Audio Buffer Management (+S Option Only) - evertz 7700 Manual

7700 MultiFrame Manual
7736CD2 Dual Composite Decoder
6.

AUDIO BUFFER MANAGEMENT (+S OPTION ONLY)

The audio is delayed, through a large memory buffer, by the same time value as experienced by the video
through the video frame synchronizer. This is done to maintain a consistent video/audio relationship.
Unfortunately, when the video frame buffer drops or repeats frames of video, we cannot do the same with
the audio. The rate of the ingoing audio data is changed and over a period of time, the buffer is returned to
a value that matches the video. TA sample rate converter is used to change the rate of the incoming AES
on the -AES version. The 48kHz sampling clock is changed slightly from its nominal value on the analog
audio input (-A4 or –A8) version. This method is also used when video is first applied or there is a sudden
change in either the input video phase or genlock video phase.
When the audio buffer delay needs to be changed, there are two rates used; +/- 1% and +/- 30ppm.
Initially, a 1% rate change is used to quickly match the audio delay to the video delay. This correction will
only take a couple of seconds but will produce a pitch change. The pitch change will be discernable,
particularly on sustained musical notes. When the video/audio delays are close to each other and only
small corrections are required, the audio buffer will be adjusted with a rate change of 30ppm (30 Parts-
Per-Million is equivalent to 0.003%). This rate change will produce a pitch change that is near the
threshold of where a human can detect it and should not be discernable with normal content.
Unfortunately, the audio buffer management relies on a sample rate converter (-AES versions only) to
adjust the audio buffer size. This precludes the synchronization of non-PCM AES data (i.e. Dolby-E™). In
the future, a non-sample rate converter mode may be available.
Audio A to D converters and sample rate converters have inherent delays that are much longer than what
the minimum acceptable video I/O delay is. To allow the user to operate the video synchronizer with small
video delays, we will hold the audio buffer delay at its minimum value when the video is less than this
minimum.
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