Keywords: GNU, Linux, DV, video, editor, non-linear, capture, Firewire, IEEE 1394

2.16. Locked Audio
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## 2.16. Locked Audio

Audio in DV is either locked or unlocked. Each video frame of DV contains the audio that accompanies that frame. For 48KHz PAL, 48000 samples/second divided by 25 frames/second equals 1920 samples/frame. If the audio is locked, each frame has exactly 1920 stereo samples. If the audio is unlocked, it can vary about the ideal sample count from one frame to the next--some frames will have a little more and some a little less. Over a fair amount of time, even when unlocked, the total number of samples should be very close to the ideal sample count. However, some devices are not only unlocked but downright sloppy--maybe due to an inaccurate oscillator--that produces an incorrect amount.

Due to the frame rate of NTSC, there is not an integer number of samples per frame: 48000 / ( 30000 / 1001 ) = 1601.6. Therefore, locked audio on NTSC follows a repeating sequence of 1600 and 1602 samples. Like unlocked audio, over a fair amount of time, the total samples is very close to the ideal.

When working with unlocked audio or NTSC clips, these inaccuracies accumulate during editing and can introduce an audio/video synchronization problem when exported to a non-DV device or format. This happens because often the transcoding tools are reading separate uncompressed audio and video streams with no timestamps.

When Kino exports a movie (unless you disable a resampling option, where available) it generates locked audio. It compares the total, actual samples to the expected, ideal amount and calculates an audio frequency adjustment. During this initial stage of export, Kino displays "Locking audio" in the status bar. Then, it resamples the audio to this new frequency as it performs the actual export.

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