HDMI, DisplayPort, and SDI — What Engineers Keep Getting Wrong
If you’ve ever seen:
- Gray blacks
- Crushed shadow detail
- Blown highlights
- A “washed-out” LED wall
- A QC rejection for “illegal levels”
There’s a very good chance you were dealing with a range mismatch.
Limited vs Full range video is one of the most misunderstood topics in AV engineering. And the confusion only gets worse when you move between HDMI, DisplayPort, and SDI.
Let’s fix that.
First: What “Range” Actually Means
Range refers to quantization levels — the numeric values used to represent brightness and color inside a digital video signal.
It does not change:
- Resolution
- Bit depth
- Chroma subsampling
- Bandwidth
It changes how code values map to black and white.
The Numbers (8-bit and 10-bit)
8-bit RGB
| Mode | Black | White |
|---|---|---|
| Full | 0 | 255 |
| Limited | 16 | 235 |
10-bit RGB
| Mode | Black | White |
|---|---|---|
| Full | 0 | 1023 |
| Limited | 64 | 940 |
Limited range reserves headroom and footroom. Full range uses the entire container.
That’s it. Nothing mystical.
HDMI & DisplayPort: The Flexible Interfaces
Defined by:
- HDMI 2.1
- DisplayPort 2.1
These interfaces were designed to serve both:
- Computer graphics systems (full range RGB)
- Consumer video systems (limited range YCbCr)
That flexibility is where the confusion starts.
RGB over HDMI / DP
- PC GPUs default to Full Range RGB
- TVs often expect Limited Range
- Monitors usually expect Full
If you send Full to a display expecting Limited:
👉 Blacks get crushed
👉 Highlights clip
👉 Contrast spikes unnaturally
If you send Limited to a display expecting Full:
👉 Blacks look gray
👉 Image looks flat
👉 Contrast is reduced
This is the classic “washed-out HDMI” complaint.
YCbCr over HDMI / DP
Here’s what many engineers miss:
YCbCr is almost always limited range.
Even when you switch a GPU from RGB to YCbCr 4:4:4, the quantization typically changes to legal/video levels.
So if someone says:
“Switch to YCbCr for better blacks.”
That’s not how it works. You just changed color encoding and range behavior.
HDMI Signaling (And Why It’s Not Reliable)
HDMI actually includes a quantization flag in its InfoFrame.
It can signal:
- Default
- Limited
- Full
But in the real world:
- Some TVs ignore it
- Some GPUs misreport it
- Some devices apply it only to RGB
- Some assume limited if YCbCr
Auto-detect works… until it doesn’t.
That’s why serious setups often require manual matching.
DisplayPort: More PC-Centric
DisplayPort evolved primarily for computer displays.
Default expectation:
👉 RGB Full Range
Most DP monitors assume full unless configured otherwise.
DP behaves more predictably than HDMI — but it still supports limited modes for compatibility.
HDR Does Not Mean Full Range
Important myth to kill:
HDR does not equal full range.
HDR10 over HDMI is still typically:
- YCbCr
- Limited range
- 10-bit
What changes is the transfer function (PQ or HLG), not the quantization container.
Now Let’s Talk SDI
Defined by standards like:
- SMPTE 292M
- SMPTE ST 2082
SDI is different.
Very different.
SDI: The Broadcast Assumption
SDI does not negotiate.
It does not signal range.
It does not auto-detect.
It transports numbers.
In 10-bit SDI:
| Mode | Black | White |
|---|---|---|
| Legal | 64 | 940 |
| Full | 0 | 1023 |
Broadcast workflows assume legal range.
That assumption is baked into:
- Cameras
- Switchers
- Routers
- Recorders
- QC systems
- Transmission specs
Is Full Range SDI “Illegal”?
Electrically? No.
Broadcast-compliant? Usually yes.
If you deliver a program exceeding 64–940:
QC will flag it as illegal video.
But inside:
- Color grading pipelines
- GPU render systems
- Unreal / LED workflows
- Engineering test environments
Full-range SDI is often used intentionally.
The transport allows 0–1023. The workflow determines legality.
Why SDI Is Stricter Than HDMI
| Feature | HDMI / DP | SDI |
|---|---|---|
| Range Negotiation | Yes | No |
| Metadata Flag | Yes | No |
| Broadcast Assumption | Flexible | Legal |
| Consumer Use | Yes | No |
| Engineering Tolerance | Mixed | Strict |
HDMI and DP are hybrid bridges between PC and video worlds.
SDI lives in broadcast territory.
Real-World Failure Scenarios
1️⃣ LED Wall Looks Washed Out
Media server outputs Full RGB
Processor expects Limited
Blacks float.
2️⃣ Broadcast Feed Fails QC
Graphics engine outputs super-white highlights
Signal exceeds 940
Rejected for illegal luminance.
3️⃣ Capture Card Records Flat Image
Camera outputs legal
Software interprets full
Everything looks lifted.
The Deep Technical Truth
Switching from Limited to Full:
• Does not increase dynamic range
• Does not increase bit depth
• Does not increase bandwidth
It changes how values map to luminance.
If the entire chain agrees, both are valid.
If one device disagrees, your image is wrong.
Practical Engineering Rules
HDMI / DP
- PC monitor → RGB Full
- TV → Usually RGB Limited
- Broadcast gear → Limited
- YCbCr → Assume Limited
SDI
- Broadcast chain → Legal
- Grading / VFX → Possibly Full internally
- On-air delivery → Legal only
The Big Takeaway
HDMI and DisplayPort are flexible.
SDI is rigid.
Full range is not “better.”
Limited range is not “worse.”
Mismatch is the enemy.
And most range problems aren’t color science issues — they’re configuration issues.