CA1118104A - Lattice filter for waveform or speech synthesis circuits using digital logic - Google Patents

Abstract

ABSTRACT
A digital filter of the type which may be used in circuits for generating complex waveforms, such as human speech. The filter has a multiplier, an adder coupled to the output of the multiplier and various delay circuits coupled to the output of the adder. A
latch memory is coupled to the output of one of the delay circuits.
Switching circuits are provided for the output of the delay and the latch memory to inputs of the multiplier and the adder to selected times. Coefficients of the filter are preferably stored in a memory coupled to another input of the multiplier. The excitation signal is coupled to the adder in one embodiment and to the multiplier in another embodiment. In either embodiment, the digital filter may be implemented on a single integrated circuit chip.

Description

TI-6890 tCa.) ~8~

sACKGROUND OF THE INVENTION
This invention relates to the generation of complex waveforms using digital signals and more specifically to the synthesis of speech by digital circuits using linear prediction methods. Disclosed is a digital filter having an array multi-plier for use in speech synthesis or waveform generation circuits.
The disclosed speech synthesis circuit may be integrated on a single integrated circuit, thereby facilitating its use in various applications in the communication handling industry, including such applications as: teaching machines, communication equipment (i.e., telephones, voice cryptographic equipment, radios, televisions, etc.), and other equipment which generate the sound of a human's voice.
Several methods are currently being used and experi-mented with to digitize human speech. For example, pulse code modulation, differential pulse code modulation, adaptive pre-dictive coding, delta modulation, channel vocoders, cepstrum vocoders, formant vocoders, voice excited vocoders, and linear predictive coding methods of speech digitalization are known.
These methods are briefly explained in "Voice Signals: Bit by Bit" at pages 28-34 in the October 1973 issue of IEEE Spectrum.
Computer simulations of the various speech digitaliza-tion methods have generally shown that the linear predictive methods of digitizing speech can produce speech having greater voice naturalness than the previous vocoder systems (i.e., channel vocoders) and at a lower data rate than the pulse coded modulation systems. As will be seen, the linear predictive systems often make use of a multi-stage digital filter and as the number of stages of the digital filter increases, the more natural sounding becomes the resulting generated speech.

.~ .

An early application of linear predictive methods to digital speech synthesis occurred in the late 1960's and early 1970's. A historical analysis of some of this early work is set forth in Markel and Gray, "Linear Prediction of Speech" (Springer-Verlag: New York, 1976) at pages 18-20.

TI-6890 (Ca.) 1~8~V4 The multi-stage digital filter used in linear predictive coding is preferably an all pole filter with all roots preferably occurring within the unit circle ¦z¦ =l when the mathematical transfer function of the filter is expressed as a Z-transform.
The filter itself may take the form of a lattice filter of a typical type. However, other filters including ladder filters, normalized ladder filters and others are known, as set forth in Chapter 5 of "Linear Prediction of Speech". As will be seen, each stage of the lattice filter requires two addition opera-tions, two multiplication operations and a delay operation.The filter is excited from either a periodic digital source for voiced sounds or a random digital source for unvoiced sounds.
The filter coefficients are preferably updated every few milli-seconds while the excitation signal is updated at a faster rate.
In the prior art, a lattice filter network has been implemented by appropriately programming large digital computers.
Exemplary Fortran programming of a computer for speech synthesis purposes is set forth in the aforementioned "Linear Prediction of Speech". Given the data rate of the excitation signal and the large number of arithmetic operations, i.e., two multi-plications and two additions for each stage of a multi-stage filter and given that increasing the number of stages thereof increases the naturalness of the generated speech, high speed digital computers have been utilized in most speech synthesis work done to date. However, Dr. J. G. Dunn, J. R. Cowan and A. J. Russo of the ITT Defense Communications Division in Nutley, New Jersey have attempted to implement a multi-stage filter using metal oxide silicon (MOS) large scale integration techniques. They attempted using a multi-processing approach, wherein many arithmetic units are operated simultaneously;

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,~ .

however, this technique requires a very large number of multiplier and adder circuits be implemented on a semiconductor chip. Some discussion of the work done by Dr. Dunn et al is set forth in ''Progress in the Development of Digital Vocoder Employing an Itakura Adaptive Predictor" published in "Telecommunications Conference Records, I.E.E.E. Publ. No. 73" (1973). Replacing t~ lattice A structure of-PICUnE 2a with various adders and multipliers results in a complex and large size semiconductor chip.
It was one object of this invention, therefore, to implement 10 a lattice type filter for generating complex wave forms, such as human speech, on a single semiconductor chip.
It was another object of this invention, that the filter components be implemented with MOS devices.
It is still yet another object of this invention that the resulting MOS filter be of smaller size than that heretofore known in the prior art.
The foregoing objects are achieved as is now described.
The digital filter includes a multiplier, one input of which receives the filter coefficients from a memory. The output of the multiplier 20 is applied to one input of an adder/subtractor, whose output is applied to a short delay circuit. The output of the short delay circuit is applied to a long delay circuit. The short and long delay circuits preferably comprise shift registers of short and long lengths, respectively. The output of the long delay circuit is coupled to a latch memory via a switch. The other input to the multiplier is selectively coupled to the output of the adder/
subtractor, the output of the short delay circuit or the output of the latch memory. The other input to the adder/subtractor is selectively coupled to the output of the latch memory, the output 3 of the long delay circuit or the output of the adder/subtractor.

TI-6890 (Ca.) 8~04 The multiplier is preferably an array multiplier. The output of the filter is provided at the output of the latch memory and the input is either coupled to the adder/subtractor or the multi-plier in the two disclosed embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS
_ The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use and further objects and features thereof, will be best understood by reference to the following detailed description of illustrated embodiments thereof when read in conjunction with the accom-panying drawings, wherein:
FIGURE la is a block diagram of the basic elements of a voice synthesizer;
FIGURE lb depicts the presence of the excitation signal and Kn coefficients with respect to time;
FIGURES 2a and 2b show a typical lattice filter of the type used in speech synthesis circuits;
FIGURES 3 shows a timing arrangement for the genera-tion of intermediate results in a lattice filter having N stages;
FIGURE 4 depicts a timing arrangement for the genera-tion of intermediate results in a lattice filter having ten stages;
FIGURE 5 shows one embodiment of a digital filter equivalent to a lattice filter;

FIGURE 6 lists the various intermediate results available in the filter of FIGURE 5 at various time periods of a cycle;
FIGURE 7 shows another embodiment of a digital filter equivalent to a lattice filter;

11~8:~04 FIGURE 8 lists the various intermediate results available in the filter of FIGURE 7 at various time periods of a cycle;
F~GURE 9 depicts the array multiplier used in the digital equivalent filter;
FIGURES lOa-lOd are logic diagrams of the various elements depicted in FIGURE 9s and ~ IrunE ll clcpicts a generalized Eorm of the digital ,~
filter.

DETAILED DESCRIPTION
Referring now to FIGURE la, there is shown, in block diagram form, the basic elements of a voice synthesizer system.
The voice synthesis circuit comprises a multi-stage lattice filter 10, which digitally filters an excitation signal 11 using filter coefficients Kl-Kn. Lattice filter lO outputs a digital signal 12 which is converted to analog form by a digital/analog converter 13.
The output of converter 13 is changed to audible sounds by a speaker 14 or other such sound conversion means; it is to be understood, of course, that an amplifier may be used between converter 13 and speaker 14 to amplify the analog output of converter 13 to levels required by speaker 14.
The excitation signal (U) 11 is generally derived from one of two sources, voicing source 15 or unvoicing source 16. The particular source used is determined by a digital switch 17. Voicing source 15 is utilized when generating those sounds for which the human vocal cords or vocal folds vibrate during speech, such as the sound of the first E in Eve. The rate at which the vocal folds open and close determines the pitch of the sound generated.
Unvoicing source 16 is used when generating those sounds, such as the F in Fish, where the vocal folds are held open and air is 30 forced past them to the vocal tract. Thus, the particular source, 15 and 16, utilized depends upon the source to be generated.

