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Digital waveform generation using a computer, Pt.3 This month we continue our mini series on digital signal generation with a couple of interesting software listings. By putting in a little work at the keyboard, you can turn your PC into an assortment of audio test gear. By STEVE PAYOR First up, we have an audio tone burst generator capable of generating signals to the IHF standard, or any other you might care to program. Listing 1 and the CRO photographs of Fig.2 provide all the details. The program is written in Borland's TURBO BASIC and achieves a data rate of 20k samples/sec. If you have a copy of TURBO BASIC, the only change you may need to make to the program is the address of the parallel printer port. If you have a different BASIC compiler (eg, Microsoft's QUICK BASIC), try it. Once you have sorted out any minor syntax clifferences, and provided the compiler is not pathetically slow, the only alteration you will have to make is to the SAMPLE.TIME! variable (here set to 50E-6 or 50µs). By the way, the"!" on the end of a variable name makes it a real variable instead of an integer. If you want to save some typing by shortening the variable names, just be careful to keep them of the same type, real or integer, as the original. You can also save a lot of typing by leaving out all the comments, although this may be a false economy in the long run. For the uninitiated, anything to the right of I Data bits D? ~----DURATION----~ BURST .DURATION to 1kHz +OdB -.1dB 0 D-A PC RESISTOR PARALLEL NETWORJ< PRINTER PORT a single quote mark is a comment, or REMark statement. Although these statements are completely ignored by the compiler and contribute nothing to the running of the program, they are worth their weight in gold to the programmer because they describe each step. The tone burst program functions in much the same way as the frequency sweep generator described last month. Once the data array has been filled with bytes, they are output repeatedly until a key is pressed. The time taken for the program to read the keyboard status leaves a tiny "gap" in the waveform. This small gap is not likely to be of any real consequence over the normal tone burst interval of half a second. It becomes an embarrassment, however, if you alter the tone burst parameters in the hope of generating continuous sine waves with this program. Generating a continuous waveform without any gaps is a much more difficult undertaking and this is where our second program excels. IIUIIIIIIIUIUIIIIIIIIUIIIIIIIIIIIIIIIIUIIIIIIII- 3RD ORDER LOW-PASS FILTER DO -STROBE i------------► CRO sync pulse ~ BURST.START Fig.1: how to set up the hardware for the tone burst program of Listing 1. The D-A converter and filter were described in the March 1990 issue. Note that all the parameters of the tone burst (eg, frequency, burst duration, etc) are fully programmable - just change the numbers in the program. 92 SILICON CHIP Listing 1: Tone Burst Generator • SILICON CHIP PROGRAMMABLE AUDIO TONE BURST GENERATOR <TURBO BASIC 1.1> • Copyright CC> Slllcon Chip Publications Pty. Ltd., 1990 • Data rate: 20k saaples/sec <running on a 4.77MHz PC-XT> DEFINT A-Z • All variables are integers unless otherwise indicated DIM TONE.BURST(lOOOO) • At 50vsec/saaple, this array can store up to 500asec • of wavefor• • (Note: The variables listed below are suitable for testing power a•plifiers • to specification IHF-A-202 1978, The Institute of High Fidelity, Inc, USA.