Specifications

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3
Random white dots on screen
These dots (aka “sparklies”) are usually caused by using an incorrectly
specified or poorly screened cable, or by signal losses caused by multiple
in-line connections such as cable joiners and wall plates. It is usually most
apparent when the equipment is operating at its maximum bandwidth
(1080p) as this is more demanding of the cable than say 1080i or 720p. To cure, either reduce the
resolution, use a shorter, better specified cable (if possible) and/or use a powered HDMI in-line signal repeater to increase the signal gain.
No Picture at all
When the equipment is first switched on there is a digital “handshake” whereby the source and display communicate with each other.
If this handshake is not successful then the display will not show anything from that source. If this happens then usually switching the
source off and then back on whilst leaving the display on will force a new handshake and solve the problem. If that fails try switching
off both devices, and if that still fails then try a different cable and/or selectively remove any switchboxes or distribution amplifiers that
might be in-line.
Picture appears for a while then randomly disappears
Like the sparklies above this is usually caused by incorrectly specified or poorly screened cable. An HDMI in-line repeater
(not passive joiner) can also be very useful in solving this problem.
General notes:
It is worth thinking of HDMI as an RF signal, (HDMI is in fact specified up to a 340MHz (UHF) level signal) and so most things that
apply to RF cable routing also make sense with HDMI. These include avoiding sharp bends and tight narrow loops in the cable and
not routing it alongside cables likely to radiate RF interference such as mains wiring etc. Tight bends can crush the cable insulation
internally, causing a change in cable impedance, RF signal reflections and all manner of horrors! HDMI does have error correction built
in but there’s only so much error it can cope with before you’ll notice drop-out and sparklies. Right angled HDMI adaptors are a good
solution to the close fitting display as they will perform much better than a tightly bent cable.
Continuing with the RF theme then you can think of everything in line between your source and your display as having an
“insertion loss”, basically something that reduces the strength or quality of the signal.
Devices that introduce an insertion loss include:
HDMI wallplates, HDMI cable couplers, HDMI switchboxes (cheap ones)
For example, if you connect a DVD player to a display with a 1m cable then the only signal losses along the way can come from within
the cable itself. Now imagine a DVD player connected via 1m cable to an HDMI wall plate. The wallplate is internally connected by a
1m cable to a wallpate in an adjacent room, and that is in turn connected via a 1m cable to a display. Now there is only 3m of cable
in-line, but because of the insertion loss of the extra connectors the signal is more akin to one that has travelled down a 20m cable.
That’s not to say that wallplates are a bad idea, they can be very useful for creating neat installations, it’s just that you need to bear
these losses in mind when planning your installation. The losses involved are usually within an acceptable range of tolerance for most
equipment but sometimes you may need to add an in-line signal booster to ensure performance at the highest resolutions.
Why do signal boosters say “power supply not normally required”?
The HDMI specification states that all HDMI sources must provide 5v at least 55mA for the handshaking process. 55mA isn’t really
enough power to be useful for an in-line device such as a signal booster, however in practice most mains powered devices usually
provide more current than this and so an external power source is not normally needed. The signal boosters do have a socket for
external power should it be required, but unless the source is something like a camcorder or a laptop then it is not normally required.
Surely all cables are the same – if it’s digital then if you’ll either get a picture or you won’t?
This isn’t unfortunately correct, as the demands on the cable are different at different resolutions and you may well find you get a
picture at say 720p but not at 1080p. Also, HDMI uses built in error correction and it follows that a pure error free signal from a quality
cable is always going to be more reliable in the long term than an on-the-limit error corrected signal from a sub-standard cable.
Anyone who has ever experienced sparklies on the picture and seen it cured by swapping to a higher grade cable would be happy to
vouch that whilst you can get a picture digitally it may not be a good one. It is true that the difference between cables is less noticeable
at shorter lengths and it is no coincidence that the longer the HDMI cable is the thicker the screening is!
To sum up:
Things that will reduce the quality of the signal:
Long cables, wall plates, passive cable couplers, non-powered switch boxes, tight bends or short loops with the cable.
Things that will improve the quality of the signal:
Short cables, signal boosters, distribution amplifiers (will act as a signal booster), powered switch boxes (will act as a signal booster).
Common Problems
and Solutions:
Top Tips to better HDMI
116513_HDMI_Brochure_Aug_2013.indd 3 11/09/2013 14:27