Short answer: raw video bandwidth is active pixels × frames per second × bits per pixel. A 1080p60 camera at 10-bit 4:2:2 needs 2.49 Gbit/s, which fits 3G-SDI (2.97 Gbit/s) but not HD-SDI (1.485 Gbit/s); the same sampling at 2160p60 needs 9.95 Gbit/s and therefore 12G-SDI, or a compressed stream instead of a raw one.
Three different numbers are called bandwidth
Datasheets mix three quantities that are not interchangeable. Uncompressed payload is what the camera pipeline produces and what an SDI or HDMI link must carry. Interface line rate is the nominal connector clock, such as 11.88 Gbit/s for 12G-SDI, and it includes blanking and synchronisation overhead, so the usable payload is always lower than the headline figure. Encoded stream rate is the H.264, H.265 or NDI bitrate that a network port or USB host handles after compression; a 8 Mbit/s stream is roughly 0.3% of a 3G-SDI link and says nothing about whether that link is adequate.
Formula, variables and units
Payload bit rate: B = W × H × f × c × d.
| Variable | Meaning | Unit |
|---|---|---|
| W, H | Active pixel dimensions | pixels |
| f | Frame rate | frames/s (Hz) |
| d | Bit depth per sample | bit (8 or 10 typical) |
| c | Component factor: 3 for 4:4:4, 2 for 4:2:2, 1.5 for 4:2:0 | ratio |
| B | Payload | bit/s, ÷1 000 000 000 for Gbit/s |
Chroma subsampling lowers the payload because the two colour-difference samples are shared between neighbouring pixels; it does not change the interface format the camera outputs. Moving from 8-bit to 10-bit adds 25% at 4:2:2 (16 to 20 bit per pixel) and moving to 4:4:4 at 8-bit adds 50%.
Worked payload figures
| Format | Payload |
|---|---|
| 1080p50, 10-bit 4:2:2 | 2.07 Gbit/s |
| 1080p60, 8-bit 4:2:2 | 1.99 Gbit/s |
| 1080p60, 10-bit 4:2:2 | 2.49 Gbit/s |
| 1080p60, 8-bit 4:4:4 | 2.99 Gbit/s |
| 2160p30, 10-bit 4:2:2 | 4.98 Gbit/s |
| 2160p60, 10-bit 4:2:2 | 9.95 Gbit/s |
Two rows decide hardware selection. 1080p60 at 8-bit 4:4:4 lands at 2.99 Gbit/s, marginally above 3G-SDI's 2.97 Gbit/s, so that combination does not fit the link even though 1080p60 10-bit 4:2:2 does. UHD at 60 fps at 9.95 Gbit/s rules out 6G-SDI's 5.94 Gbit/s.
Matching payload to a wired interface
| Interface | Nominal rate | Typical single-camera ceiling |
|---|---|---|
| HD-SDI (ST 292-1) | 1.485 Gbit/s | 1080i60, 10-bit 4:2:2 |
| 3G-SDI (ST 424) | 2.97 Gbit/s (2.967 at 59.94) | 1080p60, 10-bit 4:2:2 |
| 6G-SDI (ST 2081) | 5.94 Gbit/s | 2160p30, 10-bit 4:2:2 |
| 12G-SDI (ST 2082) | 11.88 Gbit/s | 2160p60, 10-bit 4:2:2 |
| HDMI 2.0 | 18 Gbit/s total, ~14.4 Gbit/s TMDS payload | 2160p60, 4:2:2 10-bit |
| HDMI 2.1 | 48 Gbit/s | 2160p60, 4:4:4 |
| USB 3.2 Gen 1 | 5 Gbit/s shared per controller | one or two compressed 1080p60 feeds |
| 1GbE | 1 Gbit/s nominal, ~0.95 Gbit/s usable | compressed streams only |
SDI is a point-to-point link with a fixed payload per cable, while HDMI and USB budgets are shared between devices on the same controller or hub. Cable reach behaves differently as well: loss rises with frequency, so 12G-SDI runs are the shortest in the chain and should be checked against the cable maker's distance table rather than assumed from the bit rate alone.
Frame rate and shutter settings
Payload scales linearly with frame rate, and the frame rate sets the exposure you can use. With a 180-degree shutter the exposure is 1/(2 × fps): 1/120 s at 60 fps, 1/60 s at 30 fps, 1/50 s at 25 fps. In 50 Hz lighting regions, shoot 25 or 50 fps with a 1/50 or 1/100 s shutter so lamp flicker falls on whole mains cycles.
