Cable spec sheets can look like alphabet soup: AWG, dB, MHz, RG—what does any of it actually mean for your shoots? Understanding which specifications matter (and which are mostly marketing) helps you make better buying decisions and troubleshoot problems when they arise.
Here's a practical guide to reading SDI cable specifications without needing an engineering degree.
The Specifications That Actually Matter
1. Bandwidth Rating (MHz or Gbps)
This is the most critical spec for SDI cables. It determines what signal types the cable can reliably carry.
What the numbers mean:
- 1.5 Gbps (SD/HD-SDI): Basic HD video
- 3 Gbps (3G-SDI): 1080p60, some 4K formats
- 6 Gbps (6G-SDI): 4K at lower frame rates
- 12 Gbps (12G-SDI): Full 4K 60p, 8K at lower rates
What to look for:
Always buy cables rated for the highest bandwidth you might need. A 12G-rated cable works perfectly for all lower bandwidth signals, but a 3G cable will fail with 12G signals—even if it "sort of works" initially.
The catch:
Some manufacturers list theoretical bandwidth rather than tested, reliable bandwidth. Look for specs that mention actual signal testing, not just cable construction claims.
2. Impedance (Ohms)
SDI signals require 75-ohm impedance throughout the entire signal path. This is non-negotiable.
What to look for:
- Must be 75 ohms (not 50 ohms, which is for different applications)
- Good cables specify impedance tolerance (e.g., 75 ± 3 ohms)
- Tighter tolerance = more consistent signal quality
Why it matters:
Impedance mismatches cause signal reflections that degrade picture quality. This is especially critical at higher bandwidths where there's less margin for error.
3. Attenuation (dB per 100m/ft at specific frequency)
Attenuation measures how much signal strength is lost over distance. Lower numbers are better.
How to read it:
Attenuation is always specified at a particular frequency because losses increase with frequency. A spec might read "12 dB/100m at 1.5 GHz"—this means the signal loses 12 dB of strength over 100 meters when carrying a 1.5 GHz signal.
What matters for you:
- Compare attenuation at frequencies relevant to your use (higher for 12G than 3G)
- Lower attenuation = longer possible cable runs
- Typical quality 12G cable: 20-25 dB/100m at 6 GHz
4. Return Loss (dB)
Return loss measures how much signal bounces back due to impedance inconsistencies. Higher numbers are better (yes, opposite of attenuation).
What good looks like:
- Minimum acceptable: 15 dB
- Good: 20+ dB
- Excellent: 25+ dB
Why it matters:
Poor return loss causes signal reflections that create visible artifacts, especially at higher resolutions. This spec separates professional cables from consumer-grade products.
5. Center Conductor Material
The material of the center conductor affects both signal quality and durability.
Options you'll see:
- Solid Copper (BC - Bare Copper): Best conductivity, most durable, premium price
- Copper-Clad Steel (CCS): Lower cost, adequate for short runs, more prone to issues over time
- Silver-Plated Copper: Slightly better high-frequency performance, premium option
What to choose:
For professional use, solid copper is worth the premium. CCS cables often work initially but develop problems faster, especially with repeated flexing.
6. Shield Coverage (%)
Shielding protects the signal from electromagnetic interference (EMI).
What to look for:
- Minimum acceptable: 90%
- Professional grade: 95%+
- Dual-shield (braid + foil): Better than single shield
Real-world impact:
On sets with lots of wireless devices, LED lighting, and other electronics, better shielding prevents interference that can cause sparkles, dropouts, or complete signal loss.
Specifications That Matter Less Than You'd Think
AWG (Wire Gauge)
American Wire Gauge indicates conductor thickness. While relevant, it's less important than the overall cable construction and testing.
General guidelines:
- Thicker (lower AWG number) generally allows longer runs
- 20 AWG is common for flexible cables
- 18 AWG for permanent installations or very long runs
But a well-designed 20 AWG cable can outperform a poorly-designed 18 AWG cable. Don't choose based on gauge alone.
RG Designation (RG-6, RG-59, etc.)
These designations describe general cable families but don't guarantee performance.
What they mean:
- RG-59: Smaller, more flexible, typically shorter distance capability
- RG-6: Larger, better for longer runs, less flexible
Two cables can both be "RG-6" and have dramatically different performance. The RG designation is a starting point, not a quality indicator.
Velocity of Propagation
This measures how fast signals travel through the cable (as a percentage of light speed). While it appears in specs, it rarely matters for practical video applications where cables are under a few hundred feet.
Red Flags in Spec Sheets
Watch out for:
- No frequency specified for attenuation claims
- Vague bandwidth claims ("HD compatible" instead of specific Gbps rating)
- Missing return loss specifications
- No mention of conductor material
- Specs that seem too good to be true at very low prices
How to Compare Cables
When evaluating options:
- Start with bandwidth: Does it meet your current and likely future needs?
- Check attenuation: At the frequency relevant to your use, compare apples to apples
- Verify impedance: Must be 75 ohms with reasonable tolerance
- Look at construction: Solid copper center, high shield coverage
- Consider the source: Reputable manufacturers publish complete, honest specs
The Bottom Line
You don't need to become a cable engineer, but understanding these core specs helps you:
- Avoid buying cables that won't work for your application
- Understand why some cables cost more (and whether it's justified)
- Troubleshoot issues when they arise
- Make confident purchasing decisions
When in doubt, buy from manufacturers who publish complete specifications and stand behind their products with real warranties. The best spec sheet in the world means nothing if the manufacturer won't back their claims in practice.
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