Multichannel WMA Audio Playback in DirectShow

Multichannel WMA Audio Playback in DirectShow

To play back a multichannel Windows Media Audio file in DirectShow, you must set the MFPKEY_WMADEC_HIRESOUTPUT property directly on the decoder after it has been connected to the WM ASF Reader. This property is defined in the header file wmcodecdsp.h, which is available in the Windows SDK.

• Note This configuration procedure is supported only for files that are not protected by Digital Rights Management.

The basic steps to enable multichannel output are as follows:

1. Call RenderFile to create the filter graph.
2. Obtain a pointer to the DMO Wrapper filter.
3. Disconnect the DMO Wrapper from the Audio Renderer.
4. Use the IPropertyBag interface to set the MFPKEY_WMADEC_HIRESOUTPUT property on the decoder. The property name is defined by the global constant g_wszWMACHiResOutput.
5. Reconnect the DMO Wrapper and the Audio Renderer.
6. Run the graph.

The following code snippets demonstrate these steps. This code assumes that the source file contains an audio stream and no video stream. The video codec DMO does not support the MFPKEY_WMADEC_HIRESOUTPUT property.

HRESULT BuildGraph(IGraphBuilder *pGraph, const WCHAR *wFileName)
{
    HRESULT hr = S_OK;
    IBaseFilter       *pDmoWrapper = NULL;
    IPin              *pDmoOut = NULL;
    IPin              *pRendererIn = NULL;
    IPropertyBag      *pPropBag = NULL;

    hr = pGraph->RenderFile(wFileName, NULL);

    // Get pointers to the two DMO Wrapper interfaces we need.
    // (Function bodies provided at the end of this snippet.)
    if (SUCCEEDED(hr))
    {
        hr = GetDMOWrapper(pGraph, &pDmoWrapper); 
    }

    //Disconnect the DMO Wrapper from the Audio Renderer.
    if (SUCCEEDED(hr))
    {
        hr = GetPin(pDmoWrapper, PINDIR_OUTPUT, &pDmoOut);
    }
    if (SUCCEEDED(hr))
    {
        hr = DisconnectPin(pGraph, pDmoOut, &pRendererIn);
    }

    //Set the property on the DMO.
    VARIANT varg;
    ::VariantInit(&varg);
    varg.vt    = VT_BOOL;
    varg.boolVal = TRUE;

    if (SUCCEEDED(hr))
    {
        hr = pDmoWrapper->QueryInterface(IID_IPropertyBag, (void**)&pPropBag);
    }
    if (SUCCEEDED(hr))
    {
        hr = pPropBag->Write(g_wszWMACHiResOutput, &varg);
    }

    // Reconnect the DMO Wrapper and the Audio Renderer
    if (SUCCEEDED(hr))
    {
        hr = pGraph->Connect(pDmoOut, pRendererIn);
    }

    SAFE_RELEASE(pDmoWrapper);
    SAFE_RELEASE(pDmoOut);
    SAFE_RELEASE(pRendererIn);
    SAFE_RELEASE(pPropBag);
    return hr;
}

The helper functions from the previous code snippet are implemented as follows:

HRESULT GetDMOWrapper (IFilterGraph *pGraph, IBaseFilter** ppFilter) 
{
    BOOL bFound = FALSE;

    IEnumFilters *pEnum = NULL;
    IBaseFilter *pFilter = NULL;
    IDMOWrapperFilter *pWrapper = NULL;

    HRESULT hr = pGraph->EnumFilters(&pEnum);
    if (SUCCEEDED(hr))
    {
        while(pEnum->Next(1, &pFilter, NULL) == S_OK)
        {
            // If we find the IDMOWrapperFilter interface, that means we 
            // have the DMO Wrapper filter. 
            hr = pFilter->QueryInterface(IID_IDMOWrapperFilter, (void**) &pWrapper);
            if (SUCCEEDED(hr))
            {
                bFound = TRUE;
                break;
            }
            SAFE_RELEASE(pFilter);
        }
    }

    if (bFound)
    {
        *ppFilter = pFilter;
        (*ppFilter)->AddRef();
    }
    else
    {
        hr = E_FAIL;
    }

    SAFE_RELEASE(pEnum);
    SAFE_RELEASE(pFilter);
    SAFE_RELEASE(pWrapper);
    return hr;
}

HRESULT GetPin(IBaseFilter *pFilter, PIN_DIRECTION PinDir, IPin** ppPin)
{
    BOOL bFound = FALSE;
    IEnumPins *pEnum = NULL;
    IPin *pPin = NULL;

    HRESULT hr = pFilter->EnumPins(&pEnum);
    if (SUCCEEDED(hr))
    {
        while(pEnum->Next(1, &pPin, 0) == S_OK)
        {
            PIN_DIRECTION PinDirThis;
            hr = pPin->QueryDirection(&PinDirThis);
            if (FAILED(hr))
            {
                break;
            }
            if (PinDir == PinDirThis)
            {
                bFound = TRUE;
                break;
            }
            SAFE_RELEASE(pPin);
        }
    }

    if (bFound)
    {
        *ppPin = pPin;
        (*ppPin)->AddRef();
    }
    else
    {
        hr = E_FAIL;
    }

    SAFE_RELEASE(pPin);
    SAFE_RELEASE(pEnum);
    return hr;
}

HRESULT DisconnectPin(IGraphBuilder *pGraph, IPin *pPin, IPin **ppConnected)
{
    IPin *pConnected = NULL;

    HRESULT hr = pPin->ConnectedTo(&pConnected);
    if (SUCCEEDED(hr))
    {
        hr = pGraph->Disconnect(pPin);
    }
    if (SUCCEEDED(hr))
    {
        hr = pGraph->Disconnect(pConnected);
    }
    if (SUCCEEDED(hr))
    {
        *ppConnected = pConnected;
        (*ppConnected)->AddRef();
    }

    SAFE_RELEASE(pConnected);
    return hr;
}

See Also

• Reading ASF Files in DirectShow