This program doesn't initialize X1 prior to its use. Does Qbasic
automatically initialize new variables to 0? 

It also got re-formatted, I'm assuming the program should look like this:
And I've added X1 = 0 etc...

DIM array(100)

DIM ToneFreq AS INTEGER

DIM SampleRate AS INTEGER

DIM X0 AS DOUBLE

DIM X1 AS DOUBLE

DIM X2 AS DOUBLE

ToneFreq = 440

SampleRate = 10000

PI = 3.1415

A = 5

k = 2 * COS(2 * PI * ToneFreq / SampleRate) 

X1 = 0

X2 = -A * SIN(2 * PI * ToneFreq / SampleRate) 

OPEN "LIST.TMP" FOR OUTPUT AS #1 

FOR i = 1 TO 100  

	X0 = k * X1 - X2

	X2 = X1

	X1 = X0

	array(i) = X0

	PRINT #1, array(i)

	NEXT i

CLOSE #1


Since 

sin(2 * X) = 2 * sin(X) * cos(X) 

if 

X = 2 * PI * ToneFreq / SampleRate 

And we make

A = 1

then the first 

X0 = k * X1 - X2 

Makes X0 and therefore 

X1 = SIN(X)

So the next time around (since X2 = X1 = 0)  the second

X0 = k * X1 - X2

is really 

X0 = 2 * SIN(X) * COS(X)

X0 = SIN(2*X)

X0 = SIN(4 * PI * ToneFreq / SampleRate)


So it would seem to me that it was multiplying the angle with each step.

If you added a line like:

PRINT X0 - SIN( I * (2 * PI * ToneFreq / SampleRate) )

That doesn't seem right to me. You could see the error in the approximation
vs what it is actually trying to calculate.

Or did I miss something there?

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> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Dennis Crawley
> Sent: 2004 Sep 28, Tue 05:31
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Another method of generating and detecting 
> sine waves
> 
> 
> Andy,
> I've try this QBasic program to make a file to be analized by 
> a spreadsheet
> 
> "A" seems to be the amplitud.
> The wave form is irregular until you increase the RateSample.
> The initial value of X1 is 0 in my program. Since "A" is "-A" 
> the intial value is positive.
> It reminds me  E(t)=A*SIN(wt) where w is 2pi radians.
> 
> Regards
> Dennis.
> '-----------------------------------------------
> DIM array(100)
> DIM ToneFreq AS INTEGER
> DIM SampleRate AS INTEGER
> DIM X2 AS DOUBLE
> ToneFreq = 440
> SampleRate = 10000
> PI = 3.1415
> A = 5
> X2 = -A * SIN(2 * PI * ToneFreq / SampleRate) OPEN "LIST.TMP" 
> FOR OUTPUT AS #1 FOR i = 1 TO 100  k = 2 * COS(2 * PI * 
> ToneFreq / SampleRate) X0 = k * X1 - X2
> X2 = X1
> X1 = X0
> array(i) = X0
> PRINT #1, array(i)
> NEXT i
> CLOSE #1
> 
> 
> ----- Original Message -----
> From: "Andrew Warren"
> Sent: Monday, September 27, 2004 6:40 PM
> > Harold Hallikainen <piclist@mit.edu> wrote:
> >         1.  What's "A"?
> >         2.  What's the initial value of X1?
> >         3.  Is there anything else missing?
> 
> 
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> http://www.piclist.com
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