Weird HDMI issue with projectors

[Tinribs]

Hi All,

This one is a bit odd. Our school uses a range of iMacs (from iMac 9 upwards) as PCs by running Windows via bootcamp. Nearly all the classrooms have a ceiling mounted projector connected to the computer via a VGA cable plus mini-display adapter. All works fine.

We are replacing the 9s and 10s with actual PCs running Windows. These are HP all-in-one machines that only have HDMI video output. The majority of these work fine but we currently have five rooms that refuse to connect projector to PC. The same cables and PCs work fine in other rooms (running the same or different projectors). If I move the PC in a room that works fine to a room that doesn't then that PC will not connect to the projector in that room even if I move the cables as well. Putting the old iMac back into a room that does not work with a PC works fine.

The only differences that I can see between the two are HDMI to mini-display port and/or possibly drivers but that will not explain why the same PC in a different room with the same or different projector setup will work. Is there a difference in signal, voltage or anything else between Mini DP and HDMI that could explain this.

In one room even replacing a different iMac wouldn't work - it had to be the exact same iMac that was in the room before we fitted the computer..

Any ideas??

[Zourous]

A bit tricky to read, but what is main cable running from PC/MAC to projector, VGA or HDMI?

[Tinribs]

Yeah - sorry if the original message is garbled - I tend to think faster than I type....

All the projectors are connected to a wall port using VGA. The cables from MAC/PC to the wall ports are also VGA but connected to PC via an HDMI adaptor and to MAC via a Mini-DP adaptor.

In all cases the projectors have two VGA inputs and most have two VGA ports on the wall as well. In the cases where the rooms do not work then neither VGA input on the wall port will work when using the HDMI adaptors but work with the Mini-DP.

[Zourous]

Personally when I've had issues that are confusing like this I would simplify the problem as much as possible to see what outcomes you then might get. For example run a loose long VGA cable (with adapter if needed) straight to the projector.

[Tinribs]

Indeed - that is/was my next step but getting hold of such a cable at this school is the trick... It would have to be a very long cable!

[HarryMonkey]

We have issues like this and it tends to be when the cable from the wall to the projector is more than 10m. My understanding is that HDMI doesn't like sending its signal more than 10m down the cable.

[chazzy2501]

I've found that the hdmi to VGA adapters use the HDMI power this is pretty limited and not all HDMI outputs are equal. This means that the VGA adaptor sometimes fails to work at all or has a weak signal that is easily lost on a 10m run (hdmi or VGA) depending.

[Tinribs]

Indeed - the adapters that we bought come with a USB connection to power them if needed. Even with this connected they do not work and the same setup in a working room does not need the USB connection

[Tinribs]

So if it is an HDMI signal issue (and this would make sense) would I need an HDMI booster box? Or, as the majority of the cable is actually VGA would a VGA booster work better or worse? Would it not matter at all which we got as long as the signal is amplified?

[hardtailstar]

Does the projector have hdmi? - Can you run a hdmi cable straight from the PC to the projector.

 

Ive found that if any apple product is connected via adapters it tends to not work for anything else after.

[Tinribs]

That's encouraging!!

 

Nope - most of our projectors are dinosaurs that only have component and VGA inputs. Even the short-throw models, which may well have HDMI inputs, are so mounted that only the A/V company that installed them can access them....

[nathan3388]

just be-careful of the length of the cable that you are using we had it with a few Apple TV where it wouldn't pick up and this was using 10m HDMI cables

[MkII]

I've had some strange problems similar to this. One projector flickers slowly, the others just don't show an image.

I've got around it by: separating/ isolating cable runs; adding a signal booster to VGA (cheap £10-£20 ones); adding a usb powered booster (weirdly works where a mains powered box doesn't!)..

[SamanthaD]

The official HDMI spec doesn't have a max length, though it's widely accepted that 5m is the max you can use a passive cable, and 15m for active. This doesn't really apply to you though if you're adapting it straight out of the machine.

 

Are you having problems with one specific brand of projector? I'm wondering if the PC's are outputting a resolution too high for the projectors to accept?

[Tinribs]

Nope - no consistency in projector model/make. The rooms that work can have the same projector as the room that doesn't. From what I can remember sitting here two of the rooms have very old Epson ceiling mounted projectors and three have new short throw Epsons. The resolution on all the PCs is set the same so seeing as the same PC will work in one room and not the other then it can't be that....

It sort of has to be a cabling issue from what I can tell but I need to prove it. Without being able to connect directly to the affected projectors this will be a problem.