11:181~4 Typically, the unvoicing source 16 generates a random digital signal while voicing source 15 generates a periodic digital signal.
The digital data supplied by voicing source 15 and unvoicing source 16 may, of course, be merely stored in one or more semiconductor read-only-memories (ROM's). Preferably, however, such data is stored in an encoded format, e.g., as pitch or a code actuating a random number generator. Thus, such data is usually first decoded before the random or periodic data (e.g., Signal V) is supplied to filter 10. Of course, depending on how such data is stored, the 0 need for switch 17 may be eliminated. When the data is stored as pitch or a code activating a random number generator, a amplification factor (A) is preferably also stored in the ROM. Amplification factor A adjusts the constant amplitude signal (V) from voicing source 15 or unvoicing source 16 to produce the excitation signal (V) for filter 10.
The excitation signal 11, which generally corresponds or mimics the function of the vocal folds, is altered by lattiee filter 10. Lattice filter 10 generally corresponds or mimics the funetion of the voeal traek which filters the sounds generated at 20 the voeal folds. The filter coefficients, Kl-Kn, reflect the shape (i.e.the resonances) of the vocal track during speech. Accordingly, coefficients Kl-Kn are periodically updated to reflèct the changing shape of the vocal track and may be stored along with the voicing/
~nvoicing source data in a read-only-memory.
Referring now to FIGURE lb, there is shown in graphical form the outputs of unvoicing source 16 and voicing source 15 against time. Here voicing source 16 is shown as outputting an impulse at a five millisecond period, which corresponds to a frequency of 200Hz; this pitch corresponds to voiced sounds in 80 the vocal range of many women. Since men typically have a lower pitch, a man's voicing source would output impulses less frequently.

~118~4 ~ Joicing source ~ is shown as outputting impulses at a period corresponding to the pitch of the person's voice; it is to be understood, however, that the periodic impulses may be replaced with other periodic functions, such as a decaying sine wave or the so-called "chirp function", which restart with a pitch related period. Unvoicing source 16 is shown as a random signal.
The coefficients for lattice filter 10 are be~ng shown as being updated every five milliseconds in FIGURE lb. It is to be understood, however, that the rate of which the coefficients 10 lattice filter 10 are updated is a design choice. If the coefficients are updated more frequently, the more closely lattice filter 10 will model the vocal track dynamics, but with a corresponding increase in the amount of data to be stored in the aforementioned ROM. Of course, updating the coefficients less frequently has the opposite effect. However, it has been found that by updating the w~\\ i s~c~v~ s coefficients approximately every five ~iero~e~n~s or so results in very high quality human speech being synthesized by lattice filter 10 with reasonable data storage requirements.
The time axis of FIGURE lb is shown as being divided into 20 a hundred microsecond intervals. These intervals correspond to the data rate from voicing source 15 and unvoicing source 16 as well as the data rate to and from lattice filter 10. Further, while unvoicing source 16 and voicing source 15 may appear to be analog signals in FIGURE lb, it is to be appreciated that they are in fact digital signals whose magnitudes are as shown and which are updated at the intervals shown along the time axis of FIGURE lb.
For information regarding the derivation of the magnitudes of the filter coefficients, reference should be made to the aforementioned "Linear Prediction of Speech."

11~18~04 Thus, in this embodiment, the data rate to converter 13 would be lOKHz and the upper frequency limit of the synthesized speech from converter 13 would be 5KHz. Of course, the data rate may be altered, if desired, as a matter of design choice. For , instance a 8KHz data rate would result in a synthesizer having an upper frequency response of 4KHz.
Referring now to FIGURES 2a and 2b, there is shown a block diagram of lattice filter 10. In FIGURE 2a lattice filter 10 is shown as comprising ten stages, Sl-S10, each of which is IO equivalent to the stage depicted in FIGURE 2b. For ease of illustration, only three of the stages are shown in detail in FIGURE 2a. The input to the stage S10 is the excitation signal 11 and the output 12 from the stage Sl is applied to converter 13 (FIGURE la). It will be appreciated by those skilled in the art that the output 27 from the S10 stage is not utilized and therefore adder 27a and multiplier 27b in that stage may be deleted, if desired.
Referring now to FIGURE 2b, there is shown a single stage Sn of lattice filter 10. An input to this stage, Yn+l(i), is applied as one :input to an adder 26 the output of which is Yn(i). The other input to adder 26 which is applied to a subtraction input of adder 26 is derived from the output of a multiplier 19 which multiplies the coefficient Kn times the output from a delay circuit 22, which output is bn(i-l). The output from delay circuit 22 is also applied to an adder 21 which also receives as an input the output from a multiplier 20. Multiplier 20 multiplies the coefficient Kn times the output from adder 26 which is, of course, Yn(i). The output from àdder 21 is b +l(i). As can be seen, the subscript of the Y and L data defines the stage in which that data is utilized while the number appearing in the parentheses indicates 1~81~
the cycle in which that data was generated. Delay circuit 22 provides a one time cycle deIay function, such as may be supplied by a shift register, for example. Once each time cycle a new data point U(i) (or Yll(i)) is provided to stage S10 as the excitation signal 11. Thus, for each stage in lattice filter 10, two multiplications and two additions must be accomplished during each cycle, that is, given the data rates depicted in FIGURE lb, those four operations must be accomplished in 100 microseconds in each stage of lattice filter 10. As a matter of design choice, lattice filter 10 in FIGURE 2a is shown as having ten stages; however, it will be appreciated by those skilled in the art, that the number of stages may be varied as a design choice according to the quality of sound desired to be synthesized by lattice filter 10. It has been found that a ten stage lattice filter 1~ can synthesize speech which is virtually indistinguishable from actual human speech.
It will be appreciated then that during any given time cycle, the ten stage lattice filter 10 must accomplish twenty multiplications and twenty addition/subtraction operations. It should be further appreciated that those operations cannot all be accomplished simultaneously, inasmuch as, Ylo must be calculated befcre Yg, which must be calculated before Y8, etc., during any given time cycle. Also during the same time cycle, the blo-b data must be calculated and stored in the delay circuits 22 of each stage for use during the next time cycle. The Y and ~ data defined with respect to FIGURE 2b, is also shown for stages Sl, Sg, and S10 in FIGURE 2a. Equations expressing the relationship between the various Y and ~ data are set out in Table I. It should be appreciated that Y and ~ data as well as the coefficients Kn are multi-digit numbers; that coefficients Kl-Klo may vary between a decimal equivalent of plus and minus one and are periodically updated in a manner to be described.

