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I understand that an HDMI cable contains essentially 4 shielded twisted pairs, along with additional control lines. Each of the twisted pairs is specified, to my understanding, to be a balanced transmission line with a characteristic impedance of 100Ω.

Now, since (ISO 11801) category 6/7 (shielded) twisted pairs are also balanced transmission lines, with a characteristic impedance of 100Ω, I'd assume that such a cable (or two), given enough (shielded) twisted pairs to carry all of the signals, can serve as an alternative medium to the standard HDMI cable (with the proper connectors at the end).

However, it seems that an "HDMI over Cat. 5/6/7" solution requires a device which is called a "balun" on both ends, at the interface between the proper HDMI and the ISO 11801 cable.

I understand baluns and transformers have a role at the interface of cables with different impedances, or when one is balanced and the other isn't. However, in this case, the lines in both standards are specified to be balanced 100Ω lines.

Why then are "baluns" required in this case? Are they really baluns, or do they provide some other function?

Thanks in advance.

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I suspect HDMI to Cat6 converters don't actually contain baluns, because as you point out HDMI TMDS signals are balanced signals just as UTP pairs are balanced.

In the days before HDMI, video signals were generally carried on coaxial cables. To avoid using expensive coax, many manufacturers came up with a method to use cheaper Cat5 cable instead. Since coax is unbalanced, and Cat5 is balanced, you must use a balun (a balanced to unbalanced transformer) to connect the two. A/V technicians became used to this term to mean a device that converts from one cable type to another.

I believe HDMI to Cat6 "Baluns" are simply called that because A/V technicians are familiar with the term.

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  • \$\begingroup\$ Thanks for your reasonable explanation. I gather that you aren't actually familiar with the internal circuitry of these devices, so I'll wait for a while to see if there are other responses for the question. \$\endgroup\$ – Sagie Jan 26 '15 at 13:35
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One possible reason for using a Balun is that it isn't a balun, but, instead, a 1:1 isolating transformer. This is often necessary to provide galvanic isolation so as to not exceed the allowable common-mode voltage on the differential pair.

Note that a 1:1 balun looks exactly the same as a 1:1 transformer. The only difference is that one side of the balun is connected to an unbalanced signal.

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