public void ProcessEnvelope(double val)
{
double combinedFiltered;
double decay;
// 1. Rectify the input signal
val = Math.Abs(val);
// 2. Band pass filter to ~ 100hz - 2Khz
//lpValue = lpAlpha * val + (1 - lpAlpha) * lpValue;
var lpValue = inputFilter.Process(val);
//hpSignal = hipassAlpha * lpSignal + (1 - hipassAlpha) * hpSignal
// rectify the lpValue again, because the resonance in the filter can cause a tiny bit of ringing and cause the values to go negative again
lpValue = Math.Abs(lpValue);
var mainInput = lpValue;
// 3. Compute the EMA and SMA of the band-filtered signal. Also compute the per-sample dB decay baed on the SMA
var emaValue = ema.Update(mainInput);
var smaValue = sma.Update(mainInput);
// 4. use a latching low-pass classifier to determine if signal strength is generally increasing or decreasing.
// This removes spike from the signal where the SMA may move in the opposite direction for a short period
var movementValue = movementLatch.Update(sma.GetDbDecayPerSample() > 0);
// 5. If the movement is going up, prefer the faster moving EMA signal if it's above the SMA
// If the movement is going down, prefer the faster moving EMA signal if it's below the SMA
// otherwise, use SMA
if (movementValue > 0) // going up
{
combinedFiltered = emaValue > smaValue ? emaValue : smaValue;
}
else // going down
{
combinedFiltered = emaValue < smaValue ? emaValue : smaValue;
}
// 6. use a hold mechanism to store the peak
if (combinedFiltered > hold)
{
hold = combinedFiltered;
lastTriggerCounter = 0;
}
// 7. Choosing the decay speed
// Under normal conditions, use the decay from the SMA, scaled by a fudge factor to make it slightly faster decaying.
// The reason for this is so that we gently bump into the peaks of the signal once in a while.
// If the hold mechanism hasn't been triggered for a specific timeout, then the current hold value is too high, and we need to rapidly decay downwards.
// Use the fastDecay (based on the user- specified release value) as a slew limited value
if (lastTriggerCounter > triggerCounterTimeoutSamples)
{
decay = fastDecay;
}
else
{
decay = Utils.Db2Gain(sma.GetDbDecayPerSample() * 1.2); // 1.2 is fudge factor to make the follower decay slightly faster than actual signal, so we gently bump into the peaks
}
// 7.5 Limit the decay speed in the general range of slowDecay...fastDecay, the slow decay is currently a fixed 3 seconds to -60dB value
if (decay > slowDecay)
{
decay = slowDecay;
}
if (decay < fastDecay)
{
decay = fastDecay;
}
hold = hold * decay;
// 8. Filter the resulting hold signal to retrieve a smooth envelope.
// Currently using 4x 1 pole lowpass, should replace with a proper 4th order butterworth
h1 = holdAlpha * hold + (1 - holdAlpha) * h1;
h2 = holdAlpha * h1 + (1 - holdAlpha) * h2;
h3 = holdAlpha * h2 + (1 - holdAlpha) * h3;
h4 = holdAlpha * h3 + (1 - holdAlpha) * h4;
holdFiltered = h4;
lastTriggerCounter++;
}