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https://www.bhphotovideo.com/explora/4k/4k-video-signal-guide

Quad-Link HD-SDI

Quad-Link HD-SDI transmits 4K as four separate 2K or 1080p signals. HD-SDI is a SMPTE standardized signal for 2K and 1080p video distribution. This makes Quad-Link HD-SDI a reliable, though cumbersome, way to distribute 4K video. You will need four separate 75 ohm BNC coax cables that support at least 1.5 Gb/s of bandwidth for Quad-Link HD-SDI. Quad-Link HD-SDI supports both DCI 4K and UHD 4K. The maximum quality supported by Quad-Link HD-SDI is 10-bit 4:2:2 at 30p. HD-SDI is a proven and extremely reliable signal for HD video, which is probably why Quad-Link HD-SDI is the most common way of distributing 4K on professional broadcast equipment.

Quad-Link HD-SDI以四個獨立2K或是1080p的訊號傳輸4K。HD-SDI是SMPTE標準2K與1080p影像訊號的傳輸方式，因此Quad-Link HD-SDI是可靠的，卻稍微無法扛起散播4K影像的任務。你依舊需要四條獨立的75歐姆的BNC同軸電纜，每條可在Quad-Link SDI支援至少1.5Gb/s頻寬。Quad-Link HD-SDI可支援DCI 4K與UHD 4K。Quad-Link HD-SDI最高支援到10 bit 4:2:2 30p。HD-SDI已經過認證，傳輸HD影像訊號非常穩定，這也許是為什麼Quad-Link HD-SDI在專業廣播設備裡，是最普遍傳播4K的方式

Quad-Link HD-SDI can be converted to the following signals:
Quad-Link HD-SDI可被轉換為以下訊號：

採用Blackmagic Mini Converter SDI to HDMI或AJA Hi5-4K 轉成HDMI 1.4
採用Blackmaigc Multiplex 4K轉成Dual-Link 3G-SDI
採用Blackmaigc Multiplex 4K轉成6G-SDI

•    HDMI 1.4 with either the Blackmagic Miniconverter SDI to HDMI 4K or the AJA Hi5-4K
•    Dual-Link 3G-SDI with the Blackmagic Multiplex 4K
•    6G-SDI with the Blackmagic Multiplex 4K

Quad-Link 3G-SDI

Quad-Link 3G-SDI is very similar to Quad-Link HD-SDI but, as the name implies, uses four 3G-SDI signals instead of HD-SDI. The extra bandwidth of 3G-SDI allows Quad-Link 3G-SDI to transmit 10-bit 4:2:2 4K at double the frame rate (up to 60p) or to transmit up to 12-bit 4:4:4 at the same frame rate (up to 30p). You will need four separate 75 ohm BNC coax cables that support at least 3 Gb/s of bandwidth for Quad-Link 3G-SDI. Quad-Link 3G-SDI is the most useful for post production where the extra color information in a 4:4:4 signal can be utilized. Currently no 4K cameras output a 4:4:4 signal, but some capture cards do. Because Quad-Link 3G-SDI exceeds the bandwidth of all other 4K signals currently on the market there are no ways to convert a Quad-Link 3G-SDI signal into anything else. However any device capable of Quad-Link 3G-SDI should also do Quad-Link HD-SDI.

Quad-Link 3G-SDI跟Quad-Link HD-SDI是非常相似的，但是就如同名稱所暗示的，使用四鏈路3G-SDI訊號替代了HD-SDI。3G-SDI多出來的頻寬讓Quad-Link 3G-SDI可以傳輸10 bit 4:2:2 4K的影像又多出一倍的格數(frame rate)，直達60格；又或是轉換到相同格數(最高到30格)12bit 4:4:4的影像格式。你還需要四條獨立75歐姆BNC同軸電纜線材，用來傳輸至少每秒3Gb頻寬，達成Quad-Link 3G-SDI。Quad-Link 3G-SDI是後製最有用的。

6G-SDI from Blackmagic

Blackmagic的6G-SDI

Blackmagic Design seemingly got tired of waiting for SMPTE to finalize a 4K signal and went ahead and released their own. Blackmagic Design calls their 4K signal 6G-SDI, but it is important to note that 6G-SDI is not a standard.  When or if SMPTE standardizes a 4K signal they may or may not decide to call it 6G-SDI. And there is chance whatever SMPTE 6G-SDI ends up being it will not be compatible with Blackmagic's iteration of it.

Blackmagic Design似乎厭倦了等待SMPTE宣告4K訊號，逕自發表了屬於他們的4K。Blackmagic Design宣稱他們的4K訊號為6G-SDI，但重點是6G-SDI並不是標準。當如果SMPTE設定4K為一標準訊號格式，他們可能會，也可能不會決定稱它為6G-SDI。有機會的時候，不論SMPTE 6G-SDI壽命盡了，6G-SDI就不會再跟Blackmagic Design的器材相容。

With that disclaimer out of the way it’s time to move on to the good news. 6G-SDI uses just one BNC cable to send 10 Bit 4:2:2 4K at 30p or 24p. You will need one BNC cable that supports 6 Gb/s of bandwidth for 6G-SDI. Finding a compatible cable can be tough because cables are not advertised as supporting 6G-SDI since it’s not a standard. Blackmagic says most high-quality 3G-SDI cables should work over short distances. But as of yet we haven't been able to test this. Currently only Blackmagic products utilize this signal, but Blackmagic Design has announced a number of converter boxes to convert most other 4K signals to and from 6G-SDI.

6G-SDI can be converted to the following signals:

HDMI 1.4 with the Blackmagic MiniConverter SDI to HDMI 4K
Dual-Link 3G-SDI with the Blackmagic Multiplex 4K
Quad-Link HD-SDI with the Blackmagic Multiplex 4K