chip×16 rate, and translates the signal to separate interlaced I and Q digital samples, each at a chip×8 rate, where the terms I and Q are well known as to representing In-Phase and Quadrature Phase signals. The samples are preferably between 8 and 12 bits wide. On two “data out” lines  43 , generator  28  provides the delayed I and Q samples, at a rate of chip×4. That is, delay generator  28  decimates the samples in addition to delaying them. This feature is useful in a particular implementation of simulation system  27 , although it is not material to the principles of the present invention. 
     Generator  28  preferably comprises a coarse delay unit  40 , which delays each of the incoming samples for a period which is equal to a respective, integral number of delay steps for each sample. Preferably, the delay step is equal to the period of a chip×4 cycle. 
     Generator  28  also includes an interpolation filter  42  which separately alters the values of each of the I and Q samples entering delay unit  40  in order to correct for inaccuracies introduced by the coarse delay of unit  40 . Interpolation filter  42  and delay unit  40  together form a delay line. Preferably, interpolation filter  42  corrects for the inaccuracies by calculating a value of a virtual sample (I or Q) which is then input to unit  40 . The process followed by interpolation filter  42  is explained in more detail hereinbelow with reference to FIG.  4  and FIG.  5 . Although the following description and the drawings show filter  42  at the input to delay unit  40 , those skilled in the art will be able to perform any necessary changes in order to position filter  42  at the output of unit  40 . 
     Preferably, generator  28  further comprises a processor  44  which calculates a theoretical delay for each sample transmitted from base station  24  to handset  22  due to path  31 . Preferably, an array of values of the theoretical delay of the samples is calculated at the beginning of a simulation session and stored for use during the simulation, either by processor  44  or by an external processor, which downloads the results to processor  44 . The stored values, and first differentials of the stored values, are then used for simulation initiation and at later points in the simulation, as described hereinbelow. 
     Generator  28  also comprises a delay controller  46  which iteratively calculates, during the simulation session, an actual delay to be applied to each sample. During the simulation session, controller  46  operates as an integrator, which uses a first time differential of the actual delay, in order to calculate a new actual delay. 
     FIG. 3 is a schematic block diagram of delay controller  46 , in accordance with a preferred embodiment of the present invention. Preferably, before the start of the simulation session, processor  44  stores an initial actual delay and an initial first differential of the actual delay, i.e., an initial delay change rate, in an actual delay register  154  and a first differential register  152  respectively. The initial actual delay is entered into actual delay register  154  via a multiplexer  158 . 
     During the simulation session an adder  162  iteratively integrates the actual delay at a chip×4 rate, using the first differential stored in register  152 , and stores the result in register  154 . Preferably, the value of the first differential stored in register  152  is updated at a rate of once every 125 ms, from values calculated by processor  44 . Register  154  thus contains a continuously updated value of the actual delay. 
     Returning to FIG. 2, for each sample the actual delay calculated as described above by controller  46  is divided into two parts: a coarse delay comprising an integral number of delay steps imposed by unit  40 , and a fine delay, which is compensated for by filter  42 . The coarse delay is preferably calculated by dividing the actual delay by the delay step of unit  40  (the period of the chip×4 signal), and setting the quotient of the division to be equal to the number of delay steps. The remainder of the division is the fine delay value. The fine delay value is evaluated as an integral number of parts, preferably 512 parts, of a delay step, wherein the integral number is any integer from 0 to 511. 
     Preferably, at the beginning of the simulation session, processor  44  sets a starting read address and a starting write address which are separated by the coarse delay for an initial sample. The starting read and write addresses are passed to delay unit  40 . The fine delay value of the initial sample is stored in a fine delay register  47 . 
     The fine delay value is used by filter  42  in altering the respective values of the data-in samples, as described hereinbelow. Delay controller  46  also generates control signals on a line  49  when the fine delay value reaches its maximum (corresponding to one complete delay step) or minimum value (zero delay steps). At this point, delay controller  46  generates a signal that changes the length of the coarse delay introduced by unit  40 . 
