The HDMI signal contains a digital "language" like no other, as discussed in the previous page, and it doesn't work if source and display devices don't speak the same language fluently. The bandwidth defines the capacity of the "highway" that gets the information from point A to point B. It really is just a potential value, and not one that is maximized at all times. The number of lanes on a highway may determine the potential for traffic capacity, but it doesn't mean that there it's always being used to that capacity. Likewise, the maximum power output of a car is not used at all times - it's just a maximum limit. Bandwidth is the same.

For those of you who are technically minded, here's the bandwidth characteristics of the two current HDMI formats, expressed in various common values;

Bandwidth	HDMI 1.2	HDMI 1.3
MHz - test	74.25	74.25 (Standard Speed) 165 (High Speed passive) 340 (High Speed EQ)
MHz - max	165	340
Gbps - max	4.95	10.2

To give you some idea of just how great the bandwidth of HDMI really is, check out the comparisons below;

Format	Bits/second (max)
Dial up internet	56,000
ADSL 512 internet	512,000
ADSL 1.5k internet	1,500,000
802.11b "wi-fi"	12,000,000
ADSL-2 internet	24,000,000
802.11g "wi-fi"	54,000,000
HDMI 1.2a /DVI (max)	4,950,000,000
HDMI 1.3b (max)	10,200,000,000

* NOTE: 1kbps = 1000 bits/second. 1 Byte = 8 bits

To continue the use of the car analogy, and to see how bandwidth plays an important role in HDMI, here's the simplified breakdown of what each part of the HDMI system, and it's in-car equivalent does;

• Source device is the car's driver
• HDCP is the key to start the ignition (see next page for more info)
• Bandwidth is the capacity and power output and transfer potential of the engine and drivetrain
• Bistream is the actual power generated by the engine at any given time
• The Connectors and Cable, and the PCBs inside each device, are like the many links that join the accelerator pedal (controlled by the driver/source) through to the drive train and ultimately delivers power to the tyres
• Sink/Display is equivalent to the tyres - ie: the final point of contact

The analogy goes something like this - The source/driver starts the engine with an electronic HDCP key; no key, no start. He/she then controls the power output "bitstream" by adjusting the accelerator pedal forces, together with gear selection. This starts the bitstream "power" flowing through to the system to the tyres/sink. In a car, the wheels/tyres turn, whilst in a display device a picture results. Going slow in a car may be likened to a low resolution, low bandwidth image, whilst going fast is equivalent to transmitting a high definition image - ie: more bandwidth used. It is well accepted that ALL elements in a high performance car must be present for the car to operate to its potential. This is no different in a high performance AV system - ALL elements including source, cables and display are critical. Compromise on any one may result in a compromise of the end result - SIMPLE.

Note that the bandwidth of a system is expressed as "capacity" and "potential" only, as they are not definitive of the actual flow. That is, the bandwidth potential is the capacity, being quite distinct from the actual bandwidth of the signal being transmitted. For example, the max power output of a car can be likened to it's potential bandwidth, whereas the actual power being generated in a car is invariably less than this to varying degrees. In a HDMI 1.2a spec cable, the max bandwidth capability is 165Mpixels/second, or 4.95Gbps, but the actual bandwidth of the signal being transmitted may be far less. For example, a 480i NTSC signal would be only about 13Mpixels/second, a small fraction of the 165Mpixel/second potential.

This analogy follows thorugh in that it clearly shows that the whole system depends on all components being of equivalent potential or performance. You can't put Ferrari tyres on a Daewoo to make the Daewoo go like a Ferrari (no offence Daewoo!!). Likewise you can't buy a true HD display and expect it to give you proper 1080p from anything but a true 1080p output source (we're talking true 1080p here, not a up-scaled version), and connect them with cables which are capable of passing the full 1080p with good integrity.

The bottom line...

Keep all HDMI compliant devices in your system of a similar performance potential, and connect them all using only proven cables and accessories which you know to be capable of giving you the results that you expect.

Next Page - HDCP