Keep the rate family consistent. 59.94 and 60 fps differ by the 1000/1001 factor, which is why 3G-SDI carries 2.967 Gbit/s for 59.94 content against 2.97 Gbit/s for 60; mixing the two across capture, recording and conferencing stages costs dropped frames or audio drift, not bandwidth headroom.
Encoded streams: bits per pixel as the check
For compressed output, invert the relation: bpp = bitrate ÷ (W × H × f). H.264 conference cameras commonly sit at 0.05–0.10 bit/pixel, and H.265 reaches comparable quality at roughly 30–50% lower bitrate. At 1080p60 (124.4 megapixels/s) that implies 6–12 Mbit/s with H.264 and about 4–8 Mbit/s with H.265; at 2160p30 (248.8 megapixels/s) about 12–25 Mbit/s and 8–17 Mbit/s respectively.
Full NDI is a different profile: NDI's bandwidth documentation quotes up to about 150 Mbit/s per 1080p60 stream because it uses intra-frame compression. Eight such cameras therefore need roughly 1.2 Gbit/s, more than a single 1GbE uplink, while eight H.264 streams at 8 Mbit/s total only 64 Mbit/s.
Worked example: eight cameras at 1080p50
- Payload: 1920 × 1080 × 50 × 20 bit = 2.07 Gbit/s per camera, inside 3G-SDI's 2.97 Gbit/s.
- SDI routing: eight feeds occupy eight router inputs at that rate; cables are never shared, so no statistical multiplexing applies.
- IP alternative: eight NDI HX streams at 12 Mbit/s total 96 Mbit/s, about 10% of a 0.95 Gbit/s usable link; eight full NDI streams at 150 Mbit/s total 1.2 Gbit/s and need 10GbE or segmented multicast.
- Power: IEEE 802.3af guarantees 12.95 W at the device from a 15.4 W port; a PTZ head with motors usually needs 802.3at at 25.5 W, so eight cameras consume 204 W of the switch's total budget.
Where the calculation stops being reliable
- Line rates are nominal: blanking, line coding and forward error correction reduce usable payload.
- Encoded bitrate is content-dependent, so a lectern scene and a moving audience need different values at equal quality.
- A fixed-format camera ignores the calculation: if it outputs 1080p50 on 3G-SDI, choosing a smaller payload frees no link capacity.
- PoE budget is per switch, not per port; port count × per-port watts often exceeds the total supply.
Source and version record
Interface rates are the nominal values of SMPTE ST 292-1 (HD-SDI), ST 424 (3G-SDI), ST 2081 (6G-SDI) and ST 2082 (12G-SDI); HDMI 2.0 and 2.1 bandwidths come from the HDMI specification; the 12.95 W, 25.5 W, 51 W and 71.3 W device guarantees are from IEEE 802.3af, 802.3at and 802.3bt; NDI stream rates follow NDI's published bandwidth figures for 1080p60. Payload values above are arithmetic, not measurements of a specific model, so confirm the exact output format and rated cable distance in the device datasheet before specifying hardware.
FAQ
Does switching from 4:4:4 to 4:2:2 reduce what the SDI cable carries?
No. The SDI format is set by resolution and frame rate, and the sampling structure sits inside that fixed payload; the saving is used for 10-bit precision rather than for a narrower link. Capacity is only freed when the whole format changes, for example moving 2160p60 from 12G-SDI to a 6G-SDI format.
How many compressed 1080p60 cameras fit on one 1GbE port?
Arithmetically about 100 at 8 Mbit/s against 0.95 Gbit/s of usable throughput, but the uplink, the recording server's ingest and multicast group management limit the count long before the port does. Budget 60–70% average uplink utilisation and remember that one full-NDI stream at 150 Mbit/s already takes 16% of the link.
Why does the same 4K camera run 60 fps on SDI but 30 fps on USB?
The ports carry different payloads. A 2160p60 10-bit 4:2:2 feed needs 9.95 Gbit/s, which 12G-SDI supplies; a USB 3.2 Gen 1 controller offers 5 Gbit/s shared across its devices, so the camera halves the frame rate or sends compressed MJPEG or H.264 instead. The sensor is unchanged — only the interface budget is.