Reply To: OSSC Pro: P-LM 576i passthrough gives distorted image
I figured I’d show what the error looks like. Eight R, G, B or Y, Cb, Cr samples are repeated twice and then eight samples are discarded. With a 480i signal I get no picture at all in P-LM mode with passthrough. Adjusting sync and backporch lengths does not make any difference. The HDMI TRX in the receiver just does not like the signal.
Also, the receiver’s OSD cannot be displayed, revealing that the problem occurs in the receiver and not the display. The receiver’s OSD is rendered in an FPGA and sent across with blanking, then superimposed by the HDMI TRX, as we’ll see later.
Note that the screenshot below is a photo as I don’t have a capture card.
Those stripes remind me of the original DOOM. Right-click or press and hold on your phone/tablet to open in a new tab to view the horror in detail.
So, what’s inside the receiver? Pretty much the same components as in all Japanese mid-range receivers at the time (2018-19). The main HDMI TRX is a Panasonic MN864787, which follows the HDMI 2.0 standard. This is accompanied by a Panasonic MN864788A HDMI switch. Finally, an Altera Cyclone IV FPGA handles OSD rendering as well as very basic de-interlacing and scaling. It’s actually the same version of the Cyclone IV as in the OSSC Classic, only in a different package and with 32 MB SDRAM attached.
When picture processing is disabled, the video does not pass through the FPGA. Instead the OSD is scaled and sent to the HDMI TRX along with blanking, allowing it to be superimposed into signals the FPGA picture processing can’t handle.
Since I made a diagram, I figured I’d add some trivia to it. Note the two unpopulated HDMI ports, something Yamaha decided on to avoid the RX-V685 cannibalizing sales of the RX-A880, which is otherwise the exact same product internally (aside from the European version also having a DAB tuner.)
Right-click or press and hold on your phone/tablet to open in a new tab to view the diagram at full resolution.
I’ve tried running the OSSC Pro directly through an input of the HDMI TRX itself, so the HDMI switch IC is innocent. As mentioned before, it’s perfectly happy with the output of A-LM and Scaler modes.
It would be interesting to know if other receivers from the same era and with the same HDMI TRX (Yamaha, Denon, Marantz around 2018) are affected.
What sample repetition mode does the OSSC Pro use in P-LM passthrough mode with standard definition input?
