Recovering Audio
Recovering Audio
The most typical use of the synchronizing function would be to pass capture a video frame by the means that you normally would, then to make a call to:
This function is however deceptively powerful and the key to understanding this is correctly passing the correct values into the NDIlib_audio_frame_v3_t*
p_audio_frame
parameter of this function. The following describes the values that may be specified.
Parameter | Description |
---|---|
| If this value is “0” when you pass it into the function, then if audio may be recovered then the synchronization function will automatically return the exact length of audio that matches the video frame that was passed in as the first parameter. The number of samples returned in this way will almost exactly match the values related to the timestamps but can be assumed to very accurately reflect the audio that matches the frame. Because the timestamps are often subject to noise when frames are stamped, the number of samples might vary slightly. This will result in a return code of If this value is some constant (e.g., 1601 or 1602) then the AV sync will attempt to return this number of samples if it is close to the true number of audio samples that are aligned with this frame. This is designed so that you let this class correctly recover audio that might follow some external constraint on the number of samples that are used with video frames. If the number of samples requested is sufficiently close to the number of audio samples that match this frame, then a return code of When requesting a specific number of audio samples, this is normally computed externally to this function under the assumption of some known audio sample rate. Because incoming audio might change sample rates which would render the number of requested samples invalid, please review the section below on how specifying the |
| When this function is “0”, then there is no assumed sample rate, and the function will return an audio frame that specifies the current audio sample rate. If you are specifying the number of samples to be captured as non-zero, it is likely that this was computed at a given audio sample-rate. If you specify this on the |
If the NDIlib_video_frame_v2_t
*p_video_frame
is not specified (is NULL
) then the audio parameter can be used either to capture all current audio (no_samples=0
) or a specified number of audio samples (no_samples
is not zero) and behaves exactly as specified above although all audio is handled and not simply the audio associated with the current video frame.
Please note that this function will fill all parameters of the return frame, including the timecode, timestamp, and metadata. The metadata is chosen from the closest matching audio frame and is returned just one time. If the audio frames are all much smaller in duration than the corresponding video frames, then some metadata fields might be missing since they no longer have corresponding audio data to be assigned too.
It is important that you call NDIlib_avsync_free_audio
in order to return the frames returned by NDIlib_avsync_syncronize
.
The full set of return codes from this function are documented below. Please note that these have integer values which are positive for success and negative for failure.
Error code | Meaning |
---|---|
| This function succeeded and returned audio that matches the frame, and if you specified |
* | This function succeeded, but you specified a |
| This indicates that there is currently no audio stream and so it would not be possible to return any audio data. This might indicate a video only stream. |
| This indicates that this video frame did not have matching audio data. This might be because the time at which this video frame was sent there was no corresponding audio. It might also indicate that the sending of the audio and video streams is sufficiently unaligned that they cannot easily be resynchronized; for instance, the audio data arriving is more than a second out of sync with the corresponding video data. |
| A |
| This function was called with incorrect |
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