> • Variables defining tone burst: FREQUENCYalOOO DURATION=500 BURST.DURATIONa20 BURST.START=20 MIN.AMPLITUDEl=.l • • • • • Frequency <Hz> Total duration (in cycles of sine wave) Burst duration <cycles> Tiae of burst start after CRO sync pulse (cycles> Aaplitude during •quiet• part of burst (0.1 = -20d8) • Prograa constants: PORT.A=&H378 PORT.C=PORT.A+2 Pil=3.141593 SAMPLE.TIMEl=50E-6 • Parallel port • addresses <Note: Other possible base addresses for the printer port are Hex 3BC or Hex 278> • Deterained by experiaent. Change this to suit faster • CPU's, or faster or slower BASIC coapilers • No. of saaples in one cycle SAMPLES=l/FREQUENCY/SAHPLE.TIMEI • Total no. of saaples in entire wavefor• N•SAHPLES*DURATION BURST.END=BURST.START+BURST.DURATION FOR X=O TO SAMPLES-1 • This section of code fills the SINE.WAVEl=127.5*SINCX/SAMPLES*6.2832) • wavefora array with the required FOR CYCLE=O TO DURATION-1 • signal IF CYCLE<BURST.START OR CYCLE>=BURST.END THEN AMPl=HIN.AHPLITUDEI ELSE AMPl=l TONE.BURST(CYCLE*SAMPLES+X>=AHPl*SINE.WAVEl+127.5 NEXT CYCLE • <Be patient - it takes a whole NEXT X • 4 seconds of coaputing tiae> TONE.BURST<N>=128 OUT &H21,INP<&H21) OR 1 • Disable DOS real tiae clock interrupt WHILE NOT INSTAT OUT PORT.C,O • Positive edge of CRO sync pulse FOR X•O TO N:OUT PORT.A,TONE.BURST<X>:NEXT X • Tone burst wavefora OUT PORT.C,1 • Negative edge of CRO sync pulse WEND OUT &H21,INPC&H21) AND &HFE • Restore DOS real tiae clock interrupt END Full range function generator The program of Listing 2 can fit waveform cycles together without any visible seams, thanks to some clever programming. First, it uses a high-speed machine code subroutine to do the actual outputting - we were able to tune this to a speed of exactly 100k samples/sec on an ordinary 4.77MHz XT PC. Second, the code is written in such a way that the time between OUT instructions stays the same, even while the loop counters are being reset. The problem of breaking out of the loop is solved by re-directing the standard keyboard interrupt, which accounts for the complexity of an otherwise short program. Because the interrupt is done by the hardware, no instructions are needed within the loop to check for a keypress . If you are worried about the size of this listing, note that the machine code can be typed in as a simple list of decimal numbers, as shown at the end of the listing. This conMAY 1990 93 Listing 2: Sine, Triangle & Square Wave Generator 10 • SILICON CHIP DIGITAL AUDIO SIGNAL GENERATOR (GWBASIC 3.22) 20 • Copyright CC) Silicon Chip Publications Pty. Ltd., 1990 30 • Data rate: 100k saaples/sec <running on a 4.77MHz PC-XT> 40' 50 DEFINT A-Z • All variables are 16-bit integers unless specified 60 I=O:BYTE•O • otherwise. The variables are being initialised here 70 NO.OF.BYTES=O • for the sole purpose of establishing their locations 80 START.ADDRESS=O • at the start of the GWBASIC data segaent. If an 90 PORT.ADDRESS=&H378 • unused variable happens to appear after the arrays 100 CODE.ADDRESS=O • have been diaensioned, the arrays will be physically 110 FREQ!=O:V=O:Kt=•• • aoved to aake rooa for it, requiring the address 120 • pointer for each array to be re-evaluated 130 • 140 DIM NACHINE.CODE(55) • Sufficient space for 112 bytes of aachlne code 150 DIM BYTE.ARRAY(25000)' Wavefora storage space - 50k bytes - sufficient for 160 • Ii second at a data r ,a te of 100k bytes per second 170 • 180 DEF SEG • Set Data Segaent register to GWBASIC•s data segaent 190 • 200 • MACHINE CODE SUBROUTINE TO OUTPUT A STREAM OF BYTES QUICKLY TO I/O PORT 210 • 220 • Usage: CALL CODE.ADDRESSCPORT.ADDRESS,START.ADDRESS.NO.OF.BYTES) 230 • 240 • The following section sets up