     Preferably, fine delay register  47  accepts a fixed range of integer values, for example, values between 0-511. When the fine delay value exceeds 511, it is reset to zero (i.e., reduced by the maximum value), and an overflow signal is output from control  46  to line  49 . Likewise, when the fine delay value goes below 0, 511 is added to the value in register  46 , and an underflow signal is provided. The underflow and overflow signals are provided both to delay unit  40  and to filter  42 . Unit  40  uses the underflow signal to decrease the coarse delay by one complete delay step, and uses the overflow signal to increase the coarse delay by one complete delay step. In the overflow case, filter  42  generates an additional sample to be written into a coarse delay unit memory comprised in unit  40 . The additional sample is written during an additional write operation performed during a single coarse delay unit cycle that is needed to increase the delay by one delay step, as described in more detail below. In the underflow case, filter  42  skips the next sample writing cycle, so as to decrease the delay by one delay step. 
     The fine delay value in register  47  is used as an index to a double look up table (LUT)  48  which provides interpolation coefficients to filter  42  for I and Q samples separately. LUT  48  is preferably implemented by two 2K×16 bits PROM memories, although any other suitable memory unit may be used. 
     A clock  50  provides clock signals to unit  40  and filter  42  at a rate of chip×16. Thus, samples are input on line  41  every clock period of clock  50 , and an I and a Q sample are output from simulator  28  on lines  43  every four clock periods. 
     FIG. 4 is a schematic timing diagram showing signals associated with interpolation filter  42 , in accordance with a preferred embodiment of the present invention. A chip×16 signal  200  illustrates the chip×16 clock signal regulating the operation of delay simulator  28 . Signal  200  is formed of a sequence of chip×16 periods  201 , formed of a high period  202  and a low period  203 . 
     Interpolation filter  42  alternately extracts uncorrected input I samples  210  and uncorrected input Q samples  212  from line  41  at fixed times at a total rate of chip×16. For example, the I samples may be extracted at odd chip×16 periods, and the Q samples may be extracted at even chip×16 periods. Filter  42  provides delay unit  40  with corrected I samples  220  and corrected Q samples  230 . To correct the value of incoming samples, filter  42  generates a corrected I sample  214 , and outputs it as a sample  214 ′ at a next available chip×4 time, with a value as if it were sampled from the source signal of base station  24  at time  218 . The time  218  is determined such that the period  222  between time  218  and the closest output sample  220  is equal to the fine delay value held by delay register  47 . 
     Time  218  may be located anywhere between two adjacent output samples  220 . Typically, time  218  will shift gradually relative to the times of samples  220 , due to gradual change in the total delay time. When the required delay slowly increases, time  218  moves to the left. When time  218  substantially coincides with the sample  220  on its left, the delay provided by filter  42  is maximal and an overflow signal is generated. Conversely, when time  218  coincides with the sample  220  on its right, the delay provided by filter  42  is minimal, and an underflow signal is generated. 
     Sample  214  is preferably interpolated from a set  224  of four input I samples  210 A,  210 B,  210 C, and  210 D, using a suitable interpolation method, preferably finite impulse response (FIR) filtering, wherein each of the input I samples is multiplied by a respective coefficient and the products are summed. A set of suitable coefficients for each fine delay value from 0 to 511 is stored in LUT  48  and used in the calculation, as described further hereinbelow. Alternatively or additionally, any other number of samples  210  may be used in interpolation. 
     Similarly, corrected Q sample  216 , output as a sample  216 ′ at a next available chip×4 time, is prepared by interpolation of a set  226  of four Q samples  212 A,  212 B,  212 C, and  212 D. The interpolation is performed separately because of the time difference between the I and Q input samples. 
     FIG. 5 is a schematic block diagram of interpolation filter  42 , in accordance with a preferred embodiment of the present invention. Filter  42  comprises a separator  60  which receives samples from line  41  and separates the incoming samples to I and Q samples, as is known in the art. Separator  60  receives input samples at a rate of chip×16 and provides interlaced I and Q samples, each at a rate of chip×8, to respective lines  64  and  66 . 
