showstack / glossary / biphase-mark
Biphase mark encoding 雙相符號編碼
The line code LTC uses. There is a transition at every bit boundary, and a one gets one extra transition in the middle of the bit. That gives a self-clocking, DC-free, polarity-independent signal that decodes over a wide range of speeds — which is what lets timecode survive an audio track and be read while shuttling backwards.
中文
LTC 使用的線路編碼。每個位元邊界都有一次跳變，代表 1 的位元在中間再多一次跳變。這產生一個自帶時脈、無直流成分、與極性無關的訊號，並可在很寬的速度範圍內解碼 —— 這正是時碼能存活在音軌上、並能在倒帶時被讀取的原因。
Regional usage
| Where | Term | Note |
|---|---|---|
| UK / US | Biphase mark / BMC / FM encoding | |
| HK / TW | 雙相符號編碼 |
Easy to get wrong
The encoding is polarity-independent, so a swapped pair on a timecode feed has no symptom at all. A wiring fault that would break anything else stays hidden here.
Next to this
Drop frame丟幀時碼A timecode counting scheme that skips frame numbers 00 and 01 at the start of every minute except every tenth…Frame rate影格率How many complete images a video signal carries per second. On a show it is a system-wide decision, not a per…
Where it shows up on the wire
Sources
- SMPTE ST 12-1 Time and Control Code — SMPTE primary
Understand the mechanism
Something wrong, or missing?
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