Low cost audio function generator(DDS)

[sougata]

Hi,

I need to design a low cost digital audio function generator. For the sinewave generation I would use a lookup table. To keep the control digital I need to scale (modulate) the peak sine values in software and softare PWM. Cannot use a PGA for cost restriction.
What is the best principal for that ?

I am restricted in terms of PIC throughput considering the soft pwm and other housekeeping functions. I must use a PIC16 @ 20MHz series to keep down the cost lower. So calculating the sine values on the fly (when doing a modulation) may eat up to much cycles to fit within the update routine

Please guide if it is at all possible and how ?

[mister_e]

Is there a specific reason why you want to use a PIC for that?

What is your frequency range?

[sougata]

Dear Steve,

Thanks for replying. Actually I live in a part of my country where sourcing of specialized components is a real problem. Initially I considered using a DDS chip. But set back due to unavailability. Frequency range is within 10K. I haven't done the homework yet. Just trying to get some guidelines whether to base it on a 16F. The possibility you can say. If it is technically possible then I might consider using a 18F too. That depends on the overall BOM cost.

[Acetronics2]

Hi, Sougata

ICL 8038, voltage controlled ??? ... you use the Pic as the freq regulator ... say, you build a simple PLL !!! but 8038 outputs some sinewaves ...

bad idea ???


Alain

[sougata]

Dear Alain,

Thanks for your reply. I just don't want to go the conventional (traditional)way. I have used MAX038 (big brother of XR8038) earlier but it was a pure analogue design with some fancy digital control as you mentioned. This project is all about low cost and best control. In fact with a PC side software the lookup tables could be modified to generate other waveforms as well. I am asking you guys for the possibility on a 16F. Storing one quadrant of data in the lookup table and building the sinewave is easy. But while modulating it, for a 8bit modulation index, I am doubtful on the processor througput considering 16/32 bit math. If there is an easy way out to do this in software please guide.

[Acetronics2]

Hi, Sougata

Here, you will find such a DDS module ... for the price ... just tears left !!!

http://shop.elv.de/output/controller.aspx?cid=74&detail=10&detail2=6325

or here, with a PC interface:

http://shop.elv.de/output/controller.aspx?cid=74&detail=10&detail2=10617

Alain

[Ingvar]

Hi Sougata,

I did some experimenting with DDS a few years ago, i wrote this piece of code. If my memory serves me right, it worked up to 20kHz with reasonable good looking sine on the output. I also did some testing with PWMoutput but never got good results with that, there may be some leftovers in the program from that ...... just remove.


'************************************************* ***************
'* Name : DDS16F.BAS *
'* Author : Ingvar Gillholm *
'* Date : 2004-03-11 *
'* Version : 1.0 *
'* Notes : This program produces a sinewave through PortB, *
'* : which should have a R-2R leadder connected to it. *
'************************************************* ***************


@ device pic16F876, hs_osc, wdt_off

define OSC 20
'DEFINE LOADER_USED 1

TonePort VAR PortB
CheckNew VAR PortA.3

Phase VAR BYTE[3] BANK0
Freq VAR BYTE[3] BANK0
Zero VAR BYTE BANK0

TempVar VAR BYTE

ADCON1 = 7 'Porta is digital
TrisA = $ff 'All inputs
PortB = 0 'Start with all outputs low
TrisB = 0 'All outputs
Zero = 0

PR2 = $3f
OUTPUT PortC.2
T2CON = %00000100
CCP1CON = %00001100
GOTO New

Main:
asm
;
;======================== MAIN LOOP =========================
;

bcf STATUS,RP0
bcf STATUS,RP1

Loop
BTFSC _CheckNew ; Check for a new frequency
GOTO _New ; and get new frequency
;
MOVF _Freq,W ; Get the current frequency
ADDWF _Phase,F ; and increment the phase
btfss STATUS,C ;
goto NoCarry ;burn time if no carry
incf _Phase+1 ;
btfsc STATUS,Z ;
incf _Phase+2 ;
AfterCarry ;
MOVF _Freq+1,W ;
ADDWF _Phase+1,F ;
btfsc STATUS,C ;
incf _Phase+2 ;
MOVF _Freq+2,W ;
ADDWF _Phase+2,F ;