the aachine code subroutine in an integer 250 • array MACHINE.CODE, within the GWBASIC data segaent: 260 • 270 START.ADDRESS=VARPTR<MACHINE.CODE<O>> 280 NO.OF.BYTES=t12 290 FOR I=O TO NO.OF.BYTES-1 • This section of code sets up the 300 READ BYTE • aachine code subroutine within 310 POKE START.ADDRESS+I.BYTE • the GWBASIC data segaent 320 NEXT I • 330 • 340 DATA &H8B,&HEC :• MOV BP,SP Load current stack pointer into BP 350 DATA &H8B,&H5E,&H08 :• NOV BX, CBP 1+8 Get address of 1st paraaeter 360 DATA &H8B,&H17 :• NOV DX.CBXJ Load 1/0 port address into DX 370 :• 380 DATA &H8B,&H5E,&H06 :• MOV BX,tBPJ+6 Get address of 2nd paraaeter 390 DATA &H8B,&H37 :• HOV Sl,CBXJ Load SI with starting address of bytes 400 :• 410 DATA &H8B,&H5E,&H04 :• HOV BX,tBP1+4 Get address of 3rd paraaeter 420 DATA &H8B,&HOF :• HOV CX,CBXJ Load no. of bytes into CX 430 :• Clear direction flag 440 DATA &HFC :' CLD 450 :• 460 DATA &HE4,&H21 :• IN AL,21H Get interrupt aask register 470 DATA &HOC,&HOl :' OR AL, 1 Disable tiaer interrupts 480 DATA &HE6,&H21 :• OUT 21H,AL Write interrupt aask register 490 :• 500 : ' The next section of code alters the keyboard 510 : ' interrupt pointers so that a keyboard interrupt will 520 : ' get us out of the continuous signal generation loop 530 :• 540 DATA &HE8,&HOO,&HOO :• CALL 0 Puts IP for next instruction on stack 550 DATA &H5D :' POP BP Gets address of this instruction in BP 560 DATA &H83,&HC5,&H2F :• ADD BP.47 BP now points to the instruction after the •infinite• loop below 570 :' 580 DATA &HB8,&HOO,&HOO :• HOV AX,O Clear AX 590 DATA &H8C.&HDB :• HOV BX,DS Save DS in BX 600 DATA &H8E,&HD8 :• HOV DS,AX Clear OS (to access low aeaory) 610 DATA &HFF,&H36,&H26,&HOO:' PUSH 26H Save CS of interrupt pointer on stack 620 DATA &HFF,&H36,&H24,&HOO:' PUSH 24H Save IP of interrupt pointer on stack 94 SILICON CHIP 630 DATA &HBC,&HOE,&H26,&HOO:' NOV 26H,CS Change CS of interrupt pointer 640 DATA &H89,&H2E,&H24,&HOO:' MOY 24H,BP Change IP of interrupt pointer :• NOV DS,BX Restore OS 650 DATA &HSE,&HDB :, 660 670 DATA &H8B,&HD9 :• MOV BX,CX Make a copy of CX in BX, and SI in DI 680 DATA &HBB,&HFE :' MOV DI,SI so that these registers can be reset : 690 quickly :, 700 710 :' Allowing for aeaory refresh interrupts, the loop 720 :' below executes in alaost exactly 10 psec, on a 730 :• 4.77MHz PC-XT :, 740 750 DATA &H90 :' NOP Adjusts loop tiaing 760 DATA &HAC Load byte into accuaulator : ' LOOS Output byte (40 clocks> 770 DATA &HEE : ' OUT DX,AL 780 DATA &HE2,&HFB Loop CX tiaes : ' LOOP -5 790 DATA &H90 :' NOP :• NOV CX,BX 800 DATA &H8a.&HCB Restore loop counter 810 DATA &H88,&HF7 :• HOV SI,DI Restore string pointer (32 Locks) 820 DATA &HAC Load byte : ' LOOS 830 DATA &HEE : ' OUT DX,AL Output byte 840 DATA &HE2,&HF3 Continue looping ----\40 clocks) :' LOOP -13 :, 850 :• We get here on a keyboard interrupt. Before juaping 860 870 :' to the keyboard interrupt service routine, we need to 880 :' adjust the IRET address on the stack, to force the 890 :' interrupt service routine to return below :, 900 910 DATA &H83,&HC5,&HOC :• ADD BP,12 Adjust BP to point to the interrupt :, 920 address restoration routine below 930 DATA &H83,&HC4,&H02 :' ADD SP,2 Reaove IP for IRET from stack 940 DATA &H55 :• PUSH BP Substitute the desired address : 950 960 DATA &HBB,&HEC :• MOV BP,SP Get stack pointer into BP 970 DATA &HFF,&H6E,&H06 :' JMP CBPl+6 Juap off to service keyboard interrupt :, 980 990 :• Now restore the original keyboard interrupt pointers :, 1000 1010 DATA &HB8,&HOO,&HOO :' HOV AX,O Clear AX :• MOV BX,DS 1020 DATA &HSC,&HDB Save DS in BX 1030 DATA &H8E,&HD8 :' HOV DS,AX Clear OS 1040 DATA &H8F,&H06,&H24,&HOO:' POP 24H Restore IP of keyboard interrupt 1050 DATA &HBF,&H06,&H26,&HOO:' POP 26H Restore CS of keyboard interrupt 1060 DATA &HSE,&HDB :' MOY DS,BX Restore OS :, 1070 1080 DATA &HE4,&H21 :' IN AL,21H Get interrupt aask register 1090 DATA &H24,&HFE Re-enable tiaer interrupts : ' AND AL, FEH 1100 DATA &HE6,&H21 Write interrupt aask register :' OUT 21H,AL :, 1110 1120 DATA &HCA,&H06,&HOO :• RET 6 Return and pop GWBASIC's CALL 1130 :• paraaeters fro• stack . <----,, < <_J . 