In which I investigate tv control and accidentally stumble over the HDCP protocol.
I’ve been investigating controlling my tv from my media box recently. There are three four main possibilites:
- IR blaster
- The service port on the TV
- DDC/CI
- CEC
1 – will work, but I’d like a finer degree of control
2 – I have the TV service manual, but it doesn’t really give the pinout of the service port
3 – Is the standard DDC/CI monitor protocol.
4 – CEC is docced in the HDMI specs, but I don’t think it works over my HDMI->DVI cable (but my new video card has an HDMI output so maybe it would be accessible over that). Confirmed – CEC is not done over i2c for some irritating reason – there’s a seperate 1 wire bus for it which is not exposed over DVI. Also, I don’t think the card drivers expose an API for controlling it. You can get extra hardware that intercepts it, but that doesn’t mean the TV supports it.
So I’ve been investigating 3 first of all. The TV DOES support EDID reading, but DDC/CI is seperate protocol for the PC to control the settings on the TV (e.g. change input, power it up etc). This tool, ddccontrol implements the protocol, but says my TV doesn’t support the DDC/CI protocol. Bum.
Using i2cdetect however, shows there are mysterious i2c devices on address 0x3a and 0x51 (0x50 is the EEPROM with the EDID information):
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- 3a -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: 50 51 -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
So I did some googling. Apparently the device on 0x3a is how the HDCP protocol is tunneled over the connector. This doc documents the various HDCP registers for one chip’s implementation. Nothing very useful from my point of view in there though.
I wonder what the one at 0x51 is? — probably just part of the EDID eeprom I’d guess
adq 