"SD minimum luma vale" of ADV739x.

Hello,

I have a question about "SD minimum luma vale" of ADV739x.

There is a description about "SD minimum luma vale" on page.40 as follows:

Address : 0x8A, SD Timing Register 0, Bit 6
SD minimum luma value 0 = −40 IRE.
                                    1 = −7.5 IRE.

I tried to confirm the function of this register by ADV7180 evaluation board. (Because ADV7391 is mounted on this board.)
But I can not confirm the effect.
There is no difference with both the image and the CVBS output waveform from ADV7391 on oscilloscope between SD minimum luma value = 0 and 1.
Could you tell us the function of this register?

Thank you.

Best regards.