Extending The Effects Engine Report W3

Hello again,

I am glad to inform you that I got my first pull request[1] merged this week. It was a glitch inside EngineFitlerButterworth8Band's process method. When it was called with specific frequency corners, a constant unbearable crackling could be heard.

While I was testing if the ramping after setFrequencyCorners is working properly I stumbled upon that issue. I have immediately filed a bug[2] on Launchapd. Here is how you could reproduce it:

• go to preferences -> equalizers;
• disable "Use static Equalizers";
• set Low Shelf EQ to minimum and High Shelf EQ to maximum;
• play a track and turn up the knob responsible for EQ's "Mid" gain.

I started looking for the source of the problem. As Daniel suggested, it could have been some sort of overflow inside the process method. Throughout the code, we were using CSAMPLE (which is a typedef for float) to represent coefficients and buffers. The problem was that fidlib's fid_design_coef function returns an array whose type is double. So we were initializing a CSAMPLE array with double values. Consequently, precious information was lost during this cast.

The solution was to use double instead of CSAMPLE for representing the information we need for the filter. Anyway, a performance issue arose and we should pay attention to such kind of problems because responsiveness is very important to real time processing software. So I placed a timer inside _process*pass functions to test the before and after patch performance. Here are the results:

My notebook (with RT Scheduling disabled): Without the patch: Stat("_processBandpass()","count=2.10739e+06,sum=1.89391e+09ns,average=898.699ns,min=461ns,max=422348ns,variance=1.01698e+06ns^2,stddev=1008.45ns")

With the patch: Stat("_processBandpass()",count=2.61939e+06,sum=2.50817e+09ns,average=957.538ns,min=477ns,max=663356ns,variance=1.04328e+06ns^2,stddev=1021.41ns")

Asus eeePC with RT Scheduling enabled: Without the patch: Stat("_processBandpass()","count=3.43757e+06,sum=8.52755e+09ns,average=2480.69ns,min=1467ns,max=138215ns,variance=1.43556e+06ns^2,stddev=1198.15ns")

With the patch: Stat("_processBandpass()","count=1.85549e+06,sum=4.71517e+09ns,average=2541.2ns,min=1536ns,max=131022ns,variance=1.88703e+06ns^2,stddev=1373.69ns")

As you can see, the difference is not significant and it is a small price to pay for solving an annoying issue.

Another pull request[3] I have opened this week is about porting the static Equalizer as a separate effect. This EQ is useful for computers with poor hardware since it is not so CPU demanding. Therefore, LightweightEQ seemed a good name for such an Equalizer. While I was working at this effect, Daniel pointed out that I can use fidlib for computing the coefficients needed for this EQ. This is something I hope to do in the following week.

A topic which is harder than I thought and which has been taking me quite some time is implementing switch type parameters for the effects framework. As a reminder, I need them for EQ's kill buttons. I noticed every effect has an EffectParameter list which stores information about each parameter. EffectParameterSlot is responsible for instantiating a ControlEffectKnob for an EffectParameter. The problem with ControlEffectKnob is that it behaves only as a ControlPotmeter and we need it to behave like a ControlPushButton too. I tried to create a different list of "button parameters" for each effect and create a new class (EffectButtonParameterSlot) which instead of ControlEffectKnob features a ControlPushButton. The problem with this approach is that a lot of code is duplicated and currently I haven't managed to make it work properly, because the buttons are not updating as they should. I suspect it is a problem with the "request-response" update mechanism. I hope that by the end of the next week, with the community's help, I'll come to a resolution regarding this topic.

Here is the basic flow when creating an effect parameter slot:

EffectsManager::setupDefaults --->
EffectRack::addEffectChainSlot --->
EffectChainSlot::addEffectSlot --->
EffectSlot::EffectSlot (constructor) --->
EffectSlot::addEffectParameterSlot --->
EffectParameterSlot::EffectParameterSlot (constructor) --->
ControlEffectKnob::ControlEffectKnob (constructor)

Yours truly,
Nicu Badescu

[1] - https:*github.com/mixxxdj/mixxx/pull/270
[2] - https:*bugs.launchpad.net/mixxx/+bug/1326001
[3] - https://github.com/mixxxdj/mixxx/pull/268

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