Using IAMStreamSelect To Select a Specific Audio Stream from an MPEG File

The IAMStreamSelect is a Direct Show interface for selecting which streams are played and retrieving information about them. An example of logical stream selection would be selecting a particular language (English, German, or French for example) from a set of audio streams that encode different national languages.

To use IAMStreamSelect to select a specific audio stream from an MPEG file with multiple audio streams, perform the following steps:

1. Create an instance of ltmmPlay class:

   C API

   IltmmPlay* pPlay; 
   CoCreateInstance(&CLSID_ltmmPlay, NULL, CLSCTX_INPROC_SERVER, &IID_IltmmPlay, (void**) &pPlay); 

   C++

   IltmmPlay* pPlay;   
   CoCreateInstance(&CLSID_ltmmPlay, NULL, CLSCTX_INPROC_SERVER, &IID_IltmmPlay, (void**) &pPlay); 

2. Set the source file:

   C API

   BSTR bstr;    
   // create a string containing the source file path   
   bstr = SysAllocString(L"C:\\MultipleAudio.mpg");    
   // assign the source file path to the play object   
   IltmmPlay_put_SourceFile(pPlay,  bstr);    
   // free the string   
   SysFreeString(bstr); 

   C++

   BSTR bstr;    
   // create a string containing the source file path   
   bstr = SysAllocString(L"C:\\MultipleAudio.mpg");   
   // assign the source file path to the play object   
   pPlay->put_SourceFile(bstr);    
   // free the string   
   SysFreeString(bstr); 

3. The IAMStreamSelect is usually exposed by the file splitter (Demultiplexer), so you need to get a pointer to the splitter object:

   C API

   IUnknown* pSplitter;   
   IltmmPlay_GetSubObject (pPlay ,ltmmPlay_Object_Splitter, & pSplitter); 

   C++

   IUnknown* pSplitter;   
   pPlay->GetSubObject (ltmmPlay_Object_Splitter, & pSplitter); 

4. Query for the IAMStreamSelect interface:

   C API

   IAMStreamSelect *pStreamSel = NULL;   
   IUnknown_QueryInterface(pSplitter, &IID_IAMStreamSelect, (void**)& pStreamSel);   
   IUnknown_Release(pSplitter); 

   C++

   IAMStreamSelect *pStreamSel = NULL;   
   pSplitter->QueryInterface(&IID_IAMStreamSelect, (void**)&pStreamSel);   
   pSplitter->Release(); 

5. Use the IAMStreamSelect interface to select one of the available audio streams in the file:

   C API

   // retrieve the number of available streams in the file 
   DWORD dwCount; 
   int nIndex; 
   BOOL bAudio = FALSE; 
   BOOL bEnable = FALSE; 
   AM_MEDIA_TYPE *pmt; 
   DWORD dwFlags; 
   LCID lcid; 
   DWORD dwGroup; 
   WCHAR *pszName; 
   IUnknown *pObject; 
   IUnknown *pUnk; 
   IAMStreamSelect_Count(pStreamSel , &dwCount); 
   for( nIndex = 0; nIndex<dwCount; nIndex++ ) 
   { 
   // get information about the stream, and decide if this is an audio stream and you want to enable it: 
      IAMStreamSelect_Info(pStreamSel , nIndex, &pmt, &dwFlags, &lcid, &dwGroup, &pzsName, &pObject, &pUnk); 
       
      // based on your selection criterion 
      if( bAudio && bEnable ) 
         IAMStreamSelect_Enable(pStreamSel, nIndex, AMSTREAMSELECTENABLE_ENABLE); 
   } 
   IUnknown_Release(pStreamSel); 

   C++

   // retrieve the number of available streams in the file 
   DWORD dwCount; 
   int nIndex; 
   BOOL bAudio = FALSE; 
   BOOL bEnable = FALSE; 
   AM_MEDIA_TYPE *pmt; 
   DWORD dwFlags; 
   LCID lcid; 
   DWORD dwGroup; 
   WCHAR *pszName; 
   IUnknown *pObject; 
   IUnknown *pUnk; 
    
   pStreamSel->Count(&dwCount); 
   for( nIndex = 0; nIndex<dwCount; nIndex++ ) 
   { 
   // get information about the stream, and decide if this is an audio stream and you want to enable it: 
      pStreamSel->Info(nIndex, &pmt, &dwFlags, &lcid, &dwGroup, &pzsName, &pObject, &pUnk); 
       
      // based on your selection criterion 
      if( bAudio && bEnable ) 
         pStreamSel->Enable(nIndex,       AMSTREAMSELECTENABLE_ENABLE); 
   } 
   pStreamSel->Release(); 

6. Play the file:

   C API

   IltmmPlay_Run (pPlay); 

   C++

   pPlay->Run ();