1140' 1150 • Progra• proper starts here 1160 ' 1170 CLS 1180 INPUT•Frequency <Hz) ••• •,FREQt:IF FREQ1(2 THEN END 1190 NO.OF.BYTES=lOOOOOI/FREQt 'Calculate no. of saaples in one cycle 1200' 1210' The following section of code sets up an array of data bytes for one 1220' cycle of the wavefora. Choose the appropriate calculation for sine, 1230' triangle or square waves by turning the unwanted lines into REM 1240' stateaents with a single quote aark 1250' continued next page MAY 1990 95 Listing 2: continued from previous page 1260 1270 1280 1290 1300 1310 1320 1330 1340 1350 1360 1370 1380 1390 340 350 360 370 380 390 400 410 420 430 440 450 START.ADDRESS=VARPTR<BYTE.ARRAY<O>> FOR I=O TO NO.OF.BYTES-1 • Sine V=l27.5*<l+COS(I/NO.OF.BYTES*6.2832)) • Triangle • V=ABS<IINO.OF.BYTES*2~1)*255 • IF I<NO.OF.BYTES/2 THEN V=O ELSE V=255 • Square • White noise? • V=RND*255 POKE START.ADDRESS+l,V NEXT I • CODE.ADDRESS=VARPTR<MACHINE.CODE(O)) • Get address of subroutine • CALL CODE.ADDRESS(PORT.ADDRESS,START.ADDRESS,NO.OF.BYTES) • K$=INKEY$:GOTO 1180 • Absorb unwanted keypress and request another freq. DATA 139,236,139,94,8,139,23,139,94,6 DATA 139,55,139,94,4,139,15,252,228,33 DATA 12,l,230,33,232,0,0,93,131,197 DATA 47,184,0,0,140,219,142,216,255,54 DATA 38,0,255,54,36,0,140,14,38,0 DATA 137,46, 36,0,142,219,139,217,139,254 DATA 144,172,238,226,251,144,139,203,139,247 DATA 172,238,2 26,243,131,197,12,131,196,2 DATA 85,139,236,255,ll0,6,184,0,0,140 DATA 219,142,216,143,6,36,0,143,6,38 DATA 0,142,219,228,33,36,254,230,33,202 DATA 6,0 (Lines 460 to 1130 omitted) If you don't feel like typing in the detailed machine code in the above listing, substitute these condensed DATA statements instead. denses the whole subroutine into 12 short lines. However, we have included the full listing for those who wish to experiment with the machine code. The main body of the program is written in GW BASIC, since speed is no longer a problem. Some changes to the machine code will be needed to accommodate the differing calling protocol of other languages, such as TURBO BASIC or "C". Included in the listing are lines for generating sine, triangle or square waves. Fig.3 shows each waveform at lkHz. Is Your Product Getting The Exposure It Deserves? Consumers need to see your product if you want them to buy it Contact Paul To Reserve This Space - (02) 982 9553 96 SILICON CHIP There is no reason why other waveforms can't be added; eg, a two-tone audio test signal for SSB transmitters. You will notice that waveform selection is a little primitive - you have to disable the ones that you don't want by inserting a single quote mark - but we have done this to keep the listing short. A proper program would let you choose the waveform at run time. As shown, the program is set for generating sinewaves. To generate triangle waves, all you have to do is insert a single quote mark at the start of line 1280 and delete the quote mark at the start of line 1290. Square waves and white noise can be generated by changing the program in similar fashion. While we're on the