Capture Interruptions and Failures over FireWire

Symptom #1: "Unable to Lock Deck Servo" Message During Capture Attempt

"I keep trying to capture a clip, but I constantly get an 'Unable to Lock Deck Servo' message."

Background

The safest way to edit video is always to use timecode for capture. If you capture by logging with timecode, you can easily recapture your clips and sequences from tape if anything ever happens to your scratch disk volumes. But by its very nature, timecode places a particular demand on your deck or camera with regard to playback speed.

All decks and cameras that use timecode need the tape's playback speed to be exact, so they use precision motors called servo motors. When the motors turning the deck transport reach the precise speed required for accurate timecode, they lock the deck servo, guaranteeing that the speed will remain constant until the capture is complete and the deck stops.

To ensure that the deck has enough time to get the tape running at the right speed before a capture starts, FCP uses a preroll setting, defined in the Device Control preset of your Audio/Video Settings. When a capture is initiated, the deck rolls back the number of seconds and frames specified in the preroll setting and starts playing. By the time it gets to the first frame you want to capture, the deck is up to speed, and the servo is locked.

You can get the "Unable to Lock Deck Servo" message if you set an In point for capture that is too close to the first frame of video on the tape. For example, if your preroll setting was for 3 seconds, and you set an In point for capture at the timecode value 00:00:00:02, FCP could not back the tape up 3 seconds to include 3 seconds of preroll and would likely generate a servo alert.

Solution

Always mark your In points after the first frame that could be captured, given your current Device Control preset preroll setting. If your preroll setting is 3 seconds, you can't set an In point on any frame earlier than 00:00:03:01. Always shoot at least 10 seconds of footage (referred to as head) on your tapes before you begin recording actual useable footage.

The only other way you can encounter this message is if your camera or deck really does need more preroll time than you are giving it in the preroll setting. Some prosumer DV decks and cameras take a little longer to get up to speed and lock the servo. This is easy to correct. You simply need to create a new Device Control preset with a longer preroll setting.

Symptom #2: Interrupted Captures and Spotty Performance When Using RAID-0 Volumes of FireWire 400 or 800 Drives

"I built a four-drive RAID-0 of matched FireWire 400 or 800 drives. It should be fast enough as far as data rates are concerned. Why am I still getting interrupted captures and spotty performance?"

Background

If you're using FireWire 400 and 800 drives, you're likely to run into conflicts and bottlenecks on the FireWire bus. As mentioned earlier, sharing a bus between a FireWire drive and FireWire deck or camera isn't always reliable, and with higher-resolution formats like DVCPRO HD, even more headroom is necessary.

Solution

As with the AJA I/O uncompressed converter, when capturing DVCPRO HD through FireWire, the AJ HD1200A needs to be the only item on the FireWire bus. Adding a FireWire 400 or 800 PCI card will separate the FireWire buses and give you greater throughput. Data streaming through FireWire from the deck won't collide or interfere with data streaming from the processor to the FireWire scratch disk.

You can also choose to switch to non-FireWire drives, such as internal SATA drives, or a high-speed RAID, such as an Apple Xserve Raid.