1~18104 Referring now to FIGURE 3, there is shown, in representative form, various intermediate results attained from the multipliers B and adders of'lattice filter having N stagesr -t~e horizontal axis depicts time while the vertical axis represents the various stages , of an N stage lattice filter 10. In the N stage, for example, the intermediate results, -K .bn and Kn.Y , which may be generated by multipliers 19 and 20 (FIGURE 2b), respectively, and intermediate ~ \
results Yn and bn+l, which may be obtained from adders 26 and ~
(FIGURES 2b), respectively, are shown. Timewise, the intermediate 10 result -Kn.bn must be generated before Yn may be obtained; Yn must be generated before Kn.Yn may be generated; and Kn.Yn must be generated before bn+l may be produced. According to the depicted time scale, addition operations are shown as taking a five microsecond time period while the multiplication operations take a longer time period. As to the relationship of the generation of the intermediate results with respect to the different stages, it can be seen that the bn output from an add operation must be available before the -Kn.bn multiply operation is initiated, as is depicted by arrow 25. This fact necessitates that a "no operation"
20 period 23 be inserted between the bn+l add operation and the -Kn.bn multiply operation, if only one add operation and one multiply operation are to be initiated during any given five microsecond time period, as can be seen from FIGURE 3. "No operation" periods 24 are inserted after the other add operation before the following multiply operation for symmetry purposes. Thus, it can be seen that the operations indicated in all the stages of an N stage lattice filter may be accomplished concurrently in the order depicted in FIGURE 3 and appropriate intermediate results will become available as needed. FIGURE 3 depicts the general nature and 3~ applicability of the digital implementation of a multi-stage lattice filter to be described. It is to be appreciated that the representation of FIGURE 3 shows those operations accomplished during one of the aforementioned time cycles. The f ive microsecond ~118~04 time period for an add operation is selected as a design choice because of its compatibility with P-channel MOS integrated circuits.
Of course, other time periods may be used if desired.
Referring now to FIGURE 4, there is shown a representation similar to FIGURE 3; however, the FIGURE 4 representation is for a digital implementation of an equivalent ten stage lattice filter 10 and the horizontal time axis has been increased to show more than one time cycle. Further, the time cycle has been broken down into twenty time periods, Tl-T20, each of which preferably has a duration on the order of five microseconds; as has been previously mentioned, other periods may be selected. Also in FIGURE 4, the time cycles, e.g., i-l, i and i+l, are indicated for ease in comparing the availability of intermediate results in filter 10 with the requirement set out by the mathematical formula representation of filter 10 in Table I.
At the first time period, Tl, the excitation data U is applied as an input; the output of the filter, Yl, becomes available at time period Tll. It can be seen by comparing FIGURE 4 and Table I that the various inputs required for the multiply operations are 20 available when needed and that the various inputs for the add operations are also available when needed. It can further be seen from FIGURE 4 that an add operation (which preferably takes one time period) is initiated and completed every time period and a multiply operation is similarly initiated (and completed) every time period although the particular multiply operation then being initiated will not be completed for eight time periods. The apparatus for performing these operations will be described in detail with respect to FIGURES 5, 9 and 10a-d.
It has been mentioned that a multiply and an addition operation are each initiated preferably each time period. In fact, Y U

1~18104 the number of time periods in a cycle preferably equals twice the number of stages in the equivalent lattice filter. Thus for eight or twelve stage lattice filters, the equivalent digital filter preferably has sixteen or twenty-four time periods per cycle, respectively. It should be evident from examining FIGURES 3 and 4, that the number of time periods allotted for the multiply operation depends, in part, on the number of time periods in a cycle. Thus, eight time periods may be used for multiply operations in a ten stage equivalent digital filter while six time periods may be used lû for multiply operations in an eight stage equivalent digital filter, if the digital equivalent filter scheme of FIGURES 3 and 4 is followed. It should be evident to those skilled in the art, however, that the number of time periods for multiply operation tends to dictate the number of bits which may be multiplied, i.e., tends to limit the number of bits used to represent the Kn coefficients. In most applications, the number of bits allotted to the K~ coefficients by following the processing scheme of FIGURES 3 and 4 will yield very acceptable synthesized speech. If, however, even greater accuracy is desired in representing the Kn coefficients, a multiply and a addition operation may not be initiated every time period of a cycle and some delay should be inserted at some point during the cycle. Of course, then the cycle would take a longer time to complete, thereby lowering the data rate (and frequency response) of the system.
As can be seen from FIGURE 4, the Klo.Ylo and bll intermediate results are obtained or may be obtained; however, as mentioned with respect to FIGURE 2a, these particular intermediate results are not required for a digital implementation of the lattice filter. It will be seen with respect to FIGURE ~, however, that the Klo.Ylo and bll intermediate results (or some other numbers) ~118~04 are often more easy to generate (and ignore) than it is to inhibit the apparatus from making these calculations. Further, it will be subsequently described how the multiply operation performed by multiplier 18 (FIGURE 1) may be accomplished in lieu of calculating Klo-Yl~o by the apparatus.
In FIGURE S there is shown a block diagram of a digital implementation of an equivalent lattice filter 10. The filter includes an array multiplier 30, adder/subtractor circuit 33, one period delay circuit 34, a shift register 35, and a latch memory 36.
The data inputted to and outputted from these various units at each of the twenty time periods Tl-T20 (for an equivalent ten stage lattice filter) are listed in FIGURE 6. Referring now to FIGUR~S
5 and 6, array multiplier 30 accomplishes the multiplications performed by multipliers 19 and 20 (FIGURES 2a and 2b) in each of the stages of the lattice filter. The array multiplier receives the coeficients K~-KS~, which are stored in K-stack 31, via lines 32 and either Yn or bn data via bus 40. K-stack 31 preferably comprises ten shift registers, each of which has ten stages. The data stored in K-stack 31 i5 depicted in Table II
and transmitted to array multiplier 30 via lines 32. Array multiplier 30 initiates a different multiplication operation every time period (as indicated by FIGURE 4) i.e., approximately every five microseconds. Array multiplier 30, as will be seen with respect to FIGURE 9 preferably has eight stages; a series of addition and shift operations accomplished as the data propagates through these eight stages, the data is multiplied by the appropriate Kn coefficient stored in K-stack 31. The multiply operation takes 40 microseconds; however, since a new multiplication operation is initiated every five microseconds, eight multiplications are in various stages of completion at any given time. The eignt time period computation period of array multiplier 30 can be seen with respect to the multiplier inputs and outputs in FIGURE 6. For example, the multiplier inputs at time period Tl are outputted from multiplier eight time periods later, at T9. The coefficients stored in K-stack 31 are stored as a nine bit number plus an additional bit for sign information. As aforementioned, these nine bit numbers range from -1 to +1, (decimal equivalents), which, as will be seen, simplifies the structure of array multiplier 30.
The output of array multiplier 30 is applied to adder/
subtractor circuit 33. This output, in the preferred embodiment, is a thirteen bit parallel channel: twelve bits of data and one bit for sign information. It will be appreciated by those skilled in the art, moreo~er, that the number of bits in the data channel are a design choice. The other input to adder/subtractor circuit is provided from (1) the exictation signal 11 at time period Tl, the output of adder/subtractor circuit 33 during time periods T2-T10, the output of shift register 35 during time periods Tll-Tl9 and the output of latch 36 at T20. The particular input to adder/
subtractor circuit 33 is shown, for ease of illustration, as being controlled by various single-pole, single throw switches 37a-37d;
however, it should be appreciated that solid state switches would preferably be used to perform these switching functions, as well as the other depicted switching functions. The output of adder/
subtractor circuit 33 is applied to switch 37b, switch 38a and as the input to one period delay circuit 34. The output from adder/
subtractor circuit 33 is also a thirteen bit wide parallel channel which is delayed by one time period in circuit 34 before being applied as the input to shift register 35 and to switch 38b. Shift register 35 stores the data from the thirteen bit wide channel in thirteen shift registers, each of which has eight stages. Shift register 35 is arranged to perform shift operation only during ~18104 time periods T12-T2. The output of shift register 35 is applied to switch 37c and switch 39. Switch 39 closes at time period T20 for clocking the output of the filter, Yl, into latch memory 36.
~ 0 c,~ ~ ~\ 0,~
B The output 12 of latch memory 36 is applied to analog t3-di-~converter 13 (FIGU~E la) and to switches 37d and 38c.
~, Switch 37b is closed during time periods T2-T10, switch 37c is closed during time periods Tll-T19 and switch 37d is closed at time period T20. Switch 38a is closed during time periods T13-Tl, switch 38b is closed between time periods T3-T12 and switch 38c is closed for time period T2. The other sides of switches 38a, 3gb and 38c are connected to the input to array multiplier 30 via bus 40.
In FIGURE 6 there is listed the various intermediate results occurring in the circuit of FIGURE 5 during time periods Tl-T20. Referring briefly to FIGURE 6, it can be seen that one of the multiplier inputs is the Kn coefficient information while the other input varies according to which switch 38a-38c is closed.
0 ~
At time ~eiro~ Tl switch 38a is closed, as aforementioned, so that the output of the adder/subtractor 33, in this case ~
is applied as a multiplier input. At the same time the other adder input is the excitation signal U(i). At time period T2, the other multiplier input is bl;i-l) which, according to FIGURE
5, is being loaded from the output of latch 36 via switch 38c.
The output of latch 36 according to FIGURE 6 is then Yl(i-l), but recalling the last entry in Table I, it is to be remembered that bl(i-l) is set equal to a delayed Yl(i), i.e., Yl(i-l). Also at time period T2, the other adder input is that which is being currently outputted at the adder output, i.e., in this case, Ylo(i). At time period T3 the multiplier inputs are Klo and Ylo(i), which is derived from the output of one period delay circuit 34. Of course, the results of this multiplication will not be available until time period Tll, at which time it will be provided as one of the .. . .

