根據 Microsoft 的文件, Digital Audio Protection 可以分為在 HDMI 或是 SPDIF 兩種情況. 如果 audio 走 HDMI, 那麼應該以 HDCP 保護. 如果 audio 走 SPDIF, 應該以 SCMS 保護. SCMS 的全名是 Serial Copy Management System, 討厭它的人以諧音稱之為 SCUMS (廢物, 人渣).
那麼 SPDIF 要怎麼保護 SCMS 呢？SPDIF 有 192 bits 的 channel status, 其中 bit 2 就是 SCMS 了.
Byte 0
Bit 0: 0: S/PDIF
1: AES/EBU (ignored by consumer equipment)
Bit 1: 0: Digital Audio
1: Non-audio (ignored by consumer audio equipment)
Bit 2: 0: SCMS enabled (default state)
1: SCMS disabled (this allows unlimited generation digital copying)
…. 等等！SCMS 不是應該有 2 bits 嗎？根據 WIKI:
SCMS copy protection looks for bits written in the subcode data in a digital link. There are three states of these bits: copy allowed (00), copy once (11) and copy prohibited (10). If the source has the copy bits 00, and a copy is made of the source, the copy itself will have the bit set as 00 as well, allowing copies to be made from copies. If the source has the copy bits set as 11, every copy of this material will have the bits set to 10, and making a copy from the copy would be prohibited.
對於 SPDIF 輸出端, 只要做好 SCMS = 0 還是挺容易的. 不過這樣還是不夠, 如果 bit 2 是 0, 還要再看 bit 15.
| L bit 不能 record 時的值 | Note | |
| DVD-R and DVR-RW | 1 | |
| CD-R, CD-RW, and DVD | 0 | |
| CD-DA 也就是俗稱的 CD | 未定義, 要看 bit 2 | bit 2 以 4-10 Hz 交替 0 與 1 |
完整的 2 bits 表示法, 會出現在 DAT 或 MiniDisc 的播放或錄製裝置上, 而不是定義在介面.
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