showstack / glossary / jam-sync
Jam sync 同步鎖定
Matching a timecode generator to incoming code and then continuing to generate in step. Continuous jam keeps correcting to the incoming code for as long as it is present; one-time jam takes the value once when armed and then free-runs on its own crystal. One-time jam is what a camera needs to keep sensible code after being unplugged, and it is also how a device silently stops following the master while still displaying plausible timecode.
中文
將時碼產生器對準輸入時碼後，繼續同步產生時碼。連續鎖定會在輸入時碼存在期間持續修正；單次 鎖定則只在啟動當下取值一次，之後便依自身晶振自由運行。攝影機拔線後仍要輸出合理時碼時需要 單次鎖定；但這也是裝置在畫面仍顯示合理時碼的情況下、悄悄停止跟隨主時鐘並開始漂移的原因。
Regional usage
| Where | Term | Note |
|---|---|---|
| UK / US | Jam sync | |
| HK / TW | Jam sync | Said in English; 同步鎖定 is the written form. |
Easy to get wrong
Freewheel is not jam sync. Freewheel is how long a reader keeps counting after code disappears; jam sync is a generator that took the time and now produces its own.
Next to this
Where it shows up on the wire
LTCSMPTE time and control code encoded as a biphase-mark audio signal, 80 bits per frame, so it runs down any au…MIDI TimecodeCarries SMPTE-style hours, minutes, seconds and frames over a MIDI connection, as quarter-frame messages whil…
Sources
- SMPTE ST 12-1 time and control code — Society of Motion Picture and Television Engineers primary
Understand the mechanism
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