ILMAUserCallback2 Interface

This is a user callback interface that should be implemented to receive the audio buffers. You register the callback by setting the ILMACallback::CallbackObj2 property. This interface has one method, which will be called by the LEAD Audio Callback Filter whenever the filter receives an audio buffer.

DirectShow determines the duration of the audio buffer. If you use the LEADTOOLS Multimedia toolkit, you can change the size of the audio buffers using the AudioBufferSize property.

This interface should be used for C++, .NET or other programming languages that can cast a long to a pointer. See below for .NET examples (C# and VB.NET) of a class implementing this interface.

Interface Methods:

HRESULT CallbackProc (long pData, long lDataSize, long lBitsPerSample, long lChannels, long lSamplesPerSec, long lAvgBytesPerSec);

This function will be called each time an audio sample is received by the filter.

Parameters

Returns

Comments

Format of audio data:

The general format of the audio data (lChannels) is as follows:

Sample 0

In this case,

pData(0) is Sample 0, Channel 0
pData(1) is Sample 0, Channel 1

pData(lChannels - 1) is Sample 0, Channel lChannels -1
pData(lChannels) is Sample 1, Channel 0
pData(lChannels + 1) is Sample 1, Channel 1

The format for mono data is simple, because there is only one channel. In this case, every value in the array is one sample:

pData(0) is Sample 0
pData(1) is Sample 1

The format for stereo data is still simple: there are two values per sample (one for left channel and the other for right channel):

pData(0) is Sample 0, Left channel
pData(1) is Sample 0, Right channel
pData(2) is Sample 1, Left channel
pData(3) is Sample 1, Right channel

The format of the value is different depending on whether lBitsPerChannel is 8 or 16.

Format of 8-bit audio data:

The values in the array are unsigned and between 0 and 255. The real audio value will be obtained by subtracting 128 from the array value and should have a value between -128 and 127. If you want to change the value, you must remember to add 128 before putting back the data.

Format of 16-bit data:

The values in the array are signed and between -32768 and 32767. They contain the real audio value and it is not necessary to do the conversions like for the 8-bit data.

Examples

Example1: Fast C# code (using unsafe code)

This C# code silences the sound track by setting all the sound values to 0. Note that this sample uses pointers, which means your application must be compiled as using unsafe code. For slower, safe code, look at the second example, which is written in VB.NET.

private void Form1_Load(object sender, EventArgs e)
{
   string file = @"c:\myfile.avi";

   // Insert Audio Callback Filter
   LTMMLib.ltmmProcessor proc = GetAudioProcessor("LEAD Audio Callback Filter");
   playControl.SelectedAudioProcessors.Add(proc, 0);

   // Hook callback.
   LMACallbackLib.LMACallback audioCallback = (LMACallbackLib.LMACallback)playControl.GetSubObject((int)LTMMLib.ltmmPlay_Object.ltmmPlay_Object_SelAudioProcessor);
   audioCallback.CallbackObj2 = new AudioCallback(audioCallback);

   // Play it.
   playControl.sourcefile = file;
}

public class AudioCallback : LMACallbackLib.ILMAUserCallback2
{
   unsafe public void CallbackProc(int pData, int lDataSize, int lBitsPerSample, int lChannels, int lSamplesPerSec, int lAvgBytesPerSec)
   {
      IntPtr ptrData = new IntPtr(pData);
      int i;
      if (lBitsPerPixel == 16)
      {
         short* ptrData16 = (short*)ptrData.ToPointer();

         for(i = 0; i < lDataSize; i++)
            ptrData16[i] = 0;
      }
      else
      {
         byte* ptrData8 = (byte*)ptrData.ToPointer();

         for(i = 0; i < lDataSize; i++)
            ptrData8[i] = 0;
      }

      ' If you need to return a HRESULT other than AUDCALBK_ERR_OK, then you need to throw an exception   
      ' throw new ReturnValueException(LMACallbackLib.AudCalBkErrConstants.AUDCALBK_ERR_XXX);
   }
}
 


Example2: VB.NET code (slower, but using safe code)

This VB.NET code silences the sound track by setting all the sound values to 0. Note that this sample is safe, does not use pointers, but is slower than the unsafe code above.

Public Class VBAudioCallback : Implements LMACallbackLib.ILMAUserCallback2
   Public Sub New()
   End Sub

   Sub CallbackProc(ByVal pData As Integer, ByVal lDataSize As Integer, ByVal lBitsPerSample As Integer, ByVal lChannels As Integer, ByVal lSamplesPerSec As Integer, ByVal lAvgBytesPerSec As Integer) Implements LMACallbackLib.ILMAUserCallback2.CallbackProc
      Dim ptrData As IntPtr = New IntPtr(pData)
      Dim i As Integer
      ' copy the data to a managed array, since VB does not support pointers.
      If lBitsPerSample = 16 Then
         Dim ptrData16(lDataSize) As Short
         ' Copy data from pData into ptrData16
         System.Runtime.InteropServices.Marshal.Copy(ptrData, ptrData16, 0, lDataSize)
         ' set the data to 0
         For i = 0 To lDataSize - 1
            ptrData16(i) = 0
         Next
         ' copy the data back into ptrData
         System.Runtime.InteropServices.Marshal.Copy(ptrData16, 0, ptrData, lDataSize)
      Else
         Dim ptrData8(lDataSize) As Byte
         ' Copy data from pData into ptrData8
         System.Runtime.InteropServices.Marshal.Copy(ptrData, ptrData8, 0, lDataSize)
         ' set the data to 0
         For i = 0 To lDataSize - 1
            ptrData8(i) = 0
         Next
         ' copy the data back into ptrData
         System.Runtime.InteropServices.Marshal.Copy(ptrData8, 0, ptrData, lDataSize)
      End If


   End Sub
End Class