# Sample accurate host beat sync

**URL:** <https://iplug2.discourse.group/t/sample-accurate-host-beat-sync/383>\
**Category:** Uncategorized\
**Created:** [21 April 2021 00:50 UTC](https://iplug2.discourse.group/t/sample-accurate-host-beat-sync/383 "2021-04-21T00:50:30Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![kzantow](https://yyz2.discourse-cdn.com/free1/user_avatar/iplug2.discourse.group/kzantow/32/156_2.png) [@kzantow](https://iplug2.discourse.group/u/kzantow)\
**Post date:** [21 April 2021 00:50 UTC](https://iplug2.discourse.group/t/sample-accurate-host-beat-sync/383/1 "2021-04-21T00:50:30Z")

</div>

Hiya, I needed a way to determine when the host beats occur during block processing in order to trigger an event. I thought I’d post this here because it took me a little while to figure it out and maybe it will be helpful to someone. All the examples I saw looked like they were doing things per-sample and either setting an LFO phase or something similar and while I figured out a way to use a per-sample method to determine the beat changes, it seemed pretty inefficient. I’m sure you people who have been developing plugins for a long time probably have better ways to do this, but here’s one way you can do this per-block:

First, calculate the total ppq time for the current block, `nFrames` is the number of samples in the block, of course

```auto
double blockPPQ = nFrames / GetSamplesPerBeat();

```

Next, and this is the trick to find if the beat changes during this block, use `mPPQPos` (VST projectTimeMusic). Note `numBeats` is the beat divisor, e.g. it would be 1 for quarter note .5 for eighth or 2 it would repeat every 2 beats. You may need to get even more complicated by using the `mTimeInfo.mNumerator` and `mTimeInfo.mDenominator` to determine true pulses but in my case, I could just call 1 a beat no matter what the time signature. You also might want to look into `mTimeInfo.mLastBar` to see which beat of the measure this is, depending on your use case.

```auto
double beatChange = blockPPQ - fmod(mTimeInfo.mPPQPos + blockPPQ, numBeats);

```

If beatChange is \>= 0 there is a change during this block

```auto
if (beatChange >= 0) {

```

… you may just want to set the `beatChangeSample`, process and then do what you need during the processing:

```auto
long beatChangeSample = round(beatChange * GetSamplesPerBeat());

```

In other words:

```auto
double beatDiv = 1.;
double samplesPerBeat = GetSamplesPerBeat();
double block = nFrames / samplesPerBeat;
double beatChange = block - fmod(mTimeInfo.mPPQPos + block, beatDiv);
long beatChangeSample = round(beatChange * samplesPerBeat);
for (int s = 0; s < nFrames; ++s) {
  if (s == beatChangeSample) {
    // beat change, do stuff
  }
}

```

Hope that helps someone!

---

<div class="post-metadata">

**Author:** ![Ric](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@Ric](https://iplug2.discourse.group/u/Ric)\
**Post date:** [21 April 2021 02:30 UTC](https://iplug2.discourse.group/t/sample-accurate-host-beat-sync/383/2 "2021-04-21T02:30:21Z")

</div>

Hi kzantow, thanks for sharing that.

So far, I’ve been using the function `GetSamplePos()` to align the peaks of a tremolo to the beats (or 8th, 16th note, &c.) of a song. Along with `GetSamplesPerBeat()` it’s not very difficult to find the beats, but of course things get complicated with meter and tempo changes.