IAudioClient::GetDevicePeriod

IAudioClient::GetDevicePeriod

The GetDevicePeriod method retrieves the length of the periodic interval separating successive processing passes by the audio engine on the data in the endpoint buffer.

HRESULT GetDevicePeriod(
  REFERENCE_TIME  *phnsDefaultDevicePeriod,
  REFERENCE_TIME  *phnsMinimumDevicePeriod
);

Parameters

phnsDefaultDevicePeriod

[out]  Pointer to a REFERENCE_TIME variable into which the method writes a time value specifying the default interval between periodic processing passes by the audio engine. The time is expressed in 100-nanosecond units. For information about REFERENCE_TIME, see the Windows SDK documentation.

phnsMinimumDevicePeriod

[out]  Pointer to a REFERENCE_TIME variable into which the method writes a time value specifying the minimum interval between periodic processing passes by the audio endpoint device. The time is expressed in 100-nanosecond units.

Return Value

If the method succeeds, it returns S_OK. If it fails, possible return codes include, but are not limited to, the values shown in the following table.

Remarks

The client can call this method before calling the IAudioClient::Initialize method.

The phnsDefaultDevicePeriod parameter specifies the default scheduling period for a shared-mode stream. The phnsMinimumDevicePeriod parameter specifies the minimum scheduling period for an exclusive-mode stream.

At least one of the two parameters, phnsDefaultDevicePeriod and phnsMinimumDevicePeriod, must be non-NULL or the method returns immediately with error code E_POINTER. If both parameters are non-NULL, then the method outputs both the default and minimum periods.

For a shared-mode stream, the audio engine periodically processes the data in the endpoint buffer, which the engine shares with the client application. The engine schedules itself to perform these processing passes at regular intervals.

The period between processing passes by the audio engine is fixed for a particular audio endpoint device and represents the smallest processing quantum for the audio engine. This period plus the stream latency between the buffer and endpoint device represents the minimum possible latency that an audio application can achieve.

The client has the option of scheduling its periodic processing thread to run at the same time interval as the audio engine. In this way, the client can achieve the smallest possible latency for a shared-mode stream. However, in an application for which latency is less important, the client can reduce the process-switching overhead on the CPU by scheduling its processing passes to occur less frequently. In this case, the endpoint buffer must be proportionally larger to compensate for the longer period between processing passes.

The client determines the buffer size during its call to the IAudioClient::Initialize method. For a shared-mode stream, if the client passes this method an hnsBufferDuration parameter value of 0, the method assumes that the periods for the client and audio engine are guaranteed to be equal, and the method will allocate a buffer small enough to achieve the minimum possible latency. (In fact, any hnsBufferDuration value between 0 and the sum of the audio engine's period and device latency will have the same result.) Similarly, for an exclusive-mode stream, if the client sets hnsBufferDuration to 0, the method assumes that the period of the client is set to the minimum period of the audio endpoint device, and the method will allocate a buffer small enough to achieve the minimum possible latency.

If the client chooses to run its periodic processing thread less often, at the cost of increased latency, it can do so as long as it creates an endpoint buffer during the IAudioClient::Initialize call that is sufficiently large.

Requirements

Client: Windows Vista

Header: Include Audioclient.h.

See Also

• IAudioClient Interface
• IAudioClient::Initialize