The converter
The tested setup uses an RGB-Pi 2 to convert HDMI from the PC into analogue RGB for the television’s SCART input.
I already had a Bang & Olufsen BeoCenter 1 connected to a Raspberry Pi through an RGB-Pi adapter. It ran the vendor’s system image. The hunt for a BeoCenter 1 and the RGB Pi journey describes that setup. OmaCRT came later, when I wanted to drive the same television from my desktop PC.
What it is
An RGB-Pi 2, an HDMI to SCART converter built around a Chrontel receiver. It takes HDMI on one side and puts red, green and blue on the television's own pins on the other, with the audio arriving on the same cable and on a minijack. It is powered from the HDMI connector's five volt pin, so there is no supply to find room for behind the set.
The vendor sells it for the Raspberry Pi and documents that use. OmaCRT drives it from a PC.
Timing passthrough
The converter presents itself as an HDMI monitor and passes through the supplied timing. It applies no scaling or deinterlacing and does not force a fixed output resolution.
HDMI’s minimum pixel clock is 25 MHz, above the clock needed for a narrow 15 kHz mode. The graphics card sends a wide modeline above that minimum. The converter produces the analogue 15 kHz signal. The signal explains the modelines.
The pixel clock it holds
The 25 MHz minimum is a floor set by HDMI. The converter has a ceiling of its own, and it is not the same kind of limit: it was found by measurement on this device rather than read from a specification.
The converter holds 48 MHz and 72 MHz. It does not hold 74. Its lock register was polled a thousand times a second, on a still menu with nothing else running. The 240-line timing at 72 MHz lost lock not once in 60 seconds. The 288-line timing at 74 MHz lost it 80 times in 60 seconds. The same 288-line timing at 72 MHz lost it not once in 90 seconds.
Every loss lasted between 276 and 281 milliseconds, and a loss of lock is a loss of picture. The clock is the only difference between the second measurement and the third. Both shipped modelines therefore run at 72 MHz.
omacrt dac listen is the instrument. It polls the lock register at 1 kHz, timestamps every change and resets nothing, so it can be left running while somebody watches the screen. omacrt dac watch polls the same register with the vendor’s workaround attached, which resets the converter when lock drops and costs about 2.3 seconds of black each time.
Sync and audio setup
The host configures sync and routes audio when it switches the television on.
At power-on, the converter sends only horizontal sync to the television. The picture can lock horizontally while rolling vertically. OmaCRT selects composite sync over the HDMI control bus and repeats that step after each power cycle. Changing the modeline does not correct this sync setting.
The converter supports two sync-combining modes. The television determines which mode works: the BeoCenter 1 locks to one, while broadcast monitors often need the other. OmaCRT applies the configured mode when switching the television on.
The converter uses the graphics card’s HDMI audio output. With the audio profile used here, a desktop monitor on the same card loses HDMI audio while the television uses it.
Known hardware issues
This hardware revision has a phase-locked-loop decoupling problem that can make the picture jump. RetroRGB reported it, and the vendor acknowledged the cause. OmaCRT monitors the lock register and resets the converter when lock drops. The vendor’s system image uses the same workaround; it does not fix the hardware fault.
RetroRGB’s oscilloscope measurements found that green on this revision is about a tenth of a volt lower than red and blue. This can affect colour accuracy compared with a reference converter. The colour gains in the television profile can correct this difference: increase the green gain, or decrease red and blue, until the grey ramp shows no colour.
For other converters, the diagnostic check reports that sync must be configured on the device. It accepts the absence of the RGB-Pi 2 control bus. OmaCRT does not identify or control another converter through that bus.
Other converters
Other HDMI-to-RGB converters may work if they pass through the required timings. What to buy compares the HDMI and DisplayPort paths, their kernel requirements and VGA sync options.
Technical reference
The control values, the reset sequence and the register that reports lock are in the project's repository, in docs/rgb-pi-2.md. They were read from the device and from the published parts of the vendor's own system image, and they belong next to the code that uses them.