[Stk] BlitSquare bug?

[Steven Yi]

Hi Gary,

I've gone through again my analysis again and I still see a problem.
Let me see if I can explain what I'm seeing better.

In BlitSquare.h, I see the integration really happening in this line:

  lastFrame_[0] = lastBlitOutput_ - dcbState_ + 0.999 * lastFrame_[0];

where the newly calculated value for lastBlitOutput_ and the leaky add
of 0.999 * lastFrame_[0] occur.

This line:

 lastBlitOutput_ += temp;

adds temp, but the following line:

  lastFrame_[0] = lastBlitOutput_ - dcbState_ + 0.999 * lastFrame_[0];

stubtracts dcbState_.  But the way the code is, by the time temp is
added and dcbState_ is subtracted, temp and dcbState_ are always equal
to each other. If we put the two above lines together, you get:

lastFrame_[0] = lasBlitOutput_ + temp - dcbState_ + 0.999 * lastFrame_[0];

but since temp == dcbState, the above is equivalent to:

lastFrame_[0] = lastBlitOutput_ + 0.999 * lastFrame_[0];

You can see it if you walk through the code and notice:

  StkFloat temp = lastBlitOutput_;
  dcbState_ = lastBlitOutput_;

The only time when those two values would not be equal to each other
is on the initial pass, if lastBlitOutput_ was initialized to a
different value than dcbState_.  However, BlitSquare::reset() sets
both to 0.

Hopefully that's a little clearer.  If not, I can write up some code
to show the issue that way.

Thanks,
steven

On Thu, Mar 12, 2015 at 1:56 PM, Gary Scavone <gary at ccrma.stanford.edu> wrote:
> Hi Steven,
>
> The bandlimited square wave is formed by integrating a bipolar blit.  So, the line:
>
>   lastBlitOutput_ += temp;
>
> accomplishes the integration.  The subsequent filtering operation is a one-pole DC blocker and the line:
>
>   dcbState_ = lastBlitOutput_;
>
> simply saves the current input state for the subsequent iteration. The DC blocker is required to avoid a DC offset that can accumulate over time from the integration.
>
> I don’t see a bug in the code.
>
> Regards,
>
> —gary
>
> On Mar 3, 2015, at 10:11 AM, Steven Yi <stevenyi at gmail.com> wrote:
>
>> Hi All,
>>
>> I was translating the STK code for BlitSquare to another language and
>> I have a question about the code.  As far as I can tell, the code in
>> BlitSquare.h for the tick method looks to have a bug.  I noticed this:
>>
>>  StkFloat temp = lastBlitOutput_;
>>  ...
>>  lastBlitOutput_ += temp;
>>
>>  // Now apply DC blocker.
>>  lastFrame_[0] = lastBlitOutput_ - dcbState_ + 0.999 * lastFrame_[0];
>>  dcbState_ = lastBlitOutput_;
>>
>> The value of dcbState_ and temp are both set to the lastBlitOutput_.
>> The calculation adds temp, but then subtracts dbcState_, which
>> effectively makes the line calculating last_frame_[0] :
>>
>>  lastFrame_[0] = lastBlitOutput_ + 0.999 * lastFrame_[0];
>>
>> if one takes out the "lastBlitOutput_ += temp" line.
>>
>> In my translated code, I've plotted both the direct translation as
>> well as a version removing the use of temp and dcbState_ and I got the
>> same results.
>>
>> Could someone confirm my analysis about this code?
>>
>> Thanks!
>> steven
>>
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>