The
HDMI cable, like the connector, is a highly sophisticated piece of engineering.
It may look like any other AV cable from the outside (other than the
connectors, that is), but its outer appearance belies its interior.
Inside there is a whopping 15 signal conductors, lots
of shields, and 4 drain/ground wires, to match the
19 pins on the connector. It is quite stunning how so much performance
can be so effectively transmitted through something of such small relative
thinness...
What's
inside?
Above:
Cross section of a typical HDMI cable
(colors are for demo only, and may vary)
In
the diagram above you can see four main conductors groups (the 4 larger
black circles). These are the primary "TMDS"
channels (0, 1, 2 & clock), each surrounded by an AL mylar foil
wrap for EMI shielding. All of the other individual cores are for
the other special functions of HDMI, being CEC, DDC hot plug detect
and 5V line, all of which are discussed in more detail elsewhere. This cross section is valid for both HDMI type 1.2 and 1.3. The main difference between the two is that HDMI 1.3 uses a tighter twist rate for the TMDS pairs. Also, the better quality 1.3 spec cables also tend to employ a second foil wrap directly around each TMDS pair, effectively separating the pair from it's parallel drain wire. This further increases shielding and potential bandwidth. As a result of the tighter twist and better shielding, HDMI 1.3 cables will also tend to be stiffer than the 1.2 equivalent.
Cable
Thickness
You
may see many cables advertising the use of "30AWG, "28AWG", "26AWG" or "24AWG"
conductors. The AWG
part stands for "American
Wire Gauge".
It is a system of determining relative thickness, but it's a little
backwards in that the higher the number, the thinner the cable's conductors.
The basis of the system is simple - a 3AWG increase
in number represents a HALVING of the conductor's
cross sectional area, and vice versa. For example, a 21AWG conductor
is twice the size of a 24AWG conductor, and a 26AWG is half the size
of a 23AWG, etc. Simple. So in the examples given above, 30AWG is the thinnest, with 24AWG being the thickest. Note that this only applies to the actual
conductor size. It's easy to make the overall cable bigger by just
using more PVC on the outside, so don't judge a proverbial book by its cover...
Most
short length HDMI cables use 30-28AWG conductors, but as the length
of a cable increases so does the capacitance and resistance (amongst
other things), resulting in a loss of bandwidth. It is very difficult
to achieve the target "Standard" bandwidth requirements
of HDMI, and even tougher to produce the new 1.3b spec "High Speed" standard. This is especially so for longer lengths, whcih require a high level of expertise from
the manufacturer for success. A larger gauge cable is normally used,
with 24AWG being the most common for lengths of 7-10m or more. However,
just because a manufacturer makes long length HDMI cables, it does not
mean that they will work at the full 1080p or beyond. To ensure you are getting
the full intended performance of HDMI, test your desired cable brand
and length in-store with a 1080p source (such as Blu-ray),
prior to purchase.
"Test
your cable in-store with a 1080p source... prior to purchase"
The
performance of a HDMI cable is dictated by its ability to pass the required
bandwidth, not by the size. Keep this in mind as you go shopping. Choose
only based on its real performance potential. Many other factors determine
this potential, the main of which is the licensed status of the cable
- is it genuine HDMI Approved? If not, steer clear. We will soon be
testing many HDMI cables on the market for their suitability for 1080p
in various lengths, and posting the results on this site... stay tuned!
Conductor
Material - TC vs SPC
Most
HDMI cables will utilize Tinned Copper conductors, often abbreviated
to "TC". It is polular due to
the increased structural integrity of TC, compared to straight OFC
(copper only), as well as the enhanced longevity characteristics.
Are
Silver plated cables better?
This
is a very common question, and a simple one to answer - YES.
Silver is the BEST conductor of all, and its use in the plating of copper
produces an exceptional yet relatively economical performer. Silver
Plated Copper (or SPC) provides a purer signal, whilst adding the structural
integrity in much the same way as tin does in the TC version. "...but
it's all digital - 1's and 0's", I hear you say!
Yes, it is, but billions of 1's and 0's called data bits. In
many cables these data bits can change order, change amplitude or drop
out entirely. This is known as digital jitter. Silver plating transmits
the digital bitstream more intact than TC, reducing digital jitter and
increasing performance.
Stranded cores are also more common than solid, also due to enhanced
longevity and also flexibility. Solid conductors can however provide
two advantages - 1. they are technically superior when plated with silver,
especially in coaxial style cables, eg analogue video, but less so in
HDMI due to the different electromagnetic interaction in HDMI, and 2.
they are cheaper to produce. Stranded vs solid core should generally
not be regaded as a deciding factor in buying a HDMI cable.
