4 ms·

Nice read! I have 2 questions: When calculating K, the author says “It can be shown through the use of trigonometric identities that:” and proceeds to show a f

Nice read! I have 2 questions: When calculating K, the author says “It can be shown through the use of trigonometric identities that:” and proceeds to show a formula. How exactly does this happen? After calculating K, the author assigns it to c in the cordic function, but not to s. Why?

edflsafoiewq 8y ago
To your first question: tan β_i = sin β_i / cos β_i = 2^{-i}. So cos β_i = 2^i sin β_i. Square and use Pythagoras for cos^2 β_i = 2^{2i} (1 - cos^2 β_i). Solve for cos^2 β_i and take the positive square root for 2^i / √(1 + 2^{2i}) = 1 / √(2^{-2i} + 1). To your second: I presume the author multiplied 1 by K to get c <- K. When you multiply 0 by K you still have s <- 0 though. I haven't figured out the justification for moving the multiplication from after to before the loop but if it works for c it works for s too. edit: If you write c_new ← cos α[i] × (c - tan α[i] × s) s_new ← cos α[i] × (s + tan α[i] × c) c ← c_new s ← s_new with a complex variable z = c + is (changing the index variable to j) you get z ← cos α[j] × (1 + i × tan α[j]) × z which makes it more obvious (to me) that you can pull the multiplication by cosines out of the loop. z <- 1 # c=1, s=0 z <- K × z # c=K, s=0 ...loop...
- seedless-sensat 8y ago
  To the second question, I am also struggling to follow the step "Let’s move the multiplications by cos β_i into a separate loop". The recursive construction of `c` doesn't allow the betas to be pulled out. The i-th term is multiplied by beta_i to beta_n, but not beta_0 to beta_{i-1}. Clearly the method must work, but I'm not sure what I'm missing. Edit: ignoring `direction`, I can prove this step by induction for both c(i) and s(i) together. At least to me though, this it isn't a simple algebraic simplification.
  - edflsafoiewq 8y ago
    I got it. In terms of a complex variable, mathematically the loop before that step is z = Π_j cos β[j] (1 + i direction_j 2^{-j}) So the step corresponds to the trivial rearrangement z = ( Π_j cos β[j] ) ( Π_j (1 + i direction_j 2^{-j}) )
    - edflsafoiewq 8y ago
      This is what's happening mathematically: https://i.imgur.com/MclfpEN.png https://i.imgur.com/MclfpEN.png.

edflsafoiewq 8y ago

To your first question: tan β_i = sin β_i / cos β_i = 2^{-i}. So cos β_i = 2^i sin β_i. Square and use Pythagoras for cos^2 β_i = 2^{2i} (1 - cos^2 β_i). Solve for cos^2 β_i and take the positive square root for 2^i / √(1 + 2^{2i}) = 1 / √(2^{-2i} + 1). To your second: I presume the author multiplied 1 by K to get c <- K. When you multiply 0 by K you still have s <- 0 though. I haven't figured out the justification for moving the multiplication from after to before the loop but if it works for c it works for s too. edit: If you write c_new ← cos α[i] × (c - tan α[i] × s) s_new ← cos α[i] × (s + tan α[i] × c) c ← c_new s ← s_new with a complex variable z = c + is (changing the index variable to j) you get z ← cos α[j] × (1 + i × tan α[j]) × z which makes it more obvious (to me) that you can pull the multiplication by cosines out of the loop. z <- 1 # c=1, s=0 z <- K × z # c=K, s=0 ...loop...

seedless-sensat 8y ago
To the second question, I am also struggling to follow the step "Let’s move the multiplications by cos β_i into a separate loop". The recursive construction of `c` doesn't allow the betas to be pulled out. The i-th term is multiplied by beta_i to beta_n, but not beta_0 to beta_{i-1}. Clearly the method must work, but I'm not sure what I'm missing. Edit: ignoring `direction`, I can prove this step by induction for both c(i) and s(i) together. At least to me though, this it isn't a simple algebraic simplification.
- edflsafoiewq 8y ago
  I got it. In terms of a complex variable, mathematically the loop before that step is z = Π_j cos β[j] (1 + i direction_j 2^{-j}) So the step corresponds to the trivial rearrangement z = ( Π_j cos β[j] ) ( Π_j (1 + i direction_j 2^{-j}) )
  - edflsafoiewq 8y ago
    This is what's happening mathematically: https://i.imgur.com/MclfpEN.png https://i.imgur.com/MclfpEN.png.