. .

1~1810~

inputs to adder/subtractor circuit 33. At time period Tll the other input to adder/subtractor clrcuit 33 is taken from the output of shift register 35. The first term loaded from shift ~register 35 is the hlo(i-l) term which was first outputted from shift register 35 at time period T2 and remained at the output thereof since shift register 35, as aforementioned, does not shift between time periods T3 and Tll.
At time period T13 the input to array multiplier 30 is again provided from the output of adder/subtractor circuit 33 via switch 38a. At time period T20 the Yl(i) term is outputted to latch memory 36 from shift register 35 and the current output of latch 36, Yl(i-l) is applied to the other input ~adder/subtractor circuit 33 via switch 37d for providing the bl(i-l) term, as aforementioned. Latch memory 36 stores the filter output (Yl) for one cycle.
The block diagram of FIGURE 5 has been previously explained. The filter of FIGURE 5 may also be utilized in an application equivalent to a N-stage filter having an M-stage multiplier (e.g., there may be M+2 bits in the Kn coefficients),

2~ if a shift register having a delay equivalent to N-M-2 time periods is inserted between adder~subtractor circuit 33 and one period delay circuit 34. The connection to switch 38A is then made from the output of the added shift register and the delay associated with shift register 3~ should then be set equal to N+M-l.
This generalized form of the digital filter is depicted in Figure 11.
In the embodiment of Figure 5, N-M-2 is equal to zero, so no dela~ is required in that embodiment. In the embodiment described with reference to FIGURES 5 and 6, N+M-l equals seventeen which reflects the number of time periods between the time data is applied to shift register 35 and the time that data exits shift register 35. For instance, in FIGURE 6, the b2 (i-l) data is inserted into shift register 35 at time period T2 and exits shift register 35 at time ~8~04 period Tl9, seventeen time period~ later. Ho~ever, shift register 35 only has eight stages in this embodiment, the additional delay occuring during the T3-Tll time periods that shift register 35 is ' not shifted. These nine time periods correspond to when the Y2-Y10 data is available at the output of one period delay circuit 34, which data need not be inputted into shift register 35, as can be seen in FIGURE ~. Thus, the number of stages in shift register 35 plus the number of time periods per cycle that data is not shifted in shift register 35 (if any) equals the N+M-l time period delay through shift register 35.

As can be seen, the equivalent ten stage lattice filter of FIGURES 5 and 6 pèrforms the filtering operation required by the lattice filter 10 of FIGURE la at reasonable data rates. For example, in the preferred embodiment, excitation data 11 is applied at a ten ~ilohertz rate (i.e. every 100 microseconds) and the basic addition operations in adder/subtractor circuit 33 as well as in array multiplier 30 and the shift operations in one period delay circuit 34 and shift register 35 are accomplished in nominal five microsecond time periods. As is well known to those skilled in the art, such speeds are well within the speed capabilities of P-channel MOS large scale integration devices so that the filter of FIGURE 5 may be incorporated in a relatively inexpensive P-channel MOS LSI speech synthesis or complex waveform generation chip.

- 17a -o~ ~
It should also be evident to those skilled in the art, tn~lt the basic arrangement of the ten stage equivalent lattice filter of FIGURE 5 is also applicable to digital filters equivalent to lattice filters having other numbers of stages. Ten stages were selected for the preferred embodiment of the filter, inasmuch as ten stage lattice filters for linear predictive coding speech synthesis circuits have been selected as the standard for use by the Department of Defense of the United States Government. However, should those wishing to practice this invention desire to utilize lO a digital lattice filter having a different number of equivalent stages, it is noted that the number of time periods into which a B cycle is divided should at least~equal to twice the number of equivalent stages. Thus, in the preferred embodiment, the number of time periods (twenty) equals twice the number of equivalent stages (ten). For example, if a twelve stage equivalent filter were desired, the number of time periods per cycle should be at least twenty-four and the basic design heretofore described would merely be expanded. It is noted that for a twelve stage equivalent digital lattice filter that the array multiplier 30 thereof could utilize a O ten time periods to complete a multiplication if the basic scheme heretofore described is followed i.e., one addition and one multiplication operation is initiated each time period. This can be seen from FIGURE 3 by setting N equal to twelve and completing the FIGURE 3 diagram accordingly. Of course, if the five microsecond period for each time period were maintained, the data rates which O- C c ~w~ o ~
may be acco~dated by the twelve stage version would be less than that for the ten stage version of the filter. It should be also noted that by increasing the delay time through the array multiplier 30, that the number of bits in the Ki-K~ coefficients could be 80 increased from a total of ten bits to a total of twelve bits.