     The I samples are passed on line  64  on odd chip×16 periods and are held in four registers  62 A,  62 B,  62 C and  62 D (referred to also as registers  62 ) which store the four most recent samples, corresponding to  210 A,  210 B,  210 C, and  210 D referred to in FIG.  4 . Each sample first enters register  62 A and upon the next odd chip×16 period passes to register  62 B. At the next two odd chip×16 periods the sample is passed to registers  62 C and  62 D and thereafter it is discarded. Similarly, the Q samples are passed on line  66  to four registers  68 A,  68 B,  68 C and  68 D (referred to also as registers  68 ), on even chip×16 periods. 
     Preferably, during four chip×16 periods (at a rate of chip×4), filter  42  calculates an interpolated I output sample based on the four input samples in registers  62 . During any specific chip×4 period, the four input samples comprise two new samples in registers  62 A and  62 B, and two samples which had been in registers  62 A and  62 B during the previous chip×4 period, and which have been transferred to registers  62 C and  62 D. The four input samples thus comprise two new samples, and two “old” samples. At each chip×16 period, a multiplexer  70  passes one of the samples in registers  62  into a multiplier  72  which multiplies each of the samples from registers  62  by a respective coefficient from a register  76 , providing a weighted sample. The coefficient in register  76  is preferably received from LUT  48  according to the value of the fine delay in register  47  and according to which of the four samples is being multiplied. 
     An adder  74  sums up the four weighted samples to form the interpolated I output sample on a line  78 . Preferably, multiplier  72  and adder  74  comprise a single multiplier-accumulator chip, as is known in the art. In a similar manner to that of the I samples, filter  42  calculates an interpolated Q output sample provided on a line  88 , based on the four input samples in registers  68 . Preferably, the interpolation is performed in a similar manner to that described above, using a multiplexer  80 , a multiplier  82 , an adder  84  and a Q-coefficient register  86 . Control logic blocks  94  and  96  are used to clear adders  74  and  84  respectively, at a chip×4 rate, after each set of four input samples has been utilized to generate an interpolated value. 
     LUT  48  contains different sets of values for coefficient registers  76  and  86 , due to the phase difference (one chip×16 period) between the I and Q samples. Preferably, each set of values includes four coefficients for each of the values of fine delay register  47 . Thus, if register  47  may receive 512 values, LUT  48  stores 2×4×512 coefficients. Values of the coefficients are preferably chosen so that they weight the four samples to which they are applied so as to form a linear average, although any other sets of coefficients may be used, as is known in the art. 
     FIG. 6 is a schematic timing diagram illustrating how LUT  48  is used, in accordance with a preferred embodiment of the present invention. As described with reference to FIG. 4, filter  42  calculates the value of a virtual I sample taken at time  218 , based on samples  210 A,  210 B,  210 C, and  210 D, of set  224 . Preferably, each of the samples in set  224  is multiplied by a respective coefficient α, β, γ or δ, which coefficients are based respectively on time intervals  228 A,  228 B,  228 C, and  228 D between time  218  and the respective sample of set  224 . The value I′ of the virtual sample is given by: 
     
       
           I′=I   A α n   +I   B β n   +I   C γ n   +I   D δ n 0 &lt;n ≦511, 
       
     
     where I A , I B , I C , and I D  are respective values of samples  210 A,  210 B,  210 C, and  210 D. 
     A similar process is used by filter  42  to calculate the value of an imaginary Q sample, based on samples  212 A,  212 B,  212 C, and  212 D, of set  226 , and respective coefficients  230 A,  230 B,  230 C, and  230 D. Alternatively, the coefficients are chosen according to any other suitable interpolation method. 
     Referring back to FIG.  4  and FIG. 5, in normal operation, filter  42  provides I and Q interpolated samples every chip×4 period. The provided interpolated samples are preferably output through multiplexers  90 . However, when delay controller  46  generates an underflow control signal which indicates that the delay incurred by unit  40  must be decreased, filter  42  provides a sample as usual, but no write signal is generated for that chip×4 period. 
