Converting an HDMI signal to coaxial cable, commonly known as coax, for distributing high-definition video in the home seems like it should be straightforward. After all, coax has been used for decades to transmit cable TV and antenna signals. So why not repurpose those existing coax runs for delivering HD video from media players and cable/satellite boxes?
Unfortunately it’s not that simple. There are significant technical challenges, licensing fees, and copy protection mechanisms that make converting HDMI to coax impractical and expensive compared to better alternatives like HDBaseT.
HDMI vs Coax: Key Differences
First, it helps to understand some key differences between HDMI and coax:
- HDMI carries uncompressed digital video and audio signals along 19 separate pins protected by an outer shield. It uses transition minimized differential signaling (TMDS) which encodes the video data digitally using a high frequency serial interface.
- Coax carries information by modulating analog waveforms over a single internal conductor shielded by a braided outer conductor and insulation. Traditional coax cables use RF modulation to encode analog video signals with an RF carrier signal.
So HDMI is optimized for short cable runs carrying multi-channel uncompressed digital AV, while coax is designed for longer runs of modulated analog signals. Converting between them is no simple matter.
HDMI Copy Protection Mechanisms
HDMI sources are required to encrypt the video and audio signals using High-bandwidth Digital Content Protection (HDCP). This prevents copying the digital content as it travels across the HDMI cable.
Any device converting HDMI to another format must pay licensing fees and implement HDCP authentication between the source and display. This raises costs significantly.
What’s more, many 4K/HDR sources use the more advanced HDCP 2.2 standard. Typical HDMI to coax converters only support the older HDCP 1.4 spec. So they cannot handle the latest 4K video sources.
Resolution and Bit Depth Challenges
The maximum resolutions and bit depths supported by HDMI have increased rapidly:
- HDMI 1.4 handles up to 1080p video
- HDMI 2.0 increased this to 4K 30Hz
- HDMI 2.1 now allows 4K 120Hz, 8K 60Hz, HDR, and 10-bit color
This requires very high bandwidth beyond what typical coax can support. While modern HDMI cables have upwards of 18Gbps bandwidth, the maximum capacity of coax cables is only about 6Gbps.
Expensive HDMI to HD-SDI Conversion
The only analog video interface capable of carrying uncompressed HD video is HD-SDI. But commercially available HDMI to HD-SDI converters remain extremely costly at over $1,000.
This is because converting HDMI to HD-SDI still incurs the HDCP licensing fees. So there is little cost advantage despite using coax cabling.
Enter HDMI Over IP
A much better solution is to use HDMI over ethernet, a technology standardized by the HDBaseT Alliance. This allows extending uncompressed 4K/60 4:4:4 video, audio, ethernet, USB, and control signals up to 100m over a single CAT5e/CAT6 cable or fiber:
While a basic HDBaseT transmitter and receiver still costs over $300, it avoids the pitfalls of converting between HDMI and coax. Video remains uncompressed, full HDCP support allows copying 4K HDR signals, and CAT5e/6 cabling has enough bandwidth for 8K video.
Many AV receivers, matrix switchers, and even TVs now have HDBaseT built-in. Or you can add standalone HDBaseT extenders connected via ethernet cables.
For most applications, HDBaseT provides a much simpler and cost-effective solution compared to modulating HDMI signals for transmission over coax.
Challenges of HDMI to Coax Conversion
Now that we’ve covered why HDBaseT is better than HDMI-to-coax conversion, let’s dig deeper into the specific technical challenges:
HDMI Uses TMDS Encoding
Remember that HDMI relies on TMDS to digitally encode video pixels into a high speed serial bitstream. But coax cables carry analog waveforms at much lower modulation frequencies.
Converting TMDS to RF modulation requires expensive encoder and decoder ICs. These have to be HDCP licensed and support HDMI’s broad range of resolutions and refresh rates.
HDMI 2.1 supports up to 48Gbps bandwidth which can handle 8K video at 60Hz with full chroma sampling:
Table – HDMI resolutions, bit depths and bandwidth requirements
Even 1080p video requires ~5Gbps, greater than the ~6Gbps limit of coax cable. And 4K HDR content needs over 18Gbps.
This means HDMI signals have to be heavily compressed using codecs like H.264 to squeeze them into a compatible coax bandwidth. This defeats the purpose of HDMI’s uncompressed video capability.
HDCP authentication involves the source and display device exchanging encryption keys before allowing video transfer.
Converters have to intercept, decrypt, and re-encrypt the video stream on behalf of the HDMI source and coax display. This incurs further licensing fees and processing overhead.
Workarounds and Limitations
With all those challenges, what are the workaround solutions and their limitations?
HDMI to HD-SDI Conversion
As mentioned earlier, commercial HDMI to HD-SDI converters are costly. But some hobbyists have built DIY converters using FPGAs.
These strip away HDMI’s TMDS encoding and HDCP encryption through brute force. But they are limited to older HDMI 1.4b speeds (1080p60) and only work with specific display devices. Any future firmware updates to the source or display could break the compatibility.
HDMI Compression and Modulation
Using video compression ICs (encoders/decoders) is another option. But:
- Compression artifacts will degrade image quality
- Encoders/decoders must keep up with HDMI’s high pixel rates
- Only the older HDCP 1.4 standard is supported
So there is a severe drop in A/V quality, resolutions are limited to 1080p, and 4K HDR content will be blocked.
Analog Hole Exploitation
In rare cases, analog converter chips will output HDMI content over VGA or component video ports. This analog loophole avoids HDCP encryption.
But these analog ports are disappearing, and image quality is still reduced by digital-to-analog conversion stages. So it’s a poor band-aid solution not applicable to most modern equipment.
While using existing coax cabling seems appealing for HD video distribution, HDMI and coax were never designed to interoperate. Direct digital-to-analog conversion is cost prohibitive, and compression discards HDMI’s pristine image quality.
HDBaseT provides a far superior solution – uncompressed 4K/8K video and audio, HDCP 2.2 support, 100m distances, and over 18Gbps bandwidth comparable to HDMI 2.1. For most applications, HDBaseT or native HDMI extension is the right way forward.
So while some HDMI to coax workarounds exist, they require expensive FPGAs, offer compromised quality, resolutions, and compatibility. Going forward, multimedia over IP is the path to enabling HD video distribution across the home.