4 ms·

> Cut a HDMI cable in half and splay it out, then try switching that to another display. See what happens. If you took an ancient (but still available to buy)

> Cut a HDMI cable in half and splay it out, then try switching that to another display. See what happens. If you took an ancient (but still available to buy) 25-pin 'parallel port' 4-port switch box with a rotary dial: https://photography-909a.kxcdn.com/product-images/03292/600/03292a.jpg https://photography-909a.kxcdn.com/product-images/03292/600/... And wired up the HDMI of two machines to output ports 1 & 3[1]. Then wired up the HDMI to the display to the input port (so effectively using the switch 'backwards'). As you are just simulating unplugging and re-plugging in the HDMI cable from each machine, via the switch, then surely this would work? There would be a bit of delay after switching as the HDMI/HDCP handshake completes, but unless you are switching every few minutes it should be usable? If it does work, then you've got 6 pins in the switch left to play with which could be used to switch usb devices (via a downstream hub) I've not done this, but have thought about it for a while now. ---- [1] Not ports 1 & 2 as you want to ensure total electrical isolation between the devices during switching.

gm-conspiracy 9y ago
I think the real isssue is with HDCP. The output device "ensures" the display device is authorized via a key to output. So, you would need an intermediary device that would accept any incoming signal, while also having a legit handshake with the output device. So, as you switch "inputs", there is no additional authorization required, because the output device does not detect any changes.
- sitkack 9y ago
  I believe this is also the case. The only organizations that can get exemptions from the HDCP Alliance (DRM Cabal) for devices that effectively circumvent encryption are for high end broadcast gear. Nothing precludes making a 4k KVM. Infact, a mid range AV main unit could probably route 4k KVM. https://usa.yamaha.com/products/audio_visual/av_receivers_amps/rx-a770_u/specs.html#product-tabs https://usa.yamaha.com/products/audio_visual/av_receivers_am...
- aidenn0 9y ago
  It's absolutely not HDCP since a 4k HDMI splitter is much cheaper than a 4k KVM. My guess is that with KVMs disappearing from server racks, there isn't enough volume to get the prices we used to get.
cesarb 9y ago
Would it have enough bandwidth? High speed connections have some careful engineering (for instance, maximum distance without a twist on a differential pair), that I doubt that ancient box has. Also, there are sequencing requirements. Look carefully at a USB-A plug, and you'll see that the power and ground traces are longer than the data traces: when plugging the USB cable, the power is connected first, before the data; when unplugging, the order is reversed. HDMI probably has similar requirements, perhaps with more steps. Again, I doubt that ancient box can do that.
gvb 9y ago
Not likely. If the cables are short you might get away with it but not likely. The problem you will have is that the signals require high bandwidth and the cables are carefully designed to have low crosstalk, high immunity to noise (both via twisted pairs) and a 100 ohm characteristic (dynamic) impedance (how the cables are constructed, insulation thicknesses, how they are twisted). DB-25 switches work for simple single ended low frequency signals. The physical switch will wreck havoc with the interpair skew, far-end crosstalk, attenuation and differential impedance. HDMI 1.3 defines two cable categories: Category 1-certified cables, which have been tested at 74.5 MHz (which would include resolutions such as 720p60 and 1080i60), and Category 2-certified cables, which have been tested at 340 MHz (which would include resolutions such as 1080p60 and 2160p30). Category 1 HDMI cables are marketed as "Standard" and Category 2 HDMI cables as "High Speed". This labeling guideline for HDMI cables went into effect on October 17, 2008. Category 1 and 2 cables can either meet the required parameter specifications for interpair skew, far-end crosstalk, attenuation and differential impedance, or they can meet the required nonequalized/equalized eye diagram requirements. [1] [1] https://en.wikipedia.org/wiki/HDMI#Cables https://en.wikipedia.org/wiki/HDMI#Cables
- Jaruzel 9y ago
  Thank you for the clarification! :)

gm-conspiracy 9y ago

I think the real isssue is with HDCP. The output device "ensures" the display device is authorized via a key to output. So, you would need an intermediary device that would accept any incoming signal, while also having a legit handshake with the output device. So, as you switch "inputs", there is no additional authorization required, because the output device does not detect any changes.

sitkack 9y ago
I believe this is also the case. The only organizations that can get exemptions from the HDCP Alliance (DRM Cabal) for devices that effectively circumvent encryption are for high end broadcast gear. Nothing precludes making a 4k KVM. Infact, a mid range AV main unit could probably route 4k KVM. https://usa.yamaha.com/products/audio_visual/av_receivers_amps/rx-a770_u/specs.html#product-tabs https://usa.yamaha.com/products/audio_visual/av_receivers_am...
aidenn0 9y ago
It's absolutely not HDCP since a 4k HDMI splitter is much cheaper than a 4k KVM. My guess is that with KVMs disappearing from server racks, there isn't enough volume to get the prices we used to get.

cesarb 9y ago

Would it have enough bandwidth? High speed connections have some careful engineering (for instance, maximum distance without a twist on a differential pair), that I doubt that ancient box has. Also, there are sequencing requirements. Look carefully at a USB-A plug, and you'll see that the power and ground traces are longer than the data traces: when plugging the USB cable, the power is connected first, before the data; when unplugging, the order is reversed. HDMI probably has similar requirements, perhaps with more steps. Again, I doubt that ancient box can do that.

gvb 9y ago

Not likely. If the cables are short you might get away with it but not likely. The problem you will have is that the signals require high bandwidth and the cables are carefully designed to have low crosstalk, high immunity to noise (both via twisted pairs) and a 100 ohm characteristic (dynamic) impedance (how the cables are constructed, insulation thicknesses, how they are twisted). DB-25 switches work for simple single ended low frequency signals. The physical switch will wreck havoc with the interpair skew, far-end crosstalk, attenuation and differential impedance. HDMI 1.3 defines two cable categories: Category 1-certified cables, which have been tested at 74.5 MHz (which would include resolutions such as 720p60 and 1080i60), and Category 2-certified cables, which have been tested at 340 MHz (which would include resolutions such as 1080p60 and 2160p30). Category 1 HDMI cables are marketed as "Standard" and Category 2 HDMI cables as "High Speed". This labeling guideline for HDMI cables went into effect on October 17, 2008. Category 1 and 2 cables can either meet the required parameter specifications for interpair skew, far-end crosstalk, attenuation and differential impedance, or they can meet the required nonequalized/equalized eye diagram requirements. [1] [1] https://en.wikipedia.org/wiki/HDMI#Cables https://en.wikipedia.org/wiki/HDMI#Cables

Jaruzel 9y ago
Thank you for the clarification! :)