     Conversely, when delay controller  46  generates an overflow control signal which indicates that the delay incurred by unit  40  must be increased, filter  42  provides two samples: an interpolated sample, as in every other cycle, and one of the samples stored in filter  42  (registers  62  for I samples and registers  68  for Q). Two write signals are then generated during a single chip×4 period. Control logic blocks  94  and  96  receive signals from delay controller  46  indicating when two samples are to be provided, and pass an appropriate signal to multiplexers  90 . The first interpolated sample is provided as in every normal chip×4 period, preferably during the first chip×16 period of the chip×4 period. The second sample is provided thereafter, preferably during the second chip×16 period of the chip×4 period. The second sample is provided along lines  92  and selected by multiplexers  90 . Lines  92  provide the contents of registers  62  and  68 , respectively, which contain the values of samples  210  and  212 , respectively. 
     FIG. 7 is a schematic block diagram of delay unit  40 , in accordance with a preferred embodiment of the present invention. Delay unit  40  comprises two memory units  102  and  104  which delay the I and Q samples, respectively. Memory units  102  and  104  preferably comprise standard SRAM chips. Most preferably, each of the units comprises two 128K×8 bit chips. Memory units  102  and  104  receive the samples from filter  42  and output the samples on lines  43 . The operation of memory units  102  and  104  is controlled by a controller  106 , which operates the SRAM as a first-in first-out (FIFO) memory by managing the read and write addresses of memory units  102  and  104  and their control signals, e.g., chip select CS, read enable RD and write enable WR. 
     FIG. 8 is a schematic block diagram of controller  106 , in accordance with a preferred embodiment of the present invention. Controller  106  comprises a counter  120  which stores a common read address for both of memory units  102  and  104 , and two additional counters  122  and  124  which store the write addresses of memory units  102  and  104 , respectively. Preferably, controller  106  includes an ISA or other suitable address bus  126  and an ISA or other suitable data bus  128  used for loading initial data at the beginning of the simulation session and for debugging. Two multiplexers  132  and  134  determine the source of the address passed to memory units  102  and  104 , respectively. A control block  130  receives the control signals from line  49  and a clock signal from clock  50 , and accordingly generates appropriate control signals output to memory units  102  and  104 , as well as to counters  120 ,  122  and  124  and multiplexers  132  and  134 . A 17 bit comparator  136  signals if the read and Q write values in counters  120  and  124  are the same, and is used as an error warning generator. 
     FIG. 9 is a schematic timing diagram relating to coarse delay unit  40 , in accordance with a preferred embodiment of the present invention. During the simulation session, memory units  102  and  104  provide a delayed sample every chip×4 period, and a read pointer is incremented accordingly. Normally, as shown for chip×4 periods  250  and  254 , during each chip×4 period, each of memory units  102  and  104  also receives a single interpolated sample from filter  42 , i.e., a sample is written to each of the memory units during a chip×16 period marked WR, when a write SRAM signal is set at respective times  258  and  260 . During these periods, a write pointer counter  262  of each memory unit increments once in a chip×4 period, when a write pointer counter enable signal is set at respective times, for example, at times  264  and  266 . (The numbers  12904 ,  12905 , etc., represent dummy values of the write pointer.) 
     However, as shown for a no-write chip×4 period  256 , in chip×4 periods in which an underflow control signal is received on line  49  (FIG.  2 ), a no-write signal is set at time  268 , and no sample is written into the memory unit (either  102  or  104 ) for which the underflow signal is received. During the underflow chip×4 period, memory units  102  and  104  provide output samples as usual and increment the read pointer, and, thus, the delay provided by the underflowed memory unit is increased, since the write pointer is not incremented. 
     In a chip×4 period  252 , when an overflow signal is received on line  49 , a double write enable signal is set at time  270 . As a result, two samples are written into the memory unit for which the overflow signal is received when the write SRAM signal is set at respective times  272  and  274 . In this case, the delay provided by the overflowed memory unit is shortened, since write pointer counter  262  is incremented twice in a chip×4 period. 
     Although the above-described preferred embodiments relate generally to a satellite communications simulator, those skilled in the art will appreciate that the principles of the present invention may be applied to produce variable delay generators for use in other applications, as well. It will thus be understood that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.