Setting the Graph Clock

When you build a filter graph, the Filter Graph Manager automatically chooses a reference clock for the graph. All filters in the graph are synchronized to the reference clock. In particular, renderer filters use the reference clock to determine the presentation time of each sample.

There is usually no reason for an application to override the Filter Graph Manager's choice of reference clock. However, you can do so by calling the IMediaFilter::SetSyncSource method on the Filter Graph Manager. This method takes a pointer to the clock's IReferenceClock interface. Call the method while the graph is stopped.

If a filter provides a clock, you can get the IReferenceClock pointer by calling QueryInterface on the filter. Alternatively, you can implement an external reference clock that is not provided by a filter, as long as your external clock implements IReferenceClock. The following example shows how to specify a clock:

IGraphBuilder *pGraph = 0;
IReferenceClock *pClock = 0;

CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, 
    IID_IGraphBuilder, (void **)&pGraph);

// Build the graph.
pGraph->RenderFile(L"C:\\Example.avi", 0);

// Create your clock.
hr = CreateMyPrivateClock(&pClock);
if (SUCCEEDED(hr))
{
    // Set the graph clock.
    IMediaFilter *pMediaFilter = 0;
    pGraph->QueryInterface(IID_IMediaFilter, (void**)&pMediaFilter);
    pMediaFilter->SetSyncSource(pClock);
    pClock->Release();
    pMediaFilter->Release();
}

This example assumes that CreateMyPrivateClock is an application-defined function that creates a clock and returns an IReferenceClock pointer.

You can also set the filter graph to run with no clock, by calling SetSyncSource with the value NULL. If there is no clock, the graph runs as quickly as possible. With no clock, renderer filters do not wait for a sample's presentation time. Instead, they render each sample as soon as it arrives. Setting the graph to run without a clock is useful if you want to process data quickly, rather than previewing it in real time.

Basic DirectShow Tasks

CBaseReferenceClock Class

Time and Clocks in DirectShow