Final Cut Pro 4: A/V Sync Issues When Capturing DV Clips Over Blank or Bad Tape
Capturing over a segment of blank or bad tape will effect the A/V sync of captured DV clips in Final Cut Pro.
Products Affected
Final Cut Pro
Users may notice issues with A/V sync capturing from DV tape using FireWire, when the tape in question has blank areas, or is degraded to the point where it cannot be read by a deck or camera. When a capture spans one of these breaks in content (e.g. recorded media / blank tape / recorded media) the sync between the audio and video following the break will generally be offset by the duration of the break.
While capturing DV media over FireWire using Final Cut Pro, QuickTime will extract the audio data from the incoming DV data stream, writing it to disk in parallel with the video data as a part of a standard QuickTime movie. This process necessitates that audio and video are processed separately once the DV data stream has reached the computer, and assumes that a consistent stream is received during capture.
When capturing from a DV tape, most FireWire/DV decks and cameras will temporarily stop sending data over FireWire when they encounter blank or unreadable tape, resuming transmission when valid data is encountered once again. This is different than capturing over a traditional video connection (composite, component, SDI, etc..), where the source device is required to output a constant video signal, generated from any data read from the source tape, or using an internal signal generator when no data is present.
During this pause in transmission for DV/FireWire devices, audio continues to be processed/generated by QuickTime, but video does not. This results in the sync between the audio and video being offset by the amount of time that the source deck or camera stopped transmitting data.
It is because of this behavior that capturing over blank or bad tape using DV/FireWire devices is not supported. To work around this situation, it is suggested that the segments between the blank/bad tape sections be captured individually. Alternatively, the bad tape in question can be dubbed to another tape using a traditional video connection (composite, component, SDI, etc...) to generate empty media for the blank or bad segments of tape.
Related Documents
While capturing DV media over FireWire using Final Cut Pro, QuickTime will extract the audio data from the incoming DV data stream, writing it to disk in parallel with the video data as a part of a standard QuickTime movie. This process necessitates that audio and video are processed separately once the DV data stream has reached the computer, and assumes that a consistent stream is received during capture.
When capturing from a DV tape, most FireWire/DV decks and cameras will temporarily stop sending data over FireWire when they encounter blank or unreadable tape, resuming transmission when valid data is encountered once again. This is different than capturing over a traditional video connection (composite, component, SDI, etc..), where the source device is required to output a constant video signal, generated from any data read from the source tape, or using an internal signal generator when no data is present.
During this pause in transmission for DV/FireWire devices, audio continues to be processed/generated by QuickTime, but video does not. This results in the sync between the audio and video being offset by the amount of time that the source deck or camera stopped transmitting data.
It is because of this behavior that capturing over blank or bad tape using DV/FireWire devices is not supported. To work around this situation, it is suggested that the segments between the blank/bad tape sections be captured individually. Alternatively, the bad tape in question can be dubbed to another tape using a traditional video connection (composite, component, SDI, etc...) to generate empty media for the blank or bad segments of tape.
Related Documents
93326 "Final Cut Pro 4: Irregular Audio Rates and A/V Sync in DV Clips"