Hi,
i want to implement the equation
p3 = p3*0.5/cos(3*PI/8);
into 16bit processor, where p3 is a 16 bit value;
any pointers as to how to go about it
-vijay
fixed point implementation
Started by ●December 5, 2003
Reply by ●December 5, 20032003-12-05
n_vijay_anand wrote:> Hi, > > i want to implement the equation > > p3 = p3*0.5/cos(3*PI/8); > > into 16bit processor, where p3 is a 16 bit value; > > any pointers as to how to go about itIt looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. cos(3*PI/8) = 0.99978861 0.5/0.99978861 = 0.50010571 Depending on the algorithm you might get away with simply dividing by 2: p3 >>= 1;> -vijay-- Phil Frisbie, Jr. Hawk Software http://www.hawksoft.com
Reply by ●December 5, 20032003-12-05
Phil Frisbie, Jr. wrote:> > It looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. > > cos(3*PI/8) = 0.99978861 > 0.5/0.99978861 = 0.50010571 >Time to put new batteries in your calculator ? ;-) Paul
Reply by ●December 5, 20032003-12-05
n_vijay_anand wrote:> Hi, > > i want to implement the equation > > p3 = p3*0.5/cos(3*PI/8); > > into 16bit processor, where p3 is a 16 bit value; > > any pointers as to how to go about it > > -vijayFrom the gist, I guess that you don't use floating point. 0.5/cos(3*PI/8) is just a constant; about 1.3065623. If you sat what precision you need and how much time you can spend, I or someone else might be able to suggest something. How about an intermediate 32-bit product and a division? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 5, 20032003-12-05
No the batteries are okay, he just needs to switch over to radian mode. The constant should be more like 1.3065629649 (10 places anyway) Clay "Paul Russell" <prussell@sonic.net> wrote in message news:4q3Ab.916$XF6.21919@typhoon.sonic.net...> Phil Frisbie, Jr. wrote: > > > > It looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. > > > > cos(3*PI/8) = 0.99978861 > > 0.5/0.99978861 = 0.50010571 > > > > Time to put new batteries in your calculator ? ;-) > > Paul >
Reply by ●December 5, 20032003-12-05
"Phil Frisbie, Jr." <phil@hawksoft.com> wrote in message news:Gd3Ab.911$XF6.22096@typhoon.sonic.net...> n_vijay_anand wrote: > > > Hi, > > > > i want to implement the equation > > > > p3 = p3*0.5/cos(3*PI/8); > > > > into 16bit processor, where p3 is a 16 bit value; > > > > any pointers as to how to go about it > > It looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. > > cos(3*PI/8) = 0.99978861 > 0.5/0.99978861 = 0.50010571 > > Depending on the algorithm you might get away with simply dividing by 2: > > p3 >>= 1;I think that you got ya radians/degrees mixed on the calc. Try it in radian measure and you should get cos(3*PI/8) = 0.382683 thus 0.5/cos(3*PI/8) = 1.306563 So a basic multiplication by 1.306563 should do the trick... If you're going to have to use cos/sin with a variable argument in other parts of your code then it may be worth just using a lookup table for the sinusoidal functions and doing the maths using some basic fixed point library, it would make the code a little easier to read, though would make coding it a little harder to start with.
Reply by ●December 5, 20032003-12-05
Phil Frisbie, Jr. wrote:> n_vijay_anand wrote: > >> Hi, >> >> i want to implement the equation >> >> p3 = p3*0.5/cos(3*PI/8); >> >> into 16bit processor, where p3 is a 16 bit value; >> >> any pointers as to how to go about it > > > It looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. > > cos(3*PI/8) = 0.99978861 > 0.5/0.99978861 = 0.50010571 > > Depending on the algorithm you might get away with simply dividing by 2: > > p3 >>= 1;In my world, 3*PI/8 comes out to 67.5 degrees. Take it from there. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 5, 20032003-12-05
Clay S. Turner wrote:> No the batteries are okay, he just needs to switch over to radian mode.Yes, my son must have switched it and I did not notice! I always leave it in radian mode because I use C, but my son's homework is almost always in degrees....> The constant should be more like 1.3065629649 (10 places anyway) > > Clay-- Phil Frisbie, Jr. Hawk Software http://www.hawksoft.com
Reply by ●December 8, 20032003-12-08
On Sat, 6 Dec 2003 07:14:29 +1300, Bevan Weiss wrote:> > "Phil Frisbie, Jr." <phil@hawksoft.com> wrote in message > news:Gd3Ab.911$XF6.22096@typhoon.sonic.net... >> n_vijay_anand wrote: >> >> > Hi, >> > >> > i want to implement the equation >> > >> > p3 = p3*0.5/cos(3*PI/8); >> > >> > into 16bit processor, where p3 is a 16 bit value; >> > >> > any pointers as to how to go about it >> >> It looks like '0.5/cos(3*PI/8)' is a constant, so simplify it. >> >> cos(3*PI/8) = 0.99978861 >> 0.5/0.99978861 = 0.50010571 >> >> Depending on the algorithm you might get away with simply dividing by 2: >> >> p3 >>= 1; > > I think that you got ya radians/degrees mixed on the calc. > Try it in radian measure and you should get cos(3*PI/8) = 0.382683 > thus 0.5/cos(3*PI/8) = 1.306563 > So a basic multiplication by 1.306563 should do the trick...To add a little for the OP: Many 16-bit DSPs can't (easily) express 1.306563 as a multiplicand, so you might have to multiply by (1.306563/2.0) and then multiply the result by two, perhaps by arithmetic left-shifting the accumulator, which will preserve an extra bit of precision. -- Andrew
