FMOD_ERR_INTERNAL error message spammed after FMOD::System::init call


I am using FMOD to replace an audio engine in a video game that was originally programmed to use DirectSound. I am linking to the fmod_vc (fmodL_vc for debug) lib within a MSVC++ 2019 project, C++ 11. I am using the latest FMOD SDK, version 2.02.07.

I have followed the examples and various other resources to write an implementation, but as soon as I call FMOD::System::init, the debugger window is continuously filled with;

[ERR] assert : assertion: 'length == blockSize' failed
[ERR] Output::mix : Error during mix.  FMOD_RESULT = 28 -- An error occurred that wasn't supposed to.  Contact support.

It seems like the FMOD thread is continuously perform a task but I am unable to obtain more information as to what the issue is.

The full log is;

[LOG] SystemI::init : Initialize version=20207 (125130), maxchannels=1024, flags=0x00110000
[LOG] OutputWASAPI::init : Mix Format (WAVEFORMATEX): wFormatTag=0xFFFE, nChannels=2, nSamplesPerSec=48000, nAvgBytesPerSec=384000, nBlockAlign=8, wBitsPerSample=32, cbSize=22.
[LOG] OutputWASAPI::init : Mix Format (WAVEFORMATEXTENSIBLE): wValidBitsPerSample=32, dwChannelMask=0x00000003, SubFormat=00000003-0000-0010-8000-00AA00389B71.
[LOG] OutputWASAPI::init : Output buffer size: 4092 samples, latency: 0.00ms, period: 9.99ms, DSP buffer: 1024 * 4
[LOG] Thread::initThread : Init FMOD stream thread. Affinity: 0x4000000000000003, Priority: 0xFFFF7FFB, Stack Size: 98304, Semaphore: No, Sleep Time: 10, Looping: Yes.
[LOG] Thread::initThread : Init FMOD mixer thread. Affinity: 0x4000000000000001, Priority: 0xFFFF7FFA, Stack Size: 81920, Semaphore: No, Sleep Time: 0, Looping: Yes.
[ERR] assert : assertion: 'length == blockSize' failed
[ERR] Output::mix : Error during mix.  FMOD_RESULT = 28 -- An error occurred that wasn't supposed to.  Contact support.

Here is a trimmed-down version of my init process;



System->setSoftwareFormat(48000, FMOD_SPEAKERMODE_STEREO, 0);

System->setStreamBufferSize(32768, FMOD_TIMEUNIT_RAWBYTES);

System->init(1024, FMOD_INIT_NORMAL|FMOD_INIT_THREAD_UNSAFE, nullptr);

System->createChannelGroup("Effects", &SfxChannelGroup);
System->createChannelGroup("Effects", &MusicChannelGroup);


System->setFileSystem(FMOD_FileOpenCallback, FMOD_FileCloseCallback, FMOD_FileReadCallback, FMOD_FileSeekCallback, nullptr, nullptr, -1);

This is a single-threaded application currently, my thought process is;

  • I need to retain a DirectSound currently audio instance currently as the video games movie format is decoded in chunks and pushed into DirectSound, and I have yet to find a solution for this in FMOD. Can FMOD work alongside an existing DirectSound thread opened by the application or do I need to abandon this? EDIT: Disabling the movies/DirectSound altogether made no difference.
  • I am linking to the library directly, would there be a difference if I loaded the imports from the DLL and worked with FMOD on a C level?

I hope I have provided enough information,


On Windows 10, I have only been able to make FMOD work in my project using;


I don’t think this is a viable solution, as my understanding is that this will only work for users who run Windows 10 (Our target is Win 7+)?

I have not been able to reproduce this issue by copying your init process, which looks pretty standard/sensible to me. Are the API examples running fine for you with the default WASAPI output type? If you are finding that the API examples are running fine but not your application then I should be able to reproduce it on my machine; would it be possilbe for you to provide a basic reproduction so I can try debugging the issue on my end?

That should be fine and I don’t think there would be any difference in this case.

Hi Jeff, thanks for taking the time to reply.

My apologies, I should have mentioned in my post that the example projects work perfectly.

I will drop you a direct message and hopefully be able to prepare a package for you.