Extending The Effects Engine Report W2

#2 Weekly Report

Howdy,

My main target this week was fixing this bug. What is the deal with this bug? Well, every time one of the frequency corners was changed from the preference page, a new EngineFilterButterworth object was created. Thus, some clicks could be heard because the previous filter state was lost and it was a slight difference in the output samples between from the old filter and the newly created filter. The solution to this issue is cross fading between the old output samples and the new ones.

Here is how I tackled this problem:
* store a flag inside EngineFilterButterworth class which is set to true if ramping is needed;

• store the old coefficients and buffers for the two Channels;

   * put all the values from the current coefficients array into the old array, just before the former is updated with new coefficients;
   * old buffers are initialized with current buffers, just before current buffers are zeroed (inside initBuffers method);
* when a frequency is changed, set the flag responsible for ramping to true;
* inside the process method, if the ramping flag is true, compute the old output, the new one and cross fade between them; also, set the flag to false.

However, it is easier said than done. I was pretty confident that my first implementation will work, but it didn't. I was creating dynamically a new array of MAX_BUFFER_LEN size which was used for computing the old output samples. I must thank Owen for helping me out with this. The new output samples were stored in pOutput. After cross fading between those two, I was freeing the memory allocated for the array. The problem with this approach is that MAX_BUFFER_LEN is 160,000. Imagine the inefficiency of allocating and freeing 160,000 numbers, multiple times per second.

The next idea came to me while analyzing SampleUtil::linearCrossfadeBuffers's code. Instead of computing and storing the whole old buffer before cross fading, I was going to compute it on the fly. Consequently, I needed only two numbers, one for each channel. The formula for cross fading is : result = final * coef + initial * (1 - coef). This is a simple formula which has a lot of applications, including those from the gaming industry (they use it for linear interpolation). coef is a real number between 0 and 1 which is incremented each step. As he approaches the value 1, final is fading in and initial is fading out.

This time I was sure my code will work flawlessly. Guess what? It didn't. I was doing a rookie mistake and Daniel helped me out. A new EngineFilterButterworth was being created each time the frequency corners changed. So my logic with buffers to store the old coefficients was useless. I fixed this by updating each filter instead of creating a new one.

Unfortunately, there is a case when even after ramping, a lot of crackles can be heard. To reproduce this, turn up the mid gain. Inside equalizer preferences, turn the low frequency all the way down and the high frequency corner all the way up. Any suggestions are welcomed! My branch can be found here.

Cheers!
Nicu Badescu

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