CGMS-A 的全名是 Copy Generation Management System Analogue, 它用來保護類比電視的輸出, 避免它被錄影機錄下來.
CGMS-A 的訊號寫在類比電視的 vertical blanking interval (VBI) 當中, 不同的 TV system 會把 CGMA-A 放在不同的掃描行.
| 規格名稱 | 電視系統 | 行號 |
| IEC 61880 | 525i and 480i | 20 |
| IEC 61880-2 and in EIA-J | 520p | 41 |
| EIA/CEA-608-B | 525i and 480i | 21 |
| EIA/CEA-805 | 480p | 41 |
| 720p | 24 | |
| 1080i | 19 |
在這些不同的規格當中, CGMS-A 的 2 bits 應該還是通用的, 只是位置不同而已.
| 0 | 0 | CopyFreely | Unlimited copies may be made of the content. |
| 0 | 1 | CopyNoMore* | One generation of copies has already been made; no further copying is allowed. |
| 1 | 0 | CopyOnce | One generation of copies may be made |
| 1 | 1 | CopyNever | No copies may be made of the content. |
* – CopyNoMore was not a part of the original standard. The 0,1 value originally was "Reserved".
在 EIA/CEA-805 當中的保護方式叫做: Copy Generation Management System Analog plus Redistribution Control (CGMS-A+RC). 我猜想會有這個規範是針對 "網路 DVR" 業者而來, 因為美國法院判決, 把自家電視的內容錄到網路上暫存, 供自己日後欣賞是不侵犯著作權的. 既然給自己看是合法的, RC 就要確保從這台錄下來的東西, 即使寫到數位存儲媒體中, 還是要拿回本機才能播放. 換言之, RC bit 用來啟動做暗記的演算法.
EIA/CEA-805 又分為 type A 和 type B 兩種規格, packet 有點不同, 但可以互相轉換:
Type A packets: allows North America CGMS-A + RC every field/frame (不被認同?).
Type B packets: allows CGMS-A + RC everyfield/frame.
[reference]
http://en.allexperts.com/e/c/cg/cgms-a.htm
http://www.patentstorm.us/patents/7440677/description.html
Cash Chou's Blog 