SDI
Uncompressed broadcast video on one coax cable: 10-bit words and sync patterns.
One line of video
EAV, blanking, SAV, then pixels: every row of the grid is one piece of the line.
Whole frame
SDI sends 10-bit words, not bytes: EAV, blanking, SAV, then Cb Y Cr Y video. The sync words 3FF 000 000 mark the boundaries, and there is no header, no address, no checksum in SD, just a continuous stream.
| Field | Offset | Example value | Meaning |
|---|---|---|---|
| EAV preamble | Word 0–2 | 3FF 000 000 | The sync pattern 3FF 000 000. These values are reserved: in video data, words 000–003 and 3FC–3FF are forbidden, so this pattern cannot appear by accident, and a receiver can lock onto line boundaries anywhere in the stream. |
| EAV: XYZ | Word 3 | 0x274 = EAV, field 1, active line | The fourth TRS word packs three flags (F, V, H) and four protection bits that let a receiver correct a single flipped bit among them. H = 1 marks the End of Active Video. Common values: SAV 0x200 / EAV 0x274 for active lines of field 1. |
| Horizontal blanking | Word 4–7 | Cb=512 Y=64 Cr=512 Y=64 | The gap between the end of one line and the start of the next. Here it holds blanking levels, in the same Cb Y Cr Y order as video: chroma 0x200 (512 = neutral) and luma 0x040 (64 = black). In real signals this space also carries ancillary data: embedded audio (up to 16 channels), timecode, captions, and more. |
| SAV preamble | Word 8–10 | 3FF 000 000 | The sync pattern 3FF 000 000. These values are reserved: in video data, words 000–003 and 3FC–3FF are forbidden, so this pattern cannot appear by accident, and a receiver can lock onto line boundaries anywhere in the stream. |
| SAV: XYZ | Word 11 | 0x200 = SAV, field 1, active line | The fourth TRS word packs three flags (F, V, H) and four protection bits that let a receiver correct a single flipped bit among them. H = 0 marks the Start of Active Video. Common values: SAV 0x200 / EAV 0x274 for active lines of field 1. |
| Pixels 0–1: white | Word 12–15 | Cb=512 Y=940 Cr=512 Y=940 | Active video is multiplexed as Cb Y Cr Y: one Cb and one Cr sample are shared by two neighbouring pixels (4:2:2 sampling), each pixel has its own Y. Y = 940 (0x3AC) is video white; the 10-bit range 64–940 is 'legal' video, leaving headroom above and below. Cb = Cr = 512 means no colour. |
| Pixels 2–3: black | Word 16–19 | Cb=512 Y=64 Cr=512 Y=64 | The next pair of pixels: Y = 64 (0x040) is black. A full 625-line line has 720 pixels = 1,440 active words, then EAV again. |
Overview
Serial Digital Interface is the broadcast industry's workhorse: uncompressed digital video (and embedded audio) over a 75-ohm coaxial cable with BNC connectors. It is one-way and point-to-point; the receiver locks onto the stream and finds the picture by sync words, with no handshake and no negotiation.
Data is organised in 10-bit words. SD-SDI (SMPTE 259M) sends one stream of Cb Y Cr Y words at 270 Mbit/s. HD-SDI (SMPTE 292M) sends two interleaved streams, luma and chroma, at 1.485 Gbit/s in total, and adds a line number and a CRC after each EAV. 3G, 6G and 12G-SDI double the rate again for 1080p60, 4K30 and 4K60.
Key facts
- Medium
- 75 Ω coax, BNC
- Word
- 10 bits, 4:2:2 YCbCr
- Rates
- SD 270M · HD 1.485G · 3G 2.97G · 6G · 12G
- Sync
- TRS: 3FF 000 000 XYZ
- Direction
- One way, no handshake
- Standards
- SMPTE 259M / 292M / 424M / ST 2081 / ST 2082
Words become bits
The four sync words of EAV as a serial waveform, least significant bit first.
Whole frame
Forty bits, 148 nanoseconds at SD speed (270 Mbit/s, 3.7 ns per bit). Shown unscrambled for clarity: the real signal is first scrambled with x^9 + x^4 + 1 and NRZI-coded, so it looks like random noise with a flat spectrum, and is carried at 800 mV on 75 ohm coax.
Word 0: 3FF
Ten ones. Like a UART, the least significant bit goes out first; here every bit is 1 so the line stays high for 10 bit times.
Word 1: 000
Ten zeros: the line stays low for 10 bit times. 3FF followed by 000 is the longest run of identical bits the formatter ever creates, so the real signal is scrambled before transmission.
Word 2: 000
Another ten zeros.
Word 3: XYZ 274
XYZ: bit 9 (sent last) is 1, then F V H, then the protection bits; the two lowest bits (sent first) are 0.
Where you meet it
- Broadcast studios, OB vans and master control rooms
- Cinema cameras, live-event switchers and LED wall processors
- IP formats such as SMPTE ST 2110 and NDI are replacing SDI in new studios, but SDI still wins on latency and simplicity
Watch out for
- Cable length depends on rate: roughly 300 m for SD, 100 m for HD, 70–100 m for 3G, and far less at 12G. Use proper 75 Ω cable and reclock for long runs.
- There is no negotiation: both ends must be set to the same format (1080i50 vs 1080p60). A mismatch shows a black screen, not an error.
- A 3G-SDI source can carry 'level A' or 'level B' mapping of the same picture: receivers that expect the other one lose lock.
- Not HDMI: SDI has no EDID, no HDCP, no audio negotiation; it is a plain signal.
Standards
- SMPTE ST 259 (SD-SDI)
- SMPTE ST 292-1 (HD-SDI)
- SMPTE ST 424 (3G-SDI)
- ITU-R BT.656 / BT.601