As Brent said, restoring the signal to 5 volts doesn’t fix other problems in a circuit which I already mentioned. All because the data is re-transmitted doesn’t mean you will get a picture. Devices like the signal restorers could be added into devices but the cost is not cheap. It probably cost less than $2 in parts in big volumes, but it is not the parts that add to the cost of the products. It is the design, re-tooling and testing which cost money. One dollar in parts could easily be $100 or more retail price and that is just not going to happen in inexpensive devices.

The TMDS channels are one way so there is no communication between them. Communication would have to be on the DDC channel and if there are problems on the DDC channel then we wouldn’t get authentication and no picture. The encryption key comes from the source and used throughout. The KSV are part of authentication and its answer I used for encryption. It works like this.

A key is nothing more than an algorithm/ equation. The source uses one of its 40 keys and generates a random number and it then calculates a public key to transmit called the KSV. For simplicity let’s say the source uses key #1 and its equation is X+65. The random number is X. We’ll say X=60, so the answer is 125. The source transmits its KSV derived from key #1 and random number 65. The receiver calculates the answer of 125 from the KSV. But, it doesn’t transmit the answer back to the source directly because you never want to transmit the answer or the algorithm used.

Instead it uses one of its own key numbers lets say key #20 and a calculates the number to be used with it. Key #20 is subtract 50 from this number. So that that number becomes 175. So when the source receives the KSV that was derived from key# 20 and the number. The answer is the same 125 and the data stream will now be sent. That answer of 125 is the stream cipher encryption as a simple binary XOR operation. If you don’t know what XOR is look it up. So in reality the transmitter (source) sets the encryption and the receiver confirms it. All part of the authentication process which happen ~ every 2 seconds. I think repeaters transmitter have to use the same encryption as the source or authentication would not be valid. I don’t believe it generates its own KSV that would not be a repeater then. The repeater does re-transmit it effectively making a separate communication with the display but authentication/ encryption is set by the source.

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