# Adjust event playback speed with custom DSP

**URL:** <https://qa.fmod.com/t/adjust-event-playback-speed-with-custom-dsp/22022>\
**Category:** FMOD Engine\
**Tags:** csharp\
**Created:** [August 19, 2024, 5:33pm UTC](https://qa.fmod.com/t/adjust-event-playback-speed-with-custom-dsp/22022 "2024-08-19T17:33:47Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![theome](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@theome](https://qa.fmod.com/u/theome)\
**Post date:** [August 19, 2024, 5:33pm UTC](https://qa.fmod.com/t/adjust-event-playback-speed-with-custom-dsp/22022/1 "2024-08-19T17:33:47Z")

</div>

In my game, I want to play an audio stream that I receive over the network in an FMOD event. I’m using a custom DSP to provide the PCM data. The setup looks like this (C#, but not Unity):

```
eventInstance.getChannelGroup(out eventChannelGroup); 
FMOD.DSP_DESCRIPTION description = new() {
    numinputbuffers = 1,
    numoutputbuffers = 1,
    read = dspCallback,
    userdata = /* ... */
};
AudioEngine.Instance.coreSystem.createDSP(ref description, out streamDSP);
eventChannelGroup.addDSP(FMOD.CHANNELCONTROL_DSP_INDEX.TAIL, streamDSP);

```

The callback is essentially just copying the PCM data from a ringbuffer to the DSP’s outbuffer:

```
private static readonly FMOD.DSP_READ_CALLBACK dspCallback = DSPCallback;

static FMOD.RESULT DSPCallback(ref FMOD.DSP_STATE dsp_state, IntPtr inbuffer, IntPtr outbuffer, uint length, int inchannels, ref int outchannels) {
    /* ... */
    Marshal.Copy(myRingBuffer, offset, outbuffer, (int)length);
    return FMOD.RESULT.OK;
}

```

So far, this works fine.

I’m constantly measuring the delay between the time when I receive network packets and the time when I play back samples. I want to use a dynamic jitter buffer, so I have to adjust that latency dynamically. To do so, I need to increase or slow down the playback rate slightly when the latency gets to large or to small (similar to the “Record” scripting example at [https://www.fmod.com/docs/2.03/unity/examples-record.html](https://www.fmod.com/docs/2.03/unity/examples-record.html)).

How would I do that? I tried setting the pitch to `0.5` with `eventInstance.setPitch` or `eventChannelGroup.setPitch` just to understand what happens, but both parameters don’t seem to have an effect in my setup. I would expect that either my `DSPCallback` is called less frequently if I reduce the playback speed, or the `length` parameter is shorter, but everything is constant, so my `DSPCallback` “consumes” PCM data with a constant speed.

What’s the right approach here?

---

<div class="post-metadata">

**Author:** ![theome](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@theome](https://qa.fmod.com/u/theome)\
**Post date:** [August 21, 2024, 6:14am UTC](https://qa.fmod.com/t/adjust-event-playback-speed-with-custom-dsp/22022/2 "2024-08-21T06:14:04Z")

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I think I figured it out. The `Sound` instance in the “Record” scripting example is not just a special ring buffer that the system can record into, it’s also what makes a channel’s pitch property work in the first place.

Instead of a custom DSP with a read callback I’m now using a `Sound` and write directly into its buffer which I access with `Sound.lock`. When I change the associated channel’s pitch, the playback speed is adjusted as expected.

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<div class="post-metadata">

**Author:** ![Leah\_FMOD](https://yyz2.discourse-cdn.com/flex036/user_avatar/qa.fmod.com/leah_fmod/32/3685_2.png) [@Leah\_FMOD](https://qa.fmod.com/u/Leah_FMOD)\
**Post date:** [August 21, 2024, 6:58am UTC](https://qa.fmod.com/t/adjust-event-playback-speed-with-custom-dsp/22022/3 "2024-08-21T06:58:51Z")

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Happy to hear you found a solution!