Your cable looks fine. The connectors are tight. But something's wrong with the signal.
Is it the cable? Or something else entirely?
Here's how to know for sure.
The Quick Visual Inspection
Before you grab any tools, look at the cable:
- Connector damage: Bent pins, cracked housings, loose BNC locks
- Cable damage: Kinks, crush points, cuts in the jacket
- Strain relief: Is the cable pulling away from the connector?
- Corrosion: Green or white buildup on the connectors
If you see any of these, you've probably found your problem. But invisible damage is more common than visible damage.
The Swap Test (No Tools Required)
The simplest diagnostic: swap the cable with a known good one.
- Problem goes away? It was the cable.
- Problem stays? It's something else.
This sounds obvious, but I've watched people spend 30 minutes troubleshooting when a 30-second swap would have given them the answer.
Pro tip: Always carry at least one cable you KNOW works. Test it before every job. That's your reference cable.
The Continuity Test
A basic multimeter can tell you if your cable has an open (broken conductor) or short (conductors touching).
What you need:
- Multimeter with continuity mode
- BNC barrel adapter (to connect both ends)
Test for opens:
- Set meter to continuity/resistance
- Touch one probe to center pin, one to the other end's center pin
- Should read near 0 ohms (or beep)
- Repeat for shield (outer ring to outer ring)
No continuity = broken conductor somewhere in the cable.
Test for shorts:
- Touch one probe to center pin, one to outer shield (same end)
- Should read infinite resistance (no beep)
If you get continuity between center and shield, the cable is shorted. Garbage it.
The "Wiggle Test"
Intermittent failures are the worst. The cable works, then doesn't, then works again.
With signal flowing through the cable:
- Gently flex the cable near each connector
- Flex the cable at any visible kinks or damage points
- Rotate the BNC connectors slightly
- Watch the monitor for any dropouts
If the signal cuts out when you move the cable, you've found the failure point. Usually it's right at the connector where flex stress is highest.
The Return Loss Test (Professional)
This is the real test. Return loss measures how much signal reflects back due to impedance mismatches.
For 12G-SDI, you want return loss of at least 15dB across the frequency range. Higher is better.
The catch: proper return loss testing requires a network analyzer or specialized SDI test equipment. We're talking $5,000+ for decent gear.
If you're running a rental house or cable manufacturing operation, this equipment pays for itself. For individual shooters? The swap test and continuity test will catch 95% of problems.
Signal Generator + Monitor
If you have access to an SDI signal generator (like a Blackmagic Mini or AJA test generator):
- Connect generator → cable → monitor
- Run a 75% color bars pattern
- Look for any artifacts, sparkles, or color shifts
- Check that the monitor reports the correct format (resolution, frame rate)
A bad cable will often cause:
- Sparkles (random bright pixels)
- Horizontal lines/tearing
- Color fringing at edges
- Complete signal loss
- Format detection errors
Run 12G-SDI through the cable if that's what you'll use it for. A cable might pass 3G but fail at 12G.
The Cable Tester Option
Dedicated SDI cable testers exist, like the Phabrix units. They test:
- Continuity
- Cable length (via time delay)
- Signal integrity
- Return loss
These are expensive but invaluable for production companies that go through lots of cables. For most individual users, overkill.
Creating a Test Routine
Here's my pre-job cable check:
- Visual inspection of every cable going in the bag
- Signal test on any cable that's been stressed (wrapped around gear, stepped on, used in bad weather)
- Full test on any cable I'm suspicious of
- Replace immediately any cable that fails
Takes 10 minutes. Saves hours of on-set troubleshooting.
When to Give Up
If a cable fails any test, don't try to fix it in the field. Label it "BAD" and deal with it later.
Some cables can be re-terminated if the problem is at the connector. But most of the time, the labor cost of diagnosis and repair exceeds the cost of a new cable.
Your time is worth more than a $50 cable. Don't be precious about it.
Building Your Test Kit
For field testing, I carry:
- Basic multimeter ($20)
- BNC barrel adapters (a few)
- One known-good reference cable
- Sharpie for labeling bad cables
That's it. Total investment under $50, and it catches 99% of cable problems before they become on-set disasters.
Got a cable testing question? Drop me a line. I've probably broken that cable before.
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