MOVF _Phase+2,W ;

ANDLW 07Fh ;Reduce to 7 bits for 180 degrees
CALL SineTbl ; and get amplitude

BTFSS _Phase+2,7 ;Is amplitude supposed to be negative?
SUBWF _Zero,w ;Change polarity of sine amplitude

MOVWF _TonePort ;Output to DAC
GOTO Loop ;

NoCarry
goto AfterCarry

;*********** SINE TABLE ****************

SineTbl
ADDWF PCL
retlw 080h
retlw 083h
retlw 086h
retlw 089h
retlw 08Ch
retlw 090h
retlw 093h
retlw 096h
retlw 099h
retlw 09Ch
retlw 09Fh
retlw 0A2h
retlw 0A5h
retlw 0A8h
retlw 0ABh
retlw 0AEh
retlw 0B1h
retlw 0B3h
retlw 0B6h
retlw 0B9h
retlw 0BCh
retlw 0BFh
retlw 0C1h
retlw 0C4h
retlw 0C7h
retlw 0C9h
retlw 0CCh
retlw 0CEh
retlw 0D1h
retlw 0D3h
retlw 0D5h
retlw 0D8h
retlw 0DAh
retlw 0DCh
retlw 0DEh
retlw 0E0h
retlw 0E2h
retlw 0E4h
retlw 0E6h
retlw 0E8h
retlw 0EAh
retlw 0EBh
retlw 0EDh
retlw 0EFh
retlw 0F0h
retlw 0F1h
retlw 0F3h
retlw 0F4h
retlw 0F5h
retlw 0F6h
retlw 0F8h
retlw 0F9h
retlw 0FAh
retlw 0FAh
retlw 0FBh
retlw 0FCh
retlw 0FDh
retlw 0FDh
retlw 0FEh
retlw 0FEh
retlw 0FEh
retlw 0FFh
retlw 0FFh
retlw 0FFh
retlw 0FFh
retlw 0FFh
retlw 0FFh
retlw 0FFh
retlw 0FEh
retlw 0FEh
retlw 0FEh
retlw 0FDh
retlw 0FDh
retlw 0FCh
retlw 0FBh
retlw 0FAh
retlw 0FAh
retlw 0F9h
retlw 0F8h
retlw 0F6h
retlw 0F5h
retlw 0F4h
retlw 0F3h
retlw 0F1h
retlw 0F0h
retlw 0EFh
retlw 0EDh
retlw 0EBh
retlw 0EAh
retlw 0E8h
retlw 0E6h
retlw 0E4h
retlw 0E2h
retlw 0E0h
retlw 0DEh
retlw 0DCh
retlw 0DAh
retlw 0D8h
retlw 0D5h
retlw 0D3h
retlw 0D1h
retlw 0CEh
retlw 0CCh
retlw 0C9h
retlw 0C7h
retlw 0C4h
retlw 0C1h
retlw 0BFh
retlw 0BCh
retlw 0B9h
retlw 0B6h
retlw 0B3h
retlw 0B1h
retlw 0AEh
retlw 0ABh
retlw 0A8h
retlw 0A5h
retlw 0A2h
retlw 09Fh
retlw 09Ch
retlw 099h
retlw 096h
retlw 093h
retlw 090h
retlw 08Ch
retlw 089h
retlw 086h
retlw 083h

ENDASM
New:
' The frequency is calculated as FreqValue = DesiredFreq/((1/2^24)*(5*10^6/28))
' Same as FreqValue = DesiredFreq/((1/16777216)*(5000000/28))
' Same as FreqValue = DesiredFreq*16777216*28/5000000
' Same as FreqValue = DesiredFreq*93,9524096
' The resolution is 1/93,9524096 = 0,01064368656703404........Hz

