Specifies a time stamp as one of the SMPTE time types.
- iOS 2.0+
- macOS 10.0+
- tvOS 9.0+
- watchOS 3.0+
- Core Audio
SMPTE (Society of Motion Picture and Television Engineers, pronounced “SIMPtee”) times are used to correlate a point in an audio stream with an external event. For example, a SMPTE time can be used to correlate a sound in an audio file with a video frame in a movie file.
Note that the frames referred to by this structure are video frames, where a video frame is a single complete image. (Compare with the definition of audio frames in the discussion for
A complete SMPTE time description takes 80 bits, including 32 user bits that contain vendor-specific information. The actual time-code portion of the SMPTE time description is normally sent in several messages, each message containing a portion of the time code. (The user bits are sent in a separate message.) Typically, the SMPTE time description is divided up into 8 1-byte messages, with the first nibble of each message specifying which portion of the time code is contained in the message and the second nibble containing the time information. Four such messages are normally sent with each video frame.
Video data contains somewhere from 24 to 60 frames per second (as specified by the SMPTE time type—see
SMPTE Timecode Type Constants) and each video frame has an associated SMPTE time. SMPTE time is based on a 24-hour clock. Each frame’s SMPTE time consists of an hour, minute, and second value, plus the number of the frame within the second. Because audio data is sampled at a much higher rate (MP3 data is sampled at over 100,000 bits per second, for example), it is frequently desirable to correlate the audio data with a time within the persistence period of a single video frame. For this reason, the time period during which a single video frame is displayed is subdivided into subframes (typically 80 or 100 subframes per frame, as specified by the
m field). The
m field specifies the number of subframes into the video frame represented by this time structure.