A Question about Colorimetry when converting HD to SD

[seven_deuce offsuit]

Okay, so the more I read about it the more confused I get.

I'm trying to convert a HD 1080i source I recorded off my DVR to a SD 480i file for DVD output. This is the script I'm using:

SetmtMode(5)
MPEG2Source("file.d2v")
SetmtMode(2)
Colormatrix(mode="Rec.709 -> Rec.601", interlaced=true)
AssumeTFF()
interp=NNEDI3(field=3)
Yadifmod(Order=1, field=-1, mode=1, edeint=interp)
Spline36Resize(720,480)
Separatefields()
Selectevery(4,0,3)
Weave()
ConverttoYUY2()

My question is do I need the Colormatrix line when going from HD to SD in this particular situation? From my understanding, changing colorimetry (PC.601, PC.709, REC601, and REC709) in Avisynth is only needed when going from YUV -> RGB and vice versa. But since I'm not converting to RGB, there's no use changing colorimetry right? My HD source is YV12 and it's being converted to YUY2 for my encoder (CCE); the source stays in YUV from input to output.

What's confusing to me is that I've read a few threads similar to what I'm doing that say switching colorimetry is needed when going from HD to SD since REC709 = HD and REC601 = SD. Although, if I'm reading correctly, if you're not changing from YUV to RGB then something like Colormatrix isn't needed.

Which is right for my scenario? Can I just skip the Colormatrix line or no?

[manono]

My question is do I need the Colormatrix line when going from HD to SD in this particular situation?

Yes.

Colormatrix(mode="Rec.709 -> Rec.601", interlaced=true)

And you'll probably want to disable clamping:

Colormatrix(mode="Rec.709 -> Rec.601", interlaced=true,Clamp=0)

Or however your version does it. Check your doc.

And I don't think you really need those slow bobbers. A fast Yadif should do just fine (or even a simple Bob as far as that goes). You're resizing both fields, but taking the original one (not the interpolated one) when reinterlacing it. Someone else may know more about that than I.

ConverttoYUY2()

Since it's still interlaced you'll probably want to use:

ConverttoYUY2(Interlaced=True)

[seven_deuce offsuit]

Thanks manono, but I'm confused why I need the Colormatrix function still. I thought going from REC709 to REC601 applied only to YUV <-> RGB conversions.

[jagabo]

My question is do I need the Colormatrix line when going from HD to SD in this particular situation? From my understanding, changing colorimetry (PC.601, PC.709, REC601, and REC709) in Avisynth is only needed when going from YUV -> RGB and vice versa. But since I'm not converting to RGB, there's no use changing colorimetry right?

But when the video is displayed it will be displayed in RGB. The TV (or whatever converts to RGB) needs to know whether to use the 601 or 709 matrix to perform that conversion. MPEG 2 supports color matrix flagging so, if you use an encoder that supports flagging, you could use either matrix. What I'm not sure about is whether the DVD spec includes both the 601 and 709 matrices. To be safe you should use 601 because we know for certain that DVD supports 601.