' The eight frequencies selected for this test is........
' 67,0 = 001896
' 94,8 = 0022CA
' 118,8 = 002B99
' 146,2 = 0035A7
' 173,8 = 003FC8
' 199,5 = 004937
' 225,7 = 0052D5
' 254,1 = 005D41

TempVar = PortA & %00000111 'Only lower 3 bits
' LOOKUP TempVar,[$00,$00,$00,$00,$00,$00,$00,$00],Freq[2] 'Stuff Hibyte
' LOOKUP TempVar,[$1a,$25,$2e,$39,$44,$4e,$58,$63],Freq[1] 'Stuff Midbyte
' LOOKUP TempVar,[$58,$47,$b7,$7d,$57,$72,$c0,$eb],Freq[0] 'Stuff Lowbyte
LOOKUP TempVar,[$00,$00,$00,$00,$00,$00,$00,$00],Freq[2] 'Stuff Hibyte
LOOKUP TempVar,[$18,$22,$2b,$35,$3f,$49,$52,$5d],Freq[1] 'Stuff Midbyte
LOOKUP TempVar,[$96,$ca,$99,$a7,$c8,$37,$d5,$41],Freq[0] 'Stuff Lowbyte
GOTO Main
END


Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.

/Ingvar

[sougata]

Hi All,

Thanks Ingvar at least you share the same vision as me.


Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.


Actually my client wants to put this module into a breadboard trainer to make it compete better. he does'nt want to increase the manufacturing cost however.

Because the amplitude control will also be used to modulte the base sinewave at a lower frequency it needs to be computed on the fly through a modulation index (I suppose).

Thanks Alain for the link but I cannot read German.(I am not sure even if it is German at all)

[Acetronics2]

Hi All,


Thanks Alain for the link but I cannot read German.(I am not sure even if it is German at all)

The only interesting info here is the price reached ... ~ 50 US $ ( sold ...) .

Alain

[sougata]

Hi,

How about making one for $50 ? Possible ?

[Acetronics2]

Here is the scheme ... and all you should know !!!

http://www.elv-downloads.de/service/manuals/DDS20/47138-DDS-Board_km.pdf

and

http://www.elv-downloads.de/service/manuals/DDS10/DDS10_KM_G_041222.pdf


moreover ... all components are available at this address !!!

Alain

[sougata]

Hi Alain,

Thanks for the links.

[speck]

Hi Ingvar-

Could this be made to work using the HPWM output & a timer interrupt to output very low frequency sine waves? (< 30hz) Using a lookup table & a variable timer interrupt cycle to vary the sample rate/freq. of the sine? Perhaps amplitude could be varied by scaling the wavetable values?

Was it the case that the PWM output didn't give you a smooth sine at higher frequencies? Or it just didn't work well at all?




I did some experimenting with DDS a few years ago, i wrote this piece of code. If my memory serves me right, it worked up to 20kHz with reasonable good looking sine on the output. I also did some testing with PWMoutput but never got good results with that, there may be some leftovers in the program from that ...... just remove.
Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.

/Ingvar

[Archangel]

10-MHz-DDS-Funktionsgenerator mit PC-Softwaresteuerung-preis:69,95 euro
Das ist gut, Ya ?

[sougata]

Hi Speck,

It can always be done with a hardware PWM and low pass filter. See my sinewave inverter thread http://www.picbasic.co.uk/forum/showthread.php?t=1846 for a code snippet that does this at an update rate of 100Hz driving a half bridge. Scaling the sine table is impractical with only 8 bits of resolution. For example if you define your sinewave lookup table to produce the maximum peak value into an 8 bit one then you have to do fractional math.That is your modulation index (depth of modulation) is <= 1. (The dSPIC supports fractional math). So the trade-off is multiplying the lookup table with a 8bit value and shredding off the LSBs. This works okay for low fixed frequency. If you do a cycle by cycle computation then the overhead is manageable. But for a 20K function generator to work with a base frequency and modulating frequency it has to be updated every PWM interrupt cycle. That is too much you could ask for a 16F PIC @ 5 MIPS. The 18Fs have an extended instruction set featuring a single cycle hardware multiply that too @ 10MIPS. In fact you can use Darrel's Software PWM http://www.pbpgroup.com/modules/wfsection/article.php?articleid=12 to make your life easy. It takes of all your pain in doing it in asm.

