Electronics > Projects, Designs, and Technical Stuff
Making a HDMI audio type detector
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teddyng:
I want to make a device that goes inline on a HDMI to detech what type of audio it is (PCM, dolby digital, dolby digital+...) which I know is possible as you can normally see it on a tv or amplifier but not sure how to approach it? Can anyone help me out with some ideas?
soldar:

--- Quote from: teddyng on November 12, 2019, 01:04:28 pm --- I want to make a device that goes inline on a HDMI to detech what type of audio it is (PCM, dolby digital, dolby digital+...) which I know is possible as you can normally see it on a tv or amplifier but not sure how to approach it? Can anyone help me out with some ideas?
--- End quote ---

As all HDMI signals are digitally encoded I assume examining the contents of the digital streams will say somewhere what the frame or packet contains so it can be interpreted and decoded.

Also, different versions of HDMI have different capabilities so I suppose the version is also specified in the transmitted data.

In any case, you will need to analyze the data stream and interpret it using the HDMI standard.
Yansi:
HDMI sink signals it's supported formats to the HDMI source using the DDC/EDID, using SAD (short audio descriptors).

Can somebody fill in my gap in knowledge, how the HDMI sink then determines in what format is the audio data received?

(That will also probably be the partial answer for the OP how to grasp the problem)

Usually, HDMI receiver codec just provides a multichannel raw audio data (4x I2S or TDM), that then gets interpreted by the decoder. Question is, how the decoder decides, whether it is receiving raw LPCM, or compressed garbage?

Thx, Y.
soldar:
I see in Wikipedia that there are several audio formats which can be optionally supported. I am guessing there is a protocol between sender and receiver where the sender lists its capabilities and the receiver chooses what format it prefers.
Yansi:
There is no bidirectional communication over HDMI, other the CEC. And I'm not sure if CEC can be used to negotiate the format. (Probably not?)

The I2C lines (Vesa DDC / EDID) is only unidirectional as far as I can tell. Signal source reads (read-only) content from the sink and opts to behave accordingly to the list of supported formats, as encoded in the DDC memory.

HDMI is unfortunately a lot of mystery, as it is a heavily commercialized pay-walled interface, just like MIPI. You're not simply gonna get any specs for it, sometimes you are not even gonna get full datasheets for ICs that interface either of those two. Doh!

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