Similarly, if an eight stage equi~-alent digital filter were desired, I number of time periods in a cycle would then be at least sixteen and by setting N equal to eight in FIGURE 3 it can be seen o ~ c~
that the propagation time thorug~ multiplier 30 would then be six time periods. In that case, using the array multiplier, which is subsequently described in detail, would limit the number of bits in the coefficients from K-stack 31 to having no more than eight bits. However, as was previously mentioned with respect to FIGURE 4, even more time periods may be used to accomplish a multiply 10 operation in certain embodiments. This may be desired here, as a design choice, if additional accuracy is desired in the ~n coefficients. The additional accuracy would require more bits in the Kn coefficients, which ~n~n, requires more delay through array multiplier 30. The basic design of equivalent filter of FIGURE 5 would be modified somewhat because then a multiply and an addition operation would not be initiated every time period.
It should be evident to those skilled in the art, that in that case, some of the intermediate results obtained within the filter would have to be stored temporarily, thereby, requirins additional 20 storage elements to be included in the filter of FIGURE 5. While such modifications are not spelled out here in detail, such modification to the digital implementation of lattice filter should be within the skill of knowledgeable digital circuit designers.
It has previously been mentioned that the K1o.Ylo(i) and bll(i) intermediate results are generated by the digital filter of FIGURE 5, but these intermediate results are not utilized inasmuch as they are not required to implement lattice filter 10 of FIGURE
la. Now, recalling that the data (V) from the voicing or unvoicing source is multiplied by an amplification factor (A) by a multiplier 30 18 in the conventional speech synthesis circuit of FIGURE la, it has been ~ound that this multiplication may be done by array ltiplier 30 during the time that Klo.Ylo(i) would otherwise be generated by the array multiplier. An embodiment of the digital filter performing this multiplication V(i).A is shown in FIGURE 7.
In FIGURE 8, there is shown the various intermediate results generated in the circuit of FIGURE 7.
i Referring now briefly to FIGURES 7 and 8, it can be seen that this circuit (including the intermediate results generated thereby) is similar to the circuit of FIGURE 5, with the following 10 modifications. The identification numerals of FIGURE 7 are generally the same as used in FIGURE 5, but have a prime added thereto for ease of identification. The data (V) to be multiplied ~by m~Hby~b~Lo~ factor A is applied to one input of array multiplier 30' via a switch 38d' at time period T3 in lieu of applying the output of the one period delay circuit 34 at that time.
At time period Tll, when the multiplication has been completed to form U(i+l), i.e., A-V(i+l), logical zeroes are inputted to the other input of adder/subtractor circuit 33' in lieu of inputting the blo(i-l) in data from shift register 35. Also, of course, 20 ~oth K~ coefficient data and A amplification data must be inputted to K-stack 31'. As can be seen from FIGURES 7 and 8, this embodiment incorporates the function performed by multiplier 18 (FIGURE la) into the digital implementation of lattice filter 10.
The data stored in K-stack 31' is depicted in Table III. The amplification factor A is preferably updated at the same rate as the Kn coefficients are updated in K-stack 31'.
Referring now to FIGURE 9, there is shown, in block diagram form, array multiplier 30. Lines 32-1 through 32-9 ~ ~rrs the least significant through most significant bits, respectively, ~0 of coefficient data from K-stack 32. On lines 32-10 is received ~118104 the sign data from K-stack 31. Another input to array multiplier ~ is received via bus 40. Lines 40-l through 40-12 of bus 40 carry the least significant through most significant bits, ~o--~ ~
g respectively, and line ~3-~ carries the sign of the data on bus 40.
In FIGURE 9, there is an array of elements having reference letters A, B, C, or D (the elements with no reference letter are aiso "Al' type elements, e.g., also correspond to FIGURE lOa). These elements A-D, correspond to the circuits of FIGURES lOa-lOd, respectively. Referring briefly to FIGURES lOa-lOd, the circuits 10 thereof are each enclosed via a dashed line with certain conductors extending across the dashed line. The relative position of the conductors extending across the dashed line of FIGURES lOa-lOd correspond location-wise to the conductors contacting elements A-D
in FIGURE 9. In FIGURE 9, the elements are arranged in eight rows and twelve columns. The eight rows correspond to the eight previously mentioned eight stages of array multiplier 30. These stages are identified on the right-hand side of FIGURE 9 and include the eight shift register cells 51 coupled to line3 40-13.
The twelve columns correspond to the twelve bits of numeric data 20 (on lines 40-l through 40-12) inputted to array multiplier 30.
The data on lines 40-l through 40-13 propagate through array multiplier 30 stage-by-stage in a shift register fashion as it is being multiplied in array multiplier 30. Thus, the propagation time through any given stage is on the order of the aforementioned five microseconds or so.
~ n~32-l from K-stack 31 is coupled to one input of twelve AND gates 52-l through 52-12, the other input of each one being connected to lines 40-1 through 40-12, respectively. The outputs of AND gates 52-12 through 52-1 are applied to the partial ~0 sum inputs of the type A and B elements of stage l (See FIGURES
lOa and lOb).

Lines 32-2 through 32-8 are coupled to the K-stack inputs of the A type elements (FIGURE lOa) in stages 1-7, respectively, ~21-TI-6890 (Ca.) ~1~81~

of array multiplier 30. Line 32-9 is coupled to the input there-from in the C type elements of stage 8 (See FIGURE 10c). The data on lines 40-1 through 40-12 is coupled to the "data-in"
inputs of the stage 1 elements and coupled therethrough to stage 2 through stage 8 elements by the "data-out" terminals of those elements. The partial sum input of the stage 1 elements is derived from the outputs of AND gates 52-1 through 52-12. In the following stages, the partial sum input is derived from the partial sum output from the next more significant bit of the prior stage, with the exception of the partial sum input of element in the most significant bit position whose partial sum input is derived from the carry output from the most significant bit position in the prior stage. Otherwise, the carry-out connections from the elements are serially connected to the carry-in connections in each stage.
Referring now briefly to FIGURE 10a, the data from K-stack 31 determines whether the "partial sum" is to be directly connected to the "partial sum" via a transfer gate 60 or to the output from exclusive OR gate 62 via a transfer gate 61.
An AND gate 63 and an exclusive OR gate 64 are responsive to the data on "data-in" and "partial sum in". Exclusive OR gate 62 is responsive to the output from exclusive OR gate 64 and to "carry-in". An AND gate 65 is responsive to the output of exclusive OR gate 64 and to "carry-in" and the output thereof is provided along with the output from AND gate 63 to an OR
gate 66, whose output is "carry-out." "Data-out" corresponds to "data-in" delayed by a shift register section 67 comprising two inverters, for instance. As can be seen from FIGURE 10c, a C type element is identical to an A type element with the exception that no "data-out" connection is provided nor is a shift register 67 provided. In FIGURE 10b, a B type element is shown which merely provides a "data-out" connection coupled to TI-6890 (Ca.) ~1~8~04 a shift register 67' whose input is "data-in" and a "carry-out"
connection provided by an AND gate 68 whose inputs are "data-in"
and "partial sum in". In FIGURE lOd, the D type element provides merely a "carry-out" signal from an AND gate 68' whose inputs are "data-in" and "partial sum in".
As can be seen, a new partial sum is calculated at each stage, including a necessary transfer of carry information between elements of a stage, but the "partial sum out" remains unchanged if the data on the K-stack line is a logical zero or is added to the data on "data-in" to provide the "partial sum out"
if the data on the line from K-stack 31 is a logical 1. The partial sums are shifted to succeedingly less significant places as data is shifted through the array multiplier. Of course, a least significant bit is lost in each stage of the array multi-plier. However the Kn coefficient data from K-stack 31 corres-ponds to a number in the decimal range of -1 to +1; thus if logical zeroes appear on lines 32-1 through 32 9, the output from array multiplier 30 will be a logical zero and conversely, if the data on lines 32-1 through 32-9 are all logical ones, the data inputted on bus 40 will be outputted from array multiplier 30 unchanged. For the other possible data patterns on lines 32-1 through 32-9, the data on bus 40 will be scaled between zero and the inputted value on bus 40 in 29 possible steps, according to the magnitude of the data on lines 32-1 through 32-9.
Inasmuch as the data shifts through array multiplier 30 stage-by-stage in a shift register fashion, the data from K-stack 31 is skewed as shown in Tables II and III, for instance, to assure that the appropriate bit of the appropriate coefficient arrives at the appropriate time in array multiplier 30. In FIGURES lOa-lOc the timing pulses for operating those circuits in the aforementioned shift register fashion are not depicted here, for, as is well known to those skilled in the art, such timing fun_tion may be provided ~ adding clocked gates to the circuits of FIGURES lOa-lOc or by utilizing precharge and conditional discharge type logic, and therefore such timing considerations are not shown here in detail.
Referring again briefly to FIGURE 9, the sign data on Ll ~i~.
~ e~ 40-13 is merely delayed during the eight stage delay or array multiplier 30 via shift register elements 51 and then compared with the sign data from K-stack 31 on line 32-10 at exclusive OR gate 53, thereby providing a correct sign of the outputted data according 10 to the normal rules of multiplication.
Referring again briefly to FI~URES 5 and 7, the array multiplier 30 (or 30') thereof has been described in detail. The remaining elements, such as the adder/subtractor circuit 33 (or 33'), ~period delay circuit 34 (or 34'), shift register 35 (or 35') and latch memory 36 (or 36') are not shown in such detail, since such conventional elements are well known. The adder/subtractor circuit 33 (or 33') receives signed data on its two inputs and should determine whether a subtraction or addition operation is called for based on the particular sign inputted with the data.
Hàving described the invention with respect to several embodiments thereof, additional modification may now suggest itself to those skilled in the art. The invention is not to be limitea to the particular embodiments described, except as set forth in the appended claims.