subject of limitations, we have removed the need for an anti-aliasing filter by rounding off the number of samples per cycle to the nearest integer. However, this means that the highest frequencies are rounded to the following values: 50kHz, 33.3kHz, 25kHz, 20kHz, 16.7kHz and so on. In order to generate any in-between frequencies, the program needs to step through more than one cycle of the waveform. This will generate a "double sideband" modulated signal, as described last month. The lower frequency limit is determined by how long you are prepare to wait for the array to be filled. A suitable filter will fix this. For a start you can try the simple third order filter described in March, with the L and C values scaled (a) (b) (c) Fig.2: these waveforms were produced by the TURBO BASIC program of Listing 1 . The CRO horizontal axis is not to scale ih these photographs, however the period of each cycle is exactly 1ms. (a) This is what an IHF standard tone burst looks like, or at least a small part of it. The entire tone burst waveform lasts for a whole 500ms and the short + 20dB burst is normally quite hard to capture on a CRO. Our program provides a convenient sync pulse, which can be positioned comfortably ahead of the tone burst. (b) Here we have programmed a much shorter burst so we can check out the waveform quality. As you can see, it is virtually perfect except for a small gap between the end of one burst and the start of the next. This gap occurs when the TURBO BASIC program completes its high-speed output loop and checks the keyboard input. We used this small time interval to put out the CRO sync pulse. (c) This is how the 1kHz waveform looks without a low-pass filter. The relatively low data rate of 20k samples/sec makes a filter essential for good waveform purity (see the March 1 990 issue for complete details of a filter suitable for this data rate). (a) (b) (c) Fig.3: these three photographs show typical waveforms produced by the GW BASIC program of Listing 2, which has been optimised for continuous waveform generation. The horizontal axes are 100µs per division. The output amplitude of the D-A converter has been trimmed to 2.8V peak-to-peak, which gives a nice round figure of 1 V RMS for the sinewave. (a) This is what an unfiltered 1 kHz sinewave looks like at a data rate of 100k samples/sec. For most audio applications, filtering may not be necessary. Note that there are no gaps between the end of one cycle and the start of the next. (b) The triangle wave option, again at 1kHz. (c) The square wave option, also at 1kHz. If you want the square wave to have the same RMS amplitude as the sinewave, use the values 0 and 180 or, better still, 38 and 218 instead of 0 and 255 as in the program . down to increase the - 0. ldB passband from 4kHz to 20-25kHz (ie, divide all the L and C values by 5). This will be fine for sinewaves but square and triangle waves will show a tiny bit of ringing. To do justice to this program, we are currently working on a 5th order filter which has a smoother phase characteristic. This will be built onto a small PC board which will also accommodate the simple A-D converter featured in February. We will also be making available a 360K 5.25-inch floppy disc containing source listings of all the software described so far, along with compiled and executable versions of all the programs (so you won't need to buy a BASIC compiler). These programs will have a number of " user friendly" enhancements which had to be left out of the published listing; because they would have taken up several more pages. ~ MAY 1990 97