BTW the PIC Hardware PWM uses upto 10bits and it is a good practice to set your PR2 to $FF. That is where you get the maximum usable PWM resolution.

[Acetronics2]

10-MHz-DDS-Funktionsgenerator mit PC-Softwaresteuerung-preis:69,95 euro
Das ist gut, Ya ?

Hi,Joe

I've "built" ( laughs ...) the 20Mhz one ...

just need an extra output buffer for > 10 Mhz outputs ( there's a too low -pass response somewhere ! ) ... and because the HC132 is a bit weak on even small capacitive loads.

Otherwise it works really fine !!! ... and 4Mhz is the Max I really need ...as a variable PIC clock drive.

precision and resolution are really unbeaten for such price ...

Yawohl ... sehr gut !!!

Alain

[mister_e]

Je dit... c'est le excellent prix pour le excellent produit ... yah!

I don't know the following, and i don't know how good they are and are really far of the needed range here but...
http://www.amqrp.org/kits/dds60/
http://www.minikits.com.au/kits4.html

Kinda time saver i guess.

[Ingvar]

Hi Speck,

It should be possible to do with hardware PWM, espescially at low frequencies like that. I just never got it working very well at higher freqs, so i never really finished it off to see where the limits were. Feel free to carry out your own investigation cause it should be possible.

I think it was probably due to the fact that the loop runs at 5000000/28=178,571kHz and the highest (useable) PWM freq was around twice that ....... no resolution. However, like i said, i never took it all the way, i got the results i wanted with the R-2R ladder.

/Ingvar

[Acetronics2]

Hi,All

I saw one application in a the French "Electronique Pratique" magazine, issue 247, page 86 ...

But, halas, I didn't download the Pic ( Hex ) Program ... and it's no more available.

May be someone has it in his computer memory ???

Alain

[J. Mark Wolf]

Below is the link for an audio sine wave generator chip. Works good.

http://www.mix-sig.com/prodsh_MSLOSC.html

[Luciano]

Hi,

Low cost audio function generator?

Audio?

What about using the PC Sound Card as Signal Generator?
http://www.virtins.com/page2.html#Signal%20Generator

Tons of freeware available.

Best regards,

Luciano

[sougata]

Hi,

Thanks Luciano. I am aware of the Freewares available and the application is a product. However the issue is a little different. Its all about the scope of a 16F performing DDS. Without using much of additional circuitry.
Mark thanks for the link. The scope of the project does not permit to use it. Otherwise I would have gone for waveform generator chip. Also it is hard to find (if not impossible) in low quantities (less than 0.5K). If I at all go on with this project then I would be going for a 18F with a R-2R DAC.

[Acetronics2]

Hi, Sougata

a look at CYPRESS Application notes could give you the solution and soft ...

I know ... it's not PICs !!! but Free ICD and simple programmer.

Worth it !!!

Alain

[sougata]

Hi,

Thanks Alain for the info. I have played a little bit with the PSOC from cypress sometimes back. But not so skilled. I will though hunt the appnotes there and find out if the project is possible in low cost .

[Acetronics2]

Hi,SOUGATA

AN 2086 ...

[sougata]

Hi,

Thanks Alain for the info. I am looking forward to an all digital way. Cypress PSOCs app-note uses a bandpass filter (integrated into the chip itself) In this thread things are being overlooked. So let me clear things up. I think only Ingvar replied with something direct reply related to my thread topic.



1. I am not asking how to generate a sinewave ?
2. Not battling on issues whether to use a Hardware PWM, R-2R or external DAC ?
3. Not trying to use a dedicated DDS chip ?

