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Date: Tue, 31 Dec 2013 14:26:24 -0500
Message-ID: <CALSZ9go-H-4HnAfvdQyLsVEcbNG0pT33wEM=G-CfZmDXVcMQrw@mail.gmail.com>
Subject: [geda-user] Help sending a sine wave to a speaker
From: Rob Butts <r DOT butts2 AT gmail DOT com>
To: geda-user AT delorie DOT com
Content-Type: multipart/alternative; boundary=089e010d97303689d404eed98910
Reply-To: geda-user AT delorie DOT com

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It has been a while since I've written major C code so I'm having a hard
time coming up with the code.
I have a simple design where I use a microchip pic16f1825 to read a byte
from an eeprom via spi and send it to a dac and ultimately to a speaker.
As a dac and speaker test I want to send a simple 1000Hz sine wave to the
dac at an 8KHz rate.

Can anyone check my thoughts below?  Here's my thinking...
If I set my timer to interrupt at an 8KHz rate and if I want to play the
tone for 10 seconds the for loop might look something like this:

for(j=0; j < 80000; j++){
     // j is the sample index
    // straight sine function means one cycle per 2*pi samples
    // sample(j) = sin(j)
    // multiply by 2*pi now it is one cycle per sample
    // sample(j) = sin((2 * pi) * j)
    // multiply by 1,000 samples per second now it is cycles per second
    // sample(j) = sin(1000 * 2 * pi * j)
    // divide by 8000 samples per second now it is cycles per 8000 samples
    // sample(j) = sin(1000 * 2 * pi * j / 8000)
    // the 1000 cycles is the frequency so in those terms
    //  sample(j) = sin(freq * 2 * pi * j / 8000)
    // now if I want to normalize the amplitude to 0xff  the final value is
    // sample(j) = 0xff * sin(freq * 2 * pi * j / 8000)
    // so...    declaring sample as a float
    sample = 0xff * sin(freq * 2 * pi * j / 8000);
    while(!update_dac){    // where update_dac is set to 1 in the timer
interrupt handler
    }
    update_ dac = 0;
    send_dac((UINT8) sample);
}

When I execute the code and step through it the value for sample when j = 1
is 180.238where I expect 3.146

Can someone see my error or what is going on?

Thanks

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<div dir=3D"ltr"><div>It has been a while since I&#39;ve written major C co=
de so I&#39;m having a hard time=20
coming up with the code. <br></div><div>I have a simple design where I use =
a microchip=A0pic16f1825 to=20
read a byte from an eeprom via spi and send it to a dac and ultimately to a=
=20
speaker.=A0 As a dac and speaker test I want to send a simple 1000Hz sine w=
ave to=20
the dac at an 8KHz rate.=A0<br>=A0=A0=A0 <br>Can anyone=A0check my thoughts=
 below?=A0 Here&#39;s my thinking... <br>If I=20
set my timer to interrupt at an 8KHz rate and if I want to play the tone fo=
r 10=20
seconds the for loop might look something like this: <br>=A0 <br>for(j=3D0;=
=A0j &lt;=20
80000; j++){ <br>=A0=A0=A0=A0 //=A0j is the sample index <br>=A0=A0=A0 // s=
traight sine function=20
means one cycle per 2*pi samples <br>=A0=A0=A0 //=A0sample(j) =3D sin(j) <b=
r>=A0=A0=A0 //=20
multiply by 2*pi now it is one cycle per sample <br>=A0=A0=A0 // sample(j) =
=3D sin((2 *=20
pi) * j) <br>=A0=A0=A0 // multiply by 1,000 samples per second now it is cy=
cles per=20
second <br>=A0=A0=A0 // sample(j) =3D sin(1000 * 2 * pi * j) <br>=A0=A0=A0 =
// divide by 8000=20
samples per second now it is cycles per 8000 samples <br>=A0=A0=A0 // sampl=
e(j) =3D=20
sin(1000 * 2 * pi * j / 8000) <br>=A0=A0=A0 // the 1000 cycles is the frequ=
ency so in=20
those terms <br>=A0=A0=A0 //=A0 sample(j) =3D sin(freq * 2 * pi * j / 8000)=
 <br>=A0=A0=A0 // now=20
if I want to normalize the amplitude to 0xff=A0 the final value is <br>=A0=
=A0=A0 //=20
sample(j) =3D 0xff * sin(freq * 2 * pi * j / 8000) <br>=A0=A0=A0 // so...=
=A0=A0=A0=A0declaring=20
sample as a float <br>=A0=A0=A0 sample =3D 0xff * sin(freq * 2 * pi * j / 8=
000); <br>=A0=A0=A0=20
while(!update_dac){=A0=A0=A0 // where update_dac is set to 1 in the timer i=
nterrupt=20
handler <br>=A0=A0=A0 } <br>=A0=A0=A0 update_ dac =3D 0; <br>=A0=A0=A0 send=
_dac((UINT8) sample);=20
<br>}=A0<br>=A0 <br>When I execute the code and step through it the value f=
or sample=A0when j =3D 1 is 180.238where I expect 3.146</div><div>=A0</div>=
<div>Can someone see my error or what is going on?</div><div>=A0</div><div>=
Thanks</div>
</div>

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