AudioConverterPrimeInfo |
Specifies priming information.
struct AudioConverterPrimeInfo { UInt32 leadingFrames; UInt32 trailingFrames; };
leadingFrames
- Specifies the number of leading (previous) input frames, relative to the normal/desired start input frame, required by the converter to perform a high quality conversion. If using kConverterPrimeMethod_Pre, the client should "pre-seek" the input stream provided through the input proc by leadingFrames. If no frames are available previous to the desired input start frame (because, for example, the desired start frame is at the very beginning of available audio), then provide "leadingFrames" worth of initial zero frames in the input proc. Do not "pre-seek" in the default case of kConverterPrimeMethod_Normal or when using kConverterPrimeMethod_None.
trailingFrames
- Specifies the number of trailing input frames (past the normal/expected end input frame) required by the converter to perform a high quality conversion. The client should be prepared to provide this number of additional input frames except when using kConverterPrimeMethod_None. If no more frames of input are available in the input stream (because, for example, the desired end frame is at the end of an audio file), then zero (silent) trailing frames will be synthesized for the client.
When using AudioConverterFillBuffer() (either a single call or a series of calls), some
conversions, particularly involving sample-rate conversion, ideally require a certain
number of input frames previous to the normal start input frame and beyond the end of
the last expected input frame in order to yield high-quality results.
These are expressed in the leadingFrames and trailingFrames members of the structure.
The very first call to AudioConverterFillBuffer(), or first call after
AudioConverterReset(), will request additional input frames beyond those normally
expected in the input proc callback to fulfill this first AudioConverterFillBuffer()
request. The number of additional frames requested, depending on the prime method, will
be approximately:
kConverterPrimeMethod_Pre leadingFrames + trailingFrames kConverterPrimeMethod_Normal trailingFrames kConverterPrimeMethod_None 0
(Last Updated July 18, 2005)