In a typical DDS system if I am having large phase accumulator registers then by offsetting and incrementing them at different update rate a sinewave with amplitude modulation can be produced. (attached DDS-01.gif)
To do it real fast you possibilly need an ASIC. So lets now talk about the PIC way. To be specific PIC-PBP-ASM way.

A function generator has controls for frequency, amplitude and also modulation. This is where (modulation) things get complicated.

1. How to modulate (AM) the sinewave digitally ?


I hope I type the word digitally correctly so I am not thrown back with answers like using a different PIC to generate the modulating waveform and using traditional analogue modulation techniques. Remember low cost = less BOM = no PGA = !!??!!

I planned using the compare-match module for generating upto 16bits of usable PWM (soft ) resolution with minimal software overhead. Now I need to achieve the usable output sinewave to cover upto 20KHz. To keep the quantatization noise low I planned for a 64 point ( for each quadrant /attached 1Q-Sine.gif) lookup table. Now I have always found it impractical to generate a sinewave greater than 1/4 of the sample frequency. For fixed amplitude the lookup table could be computed and stored in an array. But for modulating it with another sine table (or arbitary) from a eeprom or an external AD input I found that I need to calculate the PWM values (sine through an obtained modulation index) on the fly. My weak maths revealed that I cannot (or the 16F PIC @20MHz) handle it cause I do not have enough instruction cycles. I was very hopeful that the Gurus out there would shed some light but this thread did not bring up much. One contributor (no offense) mentioned in a PM that he could do it better than me but remained silent there after. So now you do the MATHS and give me a Yes or No if it can be done on a 16F (no support for hardware multiply). That was the original topic.

I am sorry if I am being rude. (May be my client is also following this thread!!!!!) If cost was not the factor then I would have gone any other way. Anyway I am still looking forward to suggestions from the Pros out there.

[Acetronics2]

Hi, Sougata ...

You were looking for the cheap way ... a powerful Cy8C29466 ( 32K ) costs here 7 Euros ( 9 US$ ) !!! and the IDE is free ... and no expensive peripheral needed.

That's the reason of my answer.


Q1. How to modulate (AM) the sinewave digitally ?

maybe generating your Analogic output across a full port ( an R-2R ladder ... as simple as this ! ) could solve the problem for a honest 256 steps amplitude resolution ...

Alain

[sougata]

Hi Alain,

Using R-2R ladder does reduces software overhead. Fixed amplitude routine takes about the same code for both the PWM and the R-2R in. (8 bits) If a hardware PWM is used then the code overheads are almost the same. Write to PORT or CCPR (Ignoring LSBs). But I wanted to read an ADC or another lookup table within the PWM/R-2R update routine to create AM. I lost on the available cycles there. Then the sampling Freq is just not achievable. Now the most sad part.

In my country (India) 1 USD = 50 INR (Indian Rupees). A PIC16 comes for about 1 USD but a PSOC lands for about $15. My client has a traditional Func-Gen (below average) which costs him less than $18, that too in annual quantities less than 1K. He wanted to give it a digital face lift so that he could better sale his product. He is ready to pay me for the development provided I fit the BOM within $20. I manage to earn my living by PICing and almost always face this constraint.

I could have modified the analogue thing to display the freq./amplitude on a LCD but that was not my intention. (Actually this was the clients basic requirement). I am a 100% self taught and as Steve says there is no problem only learning opportunities.

[Acetronics2]

Hi, Sougata

Did you spent some time here ???

http://www.picbasic.org/forum//showthread.php?t=1601

some ideas to pick up.

Bingo here : http://www.romanblack.com/picsound.htm

Alain

[sougata]

Hi,

I have tried using magic sinewaves from don and also tried the 1 bit audio playback from the PIC. They just don't suit my current requirement. I did a speaking greeting card using a PIC, EEPROM and 1 bit audio!!

[Acetronics2]

Hi, Sougata

another idea ...

http://www.compile-it.com/Proton/PIC_WAV_PLAYER.zip

Alain