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While looking for some HDMI-to-RCA converters I found out that most of them are simple cheap cables from China without any conversion circuits

They come in various forms, from component only, composite + audio to component + audio

component + audio cable

Many of them have description like this

  • This cable functions as a signal transmitter, but NOT a signal converter.
  • As HDMI carries digital signals while RCA carries analog signals. In order for these two formats of signal to communicate with each other, both input and output devices needs to support the signal conversion function (code and decode), please kindly check with your user manual. If both input / output device do not support signal conversion, a signal conversion box is required.

That's highly strange because AFAIK HDMI doesn't support analog signal transmission, and indeed it's doesn't

HDMI can only be used with older analog-only devices (using connections such as SCART, VGA, RCA, etc.) by means of a digital-to-analog converter or AV receiver, as the interface does not carry any analog signals (unlike DVI, where devices with DVI-I ports accept or provide either digital or analog signals).

Cables are available that contain the necessary electronics, but it is important to distinguish these active converter cables from passive HDMI to VGA cables (which are typically cheaper as they don't include any electronics). The passive cables are only useful if you have a device that is generating or expecting HDMI signals on a VGA connector, or VGA signals on an HDMI connector; this is a non-standard feature, not implemented by most devices.

https://en.wikipedia.org/wiki/HDMI#Legacy_compatibility (line break added by me)

From the last part above it seems that there are some monitors that accepts HDMI signals in the VGA port, so no conversion is needed. But the cables I found are all RCA and component ones, not VGA, thus need some converter.

Even if passive HDMI-to-RCA devices exist, how do they map the HDMI pins to the RCA pins?

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    You're assuming everybody follows standards but that's not the case. This cables are for analogue signal only that happens to use the same connector as HDMI but isn't HDMI. Many years ago there were a few Intel Atom based Windows XP "tablets" from China that featured one of this "pseudo-HDMI" instead of the typical DSub for VGA because the smaller connector fits the tablet form much better. Those used a similar "HDMI" to VGA adapter cable, not converter, and couldn't be used with any true HDMI (digital) devices. – GabrielaGarcia Feb 8 at 16:28
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    I suspect that the device intending to output to the HDMI port (video card) is set to output analog signals. Typical devices receiving RCA video input are not smart enough to convert digital to analog internally. – Christopher Hostage Feb 8 at 17:29

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