~B~04 TABLE I ~8~

EQUATION STAGE
10 (i) Yll (i) -Kloblo (i-l) 10 ,j Yg (i)=Ylo (i) -K9bg (i-l) 9 blo(i)=bg(i-l)+KgYg(i) 9 8(i) Yg(i)-K8b8(i-1) 8 bg(i)~b8(i-l)+K8Y8(i) 7(i) Y8(i)-K7b~(i-1) b8(i)=b7(i-1)+K7Y7(i) 7 6(i) Y7(i)-K6b6(i-1) 6 b7(i)=b6(i-l)+K6Y6( ) 6 .
5(i) Y6(i)-K5b5(i-1) 5 b6(i)=b5(i-l)+K5Y5(i) 5 Y (i)=Y (i)-K b4(i-1) 4 b (i)=b (i-l)+K4Y4(i) 4

3(i) Y4(i)-K3b3(i-1) b (i)=b (i-l)+K3Y3(i) 3 2(i) Y3(i)-K2b2(i-1) 2 b3(i)=b2(i-l)+K2Y2( ) 2 Yl (i) Y2 (i) -Klbl (i-l) b2 (i) =bl (i-l) +KlYl (i) bl ( i) =Yl ( i ) 1~8104 TABLE I I

K-STACK
OUTPUTTIME PERIOD
Bit I Linel Tl T2 T3 T4 T5 T6 T7 T8 T9 TlO
_ _ _. TllTl2T13 Tl4 T15 T16 T17 T18 T19 TZO
LSB32-1 2 lKlo Kg K8 K7 K6 K5 K K
32-2 2KlKlo Kg K8 K7 K6 K5 K4 K3 32-3 3 2 1 Klo Kg K8 K7 K6 K5 K4 32-4 4 3 2 Kl K10 Kg K8 K7 K6 K
32-5 K K4 K3 K2 Kl KlO Kg K8 7 6 32-6 K6K5 K4 K3 K2 Kl K10 9 8 7 32-7 7 6 5 K4 K3 K2 Kl KlO Kg K8 32-8 8 7 6 K5 K4 K3 K2 Kl K10 Kg SIGN
BIT32-ld K K8 K7 K6 K5 K4 K3 K2 1 10 o ~ 8~ o o E~ ~ X
a~ o 0 o X ~4 X
.j X ~4 ~D C~
X ;~ p~;m U~ C~
a E~ ~ X~ x~

E~ ~ ~ ~ ~ ~ X ~ ~ ~ ~

m ~ K ~ ;~ X

H ~ a ~ ~ ~ ~ ~
H O E-~ X X ~ ~ X X ~ P~; ;Y;
H ~Y; H

~ ~ ~ ~ X~

o a ~ m u~ ~ N ~`I
O E~ ~ X

E~ ~ X ~

E~ ~ X P~ ~ X

3~ X~ X X~

E~ X ~

~ X ~ ~ ~ X
,~ ~ ~ e~l U~ ~ l` C~ C~ o , E~
D ~ -- 2 7

Claims (65)

TI-6890 (Can.) The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A digital filter responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients, said filter comprising:
first memory means for storing said plurality of digital values;
a multiplier circuit;
first circuit means for coupling said first memory means and said multiplier circuit;
an arithmetic circuit having an input coupled to said multiplier circuit, second memory means for storing data outputted from said arithmetic circuit; and second circuit means for selectively coupling the outputs of said second memory means and said arithmetic circuit to an input of said multiplier circuit.

2. The filter according to Claim 1, wherein said second memory means for storing data outputted from said arithmetic circuit comprises digital storage means.

3. The filter according to Claim 1, wherein said second memory means includes first and second delay circuit means, the delay associated with said second delay circuit means being longer than the delay associated with said first delay circuit means and wherein said second circuit means selectively couples the outputs of said first and second delay circuit means to said multiplier circuit.

TI-6890 (Can.)

4. The filter according to Claim 3, wherein said second memory means further includes latch storage means for temporarily storing data outputted from said arithmetic circuit and wherein said second circuit means further selectively couples the output of said latch storage means to said multiplier circuit.

5. The filter according to Claim 4, wherein said excitation signal is coupled to said multiplier circuit by said second circuit means and wherein an amplification factor associated with said excitation signal is stored in said first memory means along with said digital values.

6. The filter according to Claim 5, wherein each one of the digital values is updated once during a plurality of cycles, wherein the excitation signal is updated each cycle, wherein each cycle includes a plurality of time periods and wherein the multi-plier circuit initiates a new multiply operation every time period and takes a plurality of time periods to complete a multiply operation.

7. The filter according to Claim 4, wherein the excita-tion signal is coupled to said arithmetic circuit.

8. The filter according to Claim 7, wherein each one of the digital values is updated once during a plurality of cycles, wherein the excitation signal is updated each cycle, wherein each cycle includes a plurality of time periods and wherein the multi-plier circuit initiates a new multiply operation every time period and takes a plurality of time periods to complete a multiply operation.

TI-6890 (Can.)

9. The filter according to Claim 4, further including third circuit means for selectively coupling the outputs of said latch storage means, said second delay circuit means and said arithmetic circuit to an input of said arithmetic circuit.

10. A digital filter for implementing a lattice filter responsive to a digital excitation signal, an amplification factor and to a plurality of digital values representing filter coefficients, said filter comprising:
first memory means for storing said plurality of digital values and said amplification factor;
a multiplier circuit;
first circuit means for coupling said first memory means and said multiplier circuit;
an arithmetic circuit having an input coupled to said multiplier circuit;
second memory means for storing data outputted from said arithmetic circuit; and second circuit means for selectively coupling the output of said second memory means, the output of said arithmetic circuit and said excitation signal to an input of said multiplier circuit.

11. The filter according to Claim 10, wherein said second memory means for storing data outputted from said arith-metic circuit comprises digital storage means.

12. The filter according to Claim 10, further including third circuit means for selectively coupling the outputs of said second memory means and said arithmetic circuit to another input of said arithmetic circuit.

TI-6890 (Can.)

13. The filter according to Claim 10, wherein said second memory means includes first and second delay circuit means, the delay associated with said second delay circuit means being longer than the delay associated with said first delay circuit means, and wherein said second circuit means selectively couples the outputs of said first and second delay circuit means to said multiplier circuit.

