EoW November 2009

EuroWire – November 2009

technical article

If an engineer doesn’t want to carry a

50 ft cable, cable clones are available. This

is a small device that mimics a given length

of cable. It most commonly has a male

and female BNC at the ends of a cylinder,

and the type of cable and the length are

marked on it.

Consumer variations

In the consumer world, things are not as

simple and elegant as in the professional

world. The reason is content. In the

professional world, it is assumed that the

production house, broadcaster, or other

content producer, owns the content

outright. There are no legal complications.

In the consumer world, most often the

user does not own the content and is not

the copyright holder. Those who do own

the copyright are concerned about loss

of content protection and loss of revenue

and, therefore, they require a delivery

system that assures that content will not

be stolen.

The solution to this is a system called

DVI (Digital Video Interface) and its latest

version HDMI (High Definition Media

Interface). DVI allows for both analogue

and digital signals to be delivered from a

source device (cable box, disc player, for

example) but for digital delivery, there is a

complex electronic ‘handshake’ between

devices, called HDCP, (High-Definition Copy

Protection) assuring that both source and

destination devices are legal.

The HDMI cable version supports digital

signals only, not analogue. Inside an HDMI

cable there are nineteen wires between

source and destination devices including

three shielded data pairs, one shielded

clock pair, and seven more wires for HDCP,

power, and future applications.

1080p/60 in the home

Just as in professional systems, signal

delivery comes in various data rates

depending on the image size and image

quality desired, from SD-SDI (called simply

‘standard digital’ in the consumer version)

to HD-SDI (called HDTV in the consumer

version). These fall under specific stan-

dards, controlled by HDMI LLC, and shown

Table 4

Notice the immense data rate required for

1080p delivery. This includes the embed-

ded 5.1 surround-sound and 12-bit or

16-bit deep colour that will be available

in future sources and displays, compared

to the current standard 8-bit per colour.

These cables are severely distance-limited

and the standards do not address the

subject of distance.

There are two ways of testing these cables

and, if they pass either test procedure,

then they pass no matter what that length

might be.

Long HDMI cables

Since the data is split between three

pairs and recombined, there are two

specifications aimed at accurate delivery

of the parts at the other end. One is delay

skew, which addresses the difference

(skew) in the delivery time (delay) of

each pair.

The other is the length of each wire in

each pair, called ‘intra-pair skew’. This is a

new specification for many cable manu-

facturers and test facilities and has not

appeared before in other pair cables, such

as category data cables. A process called

‘bonded-pairs’ – where the wires in each

pair are stuck together without glue –

dramatically improves intra-pair skew,

and makes the delay skew easier to control

and predict.

At the time of delivery of this paper, there

is only one manufacturer of HDMI cable

in the United States. All other cables

are made offshore in Asian countries

[Note 3]

Putting nineteen wires inside a tiny

connector is difficult, which is why raw

cable and installer-friendly connectors are

not available. There is pressure to change

this, and field installable connectors

might eventually appear. However, such

connectors must pass 10.2 Gbps, high data

rates, just for the existing 1080p signal.

Any impedance variation (return loss)

would show up instantly, making HDMI

connectors even more problematic; and

1080p/60 is not the end of the line.

There is significant discussion about

1440p and even higher resolutions. These

discussions also include improving the

frame rate, also called the refresh rate, of

60 frames per second. Proposed rates of

85 frames, 100 frames, even up to 120 frames,

sometimes described as 120 Hertz, makes

signals even larger, and the delivery of

those signals even more complex.

Notes

Note 1: Network analysers are easily available beyond

3 GHz; however, they are 50 ohm input/

output devices, so matching networks must

be obtained to use this equipment at 75 ohms

at any higher frequency or bandwidth. These

matching networks must be tested and

verified (be traceable) to NIST (the National

Institutes of Standards and Technology) other-

wise the results of the testing are not verifiable

or repeatable

Note 2: Since SMPTE 292M establishes a return loss

of –15 dB at the second harmonic of the

clock (750 x 2 = 1.5 GHz) it is consistent that

a new limit for 1080p/60 be set as the second

harmonic of that clock (1.5 GHz x 2 = 3 GHz),

with a limit of –10 dB. However, it should be

stressed that any component should easily

pass this –15 dB/–10 dB requirement, and

any part that does not exceed this minimum

should be rejected. Note that –10 dB return

loss is equivalent to 10% reflection, a

significant amount

Note 3: USA-manufactured HDMI cable is just cable,

as no connectors are available in the USA or

anywhere else outside Asia. The US-produced

cable is shipped to Asia to have connectors

attached. To be completely accurate, these

finished cables are made from US– and

Asian–manufactured parts and assembled

in Asia

Note 4: At NAB two years ago, a major Canadian chip

manufacturer demonstrated a prototype chip

driving professional 1080i along 2,000 feet

of RG-6 (1694A) with a perfect eye pattern at

the end. The formula states 400 feet, further

emphasising that these distance are safe

distances and real-world applications could go

much further. If one wishes to go further than

the calculated distance, then those cables

should be individually tested for bit errors or

eye patterns

This paper was first presented at the 57

IWCS and is reproduced with the permission

of the organisers.

Belden

San Francisco, California, USA

Email

shlampen@aol.com

Website

www.belden.com

Standard

Clock pair

Data rate per pair Total data delivery