HDMI over Cat5

[revbobuk]

Folks - I know there are some very smart people on here. So can someone explain to me why some HDMI over Cat5 solutions use 2 Cat5 cables, and some seem to manage with just one? I've got spare Cat5 runs front-to-back in a building (little bit of future proofing in the design) but I'd rather not use two if I can manage with one. Are there performance limits with the single cable solution? For example - the Labgear (here from Screwfix, of all places) is single Cat5; the Lindy (here) uses two.

[mac.calder]

It is a matter if bandwidth and compression generally. Some of it is also based upon what method they used to convert the signal.

 

Compression basically follows the following formula: size reduction = loss in quality x processing time x processing power. Real time video compression - if you need 'real time' and significant size reduction that means you significantly drop quality and/or need processing power ($$$).

 

Regarding bandwidth - Cat5, Cat5e, Cat6, Cat6a - all of these can only (physicaly) carry a certain amount of data over a certain amount of distance - or rather, to meet one of the standards they must be able to carry a certain amount of data (able to transmit a certain frequency range with a set amount of noise rejection etc) over a certain distance. Cat5e is good for gigabit up to 100m. Shielded Cat6a is good for 10gig up to 100m. That doesn't mean Cat5e can't do 10G, it can - Cat5e is good to about 30m for 10G. Of course these are all ethernet figures - and depending on the technology used in the boxes these figures are a bit elastic.

 

4k@50 with a 4:4:4 colour space is just over 10Gb/s of data, 1080p@50 with 4:4:4 colour space is about 2.9Gb/s. You will generally find that many solutions use (shielded) Cat6a - which is good for 10Gb/s up to100m. One cable has more than enough bandwidth for a full HD stream - and can probably do 4k@50 with a reduced 4:2:2 colour space, or 4k@25 4:4:4. There are now systems out there that do some light "visually lossless" compression with some funky time alignment magic and can get the full 4k@60 4:4:4 down a cat6a cable.

 

If it is a Cat5e solution, generally it will use 2 cables (good for 1Gb/s each of ethernet data, but can be pushed a bit higher without the ethernet overheads), voila. Single cable solutions will generally do it down cat5a at a reduced distance.

[david.elsbury]

Eh?!?

 

As far as I know the dual cable options are a passive converter good for 10-30m.

 

Single cable options tend to be based on HDBaseT technology. I'd go for a HDBT solution myself. 70m (lite) or 100m (full) on a cat5e at 1080p

[Joe Fernand]

Dual Cat systems generally split the signal as Video (with embedded audio) down one cable and DDC, HDCP, Control (and in some designs) Power down the second cable - you can often 'get away with' one cable being of a lesser quality than the other (helpful if one of your installed cables is not great). Some of the very low cost Dual Cat systems convert the digital video to an analogue YUV signal and rebuild the digital signal at the far end of the system (it can be handy when you simply ignore/break HDCP!).

 

Single CAT solutions as Mhm says have to process the signal at the TX and rebuild it at the RX and as highlighted systems built on the HDBaseT (HDBT) standard are the most reliable - there are multiple HDBT chip sets and Features so you have to know which Features you require. In theory any HDBT TX ought to work to a basic Spec with any HDBT RX no matter the brands of the devices - you will see some Projectors and Displays have an embedded HDBT RX unit.

 

Joe