14. The filter according to Claim 13, wherein said second memory means further includes latch storage means for temporarily storing data outputted from said arithmetic circuit and wherein said second circuit means further selectively couples the output of said latch storage means to said multiplier circuit.

15. The filter according to Claim 14, wherein each one of the digital values is updated once during a plurality of cycles, wherein the excitation signal is updated each cycle, wherein each cycle includes a plurality of time periods and wherein the multiplier circuit initiates a new multiply operation every time period and takes a plurality of time periods to com-plete a multiply operation.

16. The filter according to any of Claims 10-12, wherein said digital filter is utilized in a speech synthesis circuit for producing human-like sounds in response to the exci-tation signal and the digital values and wherein said speech synthesis circuit includes means coupled to receive selected outputs of said arithmetic circuit for converting said selected outputs of said arithmetic circuit to audible sounds.

TI-6890 (Can.)

17. A digital filter for implementing a lattice filter responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients, said filter comprising:
first memory means for storing said plurality of digital values;
a multiplier circuit;
first circuit means for coupling said first memory means and said multiplier circuit;
an arithmetic circuit having an input coupled to said multiplier circuit;
second memory means for storing data outputted from said arithmetic circuit;
second circuit means for selectively coupling the outputs of said second memory means and said arithmetic circuit to an input of said multiplier circuit; and third circuit means for selectively coupling the output of said second memory means, the output of said arithmetic circuit and said excitation signal to another input of said arithmetic circuit.

18. A digital filter responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients, said filter comprising:
a multiplier circuit;
an adder/subtractor circuit coupled at a first input thereof to the output of said multiplier circuit;
first delay circuit means coupled to the output of said adder/subtractor circuit;
second delay circuit means coupled to receive data outputted from said adder/subtractor circuit, the delay associated TI-6890 (Can.) with said second delay circuit means being longer than the delay associated with said first delay circuit means;
latch storage means for temporarily storing data out-putted from said second delay circuit means;
first switching means for selectively coupling the output of said latch storage means, the output of said first delay circuit means and the output of said adder/subtractor circuit to a first input of said multiplier circuit;
second switching means for selectively coupling the output of said latch storage means, the output of said adder/
subtractor circuit and said second delay circuit means to a second input of said adder/subtractor circuit; and memory means coupled to a second input of said multiplier circuit for storing said digital values representing the filter coefficients.

19. The filter according to Claim 18, wherein said second switching means further selectively couples the excitation signal to the second input of said adder/subtractor circuit.

20. The filter according to Claim 19, wherein said multiplier circuit and said adder/subtractor circuit receive data in parallel and output data in parallel at the respective inputs and outputs thereof.

21. The filter according to Claim 20, wherein the excitation signal is updated once a cycle, wherein a cycle includes a plurality of time periods, and wherein said multiplier circuit initiates a new multiply operation every time period, but requires a plurality of time periods to complete a multiply operation.

TI-6890 (Can.)

22. The filter according to Claim 21, wherein said multiplier circuit is an array multiplier.

23. The filter according to Claim 22, wherein the number of time periods in a cycle is equal to twice the number of filter coefficients.

24. The filter according to Claim 23, wherein the number of time periods required by said array multiplier to complete a multiply operation is equal to two less than the number of filter coefficients.

25. The filter according to Claim 24, wherein the output of said latch storage means is coupled to a digital to analog converter and said digital filter is utilized in a speech synthesis circuit.

26. The filter according to Claim 19, wherein said first switching means further selectively couples the excitation signal to the first input of said multiplier circuit and wherein a digital amplification signal is inputted to said memory means.

27. The filter according to Claim 26, wherein said multiplier circuit and said adder/subtractor circuit receive data in parallel and output data in parallel at the respective inputs and outputs thereof.

28. The filter according to Claim 27, wherein the excitation signal is updated once a cycle, wherein a cycle includes a plurality of time periods, and wherein said multiplier circuit initiates a new multiply operation every time period, but requires a plurality of time periods to complete a multiply operation.

TI-6890 (Can.)

29. The filter according to Claim 28, wherein said multiplier circuit is a array multiplier.

30. The filter according to Claim 29, wherein the number of time periods in a cycle is equal to twice the number of filter coefficients.

31. The filter according to Claim 30, wherein the number of time periods required by said array multiplier to complete a multiply operation is equal to two less than the number of filter coefficients.

32. The filter according to Claim 31, wherein the output of said latch storage means is coupled to a digital to analog converter and said digital filter is utilized in a speech synthesis circuit.

33. The filter according to Claim 18, wherein said multiplier circuit is an array multiplier receiving data in parallel at the inputs thereof.

34. The filter according to Claim 33, wherein the excitation signal is updated once a cycle, wherein a cycle includes a plurality of time periods, and wherein said array multiplier initiates a new multiply operation every time period, but requires a plurality of time periods to complete a multiply operation.

35. The filter according to Claim 34, wherein the number of time periods in a cycle is equal to twice the number of filter coefficients.

TI-6890 (Can.)

36. The filter according to Claim 35, wherein said second switching means further selectively couples the excitation signal to the second input of said adder/subtractor circuit.

37. The filter according to Claim 36, wherein the output of said latch storage means is coupled to a digital to analog converter and said digital filter is utilized in a speech synthesis circuit.

38. The filter according to Claim 35, wherein said first switching means further selectively couples the excitation signal to the first input of said array multiplier and wherein a digital amplification signal is inputted to said memory means.

39. The filter according to Claim 38, wherein the output of said latch storage means is coupled to a digital to analog converter and said digital filter is utilized in a speech synthesis circuit.

40. A digital filter for a speech synthesis circuit, said filter being responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients, said filter comprising:
a multiplier circuit;
an adder/subtractor circuit coupled at a first input thereof to the output of said multiplier circuit;
delay circuit means coupled to the output of said adder/
subtractor circuit;
latch storage means for temporarily storing data out-putted from said delay circuit means;

TI-6890 (Can.) first switching means for selectively coupling the output of said latch storage means, the output of said delay circuit means and the output of said adder/subtractor circuit to a first input of said multiplier circuit;
second switching means for selectively coupling the output of said latch storage means, the output of said adder/
subtractor circuit and said delay circuit means to a second input of said adder/subtractor circuit;
memory means coupled co a second input of said multiplier circuit for storing said digital values representing the filter coefficients; and digital to analog converter means coupled to the output of said latch storage means.

41. The filter according to Claim 40, wherein said second switching means further selectively couples the excitation signal to the second input of said adder/subtractor circuit.

42. The filter according to Claim 41, wherein the excitation signal is updated once a cycle, wherein a cycle includes a plurality of time periods, and wherein said multiplier circuit initiates a new multiply operation every time period, but requires a plurality of time periods to complete a multiply operation.

43. The filter according to Claim 42, wherein said multiplier circuit is an array multiplier.

TI-6890 (Can.)

44. The filter according to Claim 40, wherein said delay circuit means includes first and second delay circuit means, the delay associated with said second delay circuit means being longer than the delay associated with said first delay circuit means, wherein said latch storage means temporarily stores data outputted from said second delay circuit means, wherein said first switching means selectively couples the output of said first delay circuit means to said first input of said multiplier circuit and wherein said second switching means selectively couples the output of said second delay circuit means to said second input of said adder/subtractor circuit.

45. The filter according to Claim 40, wherein said first switching means further selectively couples the excitation signal to the first input of said multiplier circuit and wherein a digital amplification signal is inputted to said memory means.

