Decklink SDK Scheduled Audio Playback

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Steffen Pätzold

  • Posts: 6
  • Joined: Mon Mar 30, 2015 10:35 am

Decklink SDK Scheduled Audio Playback

PostTue Jun 13, 2017 6:48 am

We are using the Decklink SDK functions ScheduleVideoFrame and ScheduleAudioSamples for Playback.

Scheduling Audio appropriate to Video is no problem when using Framrates of 25/50. The number of Samples is always equal for each frame. ( e.g. 25fps -> 1 frame = 1920 samples )

With a framerate of 29.97/59.94 it is quite impossible to schedule audio exactly to the timestap of a frame.
Currently we shedule one sample more after n frames to achieve 48000 samples per second.

Questions:
Does the driver automatically detects this uneven scheduling of audio and produces correct playback?
Is there way to request how many samples must be scheduled at a certain time to cover a defined timespan?
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Nicholas Gill

Blackmagic Design

  • Posts: 169
  • Joined: Mon May 04, 2015 10:28 pm

Re: Decklink SDK Scheduled Audio Playback

PostWed Jul 12, 2017 3:43 am

Hi Steffen,

When scheduling audio for fractional frame rates, applications will often follow the SMPTE audio cadences, e.g. for bmdModeHD1080p2997 at bmdAudioSampleRate48kHz this is defined as 8008 samples in 5 frames, distributed as 1602, 1601, 1602, 1601, 1602.

For the DeckLink API, as long as the required number of audio samples have been scheduled for the frame being prepared for output, playback will be correct, regardless of how the application scheduled those samples (e.g. 1600,2 / 801,801 etc.)

Cheers,

-nick

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