46. The filter according to Claim 45, wherein the excitation signal is updated once a cycle, wherein a cycle includes a plurality of time periods, and wherein said multiplier circuit initiates a new multiply operation every time period, but requires a plurality of time periods to complete a multiply operation.

47. The filter according to Claim 46, wherein said multiplier circuit is an array multiplier.

TI-6890 (Can.)

48. A digital filter responsive to time period timing signals and to a plurality of digital values representing filter coefficients, said filter being equivalent to an N stage lattice filter and comprising:
an array multiplier having M stages;
an adder/subtractor circuit coupled at a first input thereof to the output of said multiplier;
first delay circuit means having a delay of N-M-2 time periods and coupled to the output of said adder/subtractor circuit;
second delay circuit means coupled to the output of said first delay circuit means;
third delay circuit means coupled to the output of said second delay circuit means, the dealy associated with said third delay circuit means being equal to N+M-1 time periods;
latch storage means for temporarily storing selected data outputted from said adder/subtractor circuit;
first switching means for selectively coupling the output of said latch storage means, the output of said second delay cir-cuit means and the output of said first delay circuit means to a first input of said multiplier;
second switching means for selectively coupling the output of said latch storage means, the output of said adder/
subtractor circuit and the output of said third delay circuit means to a second input of said adder/subtractor circuit; and memory means coupled to a second input of said multiplier for applying said digital values representing filter coefficients thereto.

TI-6890 (Can.)

49. A digital filter responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients, said filter comprising:
a digital array multiplier;
memory means for storing said plurality of digital values;
circuit means for coupling said memory means to one input of said multiplier;
an arithmetic circuit having an input coupled to said multiplier for performing arithmetic operations on data outputted from said multiplier;
first delay circuit means for temporarily storing at least a portion of the results of the arithmetic operations performed by said arithmetic circuit;
first switching means for selectively coupling the output of said first delay circuit means to another input of said multiplier;
second delay circuit means for temporarily storing at least a portion of the results of the arithmetic operations performed by said arithmetic circuit, said second delay circuit means storing the results of the arithmetic operations for a longer period of time than said first delay circuit means;
second switching means for selectively coupling the output of said second delay circuit means to an input of said arithmetic circuit; and filter output means for outputting a selected portion of said results of the arithmetic operations performed by said arithmetic circuit.

TI-6890 (Can.)

50. The filter according to Claim 49, further including third switching means for selectively coupling the output of said arithmetic circuit to an input thereof.

51. The filter according to Claim 50, further including fourth switching means for selectively coupling said digital excitation signal to said another input of said multiplier.

52. The filter according to Claim 50, wherein said digital filter is further responsive to a digital amplitude signal and wherein said circuit means includes means for coupling said digital amplitude signal along with said digital values from said memory means to said multiplier.

53. The filter according to Claim 50, further including fourth switching means for selectively coupling said digital excitation signal to said arithmetic circuit.

54. A digital filter responsive to a digital excitation signal and to a plurality of digital values representing filter coefficients for implementing an N stage lattice filter comprising:
a multiplier circuit;
memory means for storing digital values representing filter coefficients;
circuit means for coupling said memory means to an input of said multiplier circuit;
an arithmetic circuit having an input coupled to said multiplier circuit for performing arithmetic operations on data outputted from said multiplier circuit;

TI-6890 (Can.) first delay circuit means for temporarily storing at least a portion of the results of the arithmetic operations performed by said arithmetic circuit;
first switching means for selectively coupling the output of said first delay circuit means to another input of said multiplier circuit;
second delay circuit means for temporarily storing at least a portion of the results of the arithmetic operations performed by said arithmetic circuit, said second delay circuit means storing said results of the arithmetic operations for a longer period of time than said first delay circuit means;
second switching means for selectively coupling the output of said second delay circuit means to an input of said arithmetic circuit; and filter output means for outputting a preselected portion of said results of the arithmetic operations performed by said arithmetic circuit.

55. The filter according to Claim 54, further including third switching means for selectively coupling the output of said arithmetic circuit to an input thereof.

56. The filter according to Claim 55, further including fourth switching means for selectively coupling a digital excita-tion signal to said another input of said multiplier circuit.

57. The filter according to Claim 55, wherein said digital filter is responsive to a digital amplitude signal and wherein said circuit means includes means for coupling said digital amplitude signal along with said digital values repre-senting filter coefficients from said memory means to said multiplier circuit.

TI-6890 (Can.)

58. The filter according to Claim 55, further including fourth switching means for selectively coupling a digital exci-tation signal to said arithmetic circuit.

59. A method of generating a complex waveform from a digital excitation signal using a number of digital values repre-senting filter coefficients, said digital excitation signal being updated once a cycle, each said cycle having a plurality of time periods, said method comprising the steps of:
initiating a multiply operation once each time period per cycle with a multiplier, said multiplier requiring a plurality of time periods to complete a multiplication;
supplying, at least during a majority of the time periods each cycle, selected digital values representing filter coefficients to a first input of said multiplier;
initiating an arithmetic operation once each time period per cycle at an adder/subtractor means, the output of the multiplier providing an input to the adder/subtractor means;
temporarily storing the output of the adder/subtractor means in a memory;
temporarily storing in a latch memory means selected data from said memory;
selectively providing data outputted from said latch memory means, from said memory and from said adder/subtractor means to a second input of said multiplier; and selectively providing data outputted from said adder/
subtractor means, from said memory and from said latch memory means to another input of said adder/subtractor means.

TI-6890 (Can.)

60. The method according to Claim 59, wherein said memory has a first output corresponding to a temporary storage equal to one time period and a second output corresponding to a temporary storage equal to a plurality of time periods, wherein said latch memory means temporarily stores selected data from the second output of said memory, wherein the data selectively provided from said memory to the second input of said multiplier is pro-vided from said first output of said memory and wherein the data selectively provided to said adder/subtractor means from said memory is provided from the second output of said memory.

61. The method according to Claim 60, wherein the digital values represent N filter coefficients and wherein each cycle has 2N time periods.

62. The method according to Claim 61, wherein said multiplier requires N-2 time periods to complete a multiplication.

63. The method according to Claim 59, where the step of selectively providing data from several recited outputs to the another input of said adder/subtractor means includes selectively providing the digital excitation signal to the another input of said adder/subtractor means.

64. The method according to Claim 59, wherein the step of selectively providing data from several recited outputs to the second input of said multiplier includes providing the digital excitation signal to the second input of said multiplier, said method further including the step of supplying a digital ampli-fication factor to the first input of said multiplier.

TI-6890 (Can.)

65. A method of generating human-like sounds from a digital excitation signal, a digital amplitude signal and N
digital filter coefficients in an electronic filter, said method comprising the steps of:
repetitively initiating 2N multiply operations with a multiplier using all but one of the digital filter coefficients stored in a memory as one operator twice during the 2N operations, using said one of the digital filter coefficients once as one operator during the 2N operations and using said digital amplitude signal once as one operator during the 2N operations;
repetitively initiating an arithmetic operation in an arithmetic circuit using the results of the multiply operation as an operator in the arithmetic operation;
temporarily storing selected data outputted from the arithmetic circuit;
repetitively using temporarily stored data outputted from the arithmetic circuit as another operator in said multiplier during N of said 2N operations;
repetitively using data outputted from the arithmetic circuit as another operator in said multiplier during N-1 of said 2N operations;
repetitively using the digital excitation signal as another operator in said multiplier once during each 2N operations, said digital excitation signal being multiplied with said digital amplitude signal; and converting selected ones of the results of the arithmetic operations performed by said arithmetic circuit to sound.