Demystifying SD video color spaces

[rgr]

So, no need to adjust anything in Hybrid atm., right?

Cu Selur

I know about the program, but I don't use it, so I don't know. However, in the new ffmpeg, setting flags with -colorspace, -primaries and -color_tbc (@ output) has partially disappeared, so such properties are now safer to set with "-vf setparams".

But I found an interesting fragment regarding the differences between BT470BG and BT601.

Exact Values in Standards
The luma Y formula in RGB for both systems is:

Y=kR⋅R+kG⋅G+kB⋅BY=k R⋅R+k G⋅G+k B⋅B

In BT.601 (digital SD PAL, NTSC, etc.): kG=0.587k G=0.587, though sometimes rounded or listed as 0.30 in older docs. For analog PAL (BT.470 system B/G/I): coefficients were closer to 0.30R + 0.59G + 0.11B, but practical VCR/VHS implementations had drifts, creating a perceived higher green level.

Why the Difference Causes "Green" Effect
The gap between 0.30 and 0.299 (or precise 0.587) is tiny (~0.3%), but during VHS capture (analog path with imperfect Y/C separation) + conversion to digital YCbCr BT.601, it causes a mismatch: the green channel gets disproportionately boosted or shifted in the color vector. In practice, grabbing VHS PAL to a BT.601 card assumes the newer coefficient, which on analog VCR output (older bias) results in a slight green boost in decoded RGB/YUV.

Practical Impact in Workflow
During VHS PAL digitization to digital SD (e.g., DV, MPEG2 720x576) without correction, the green level appears higher than in native digital PAL (studio, DVD), due to analog bias + matrix mismatch. In AviSynth/FFmpeg: use ColorMatrix with source=rec601 > dest=rec601 (or bt470 if available), plus Levels(16,1.0,235,0,255) + manual tint shift for VHS.

Correction in Tools
In FFmpeg: -vf colormatrix=bt601:bt601 with analog input flag, or custom matrix with k_G=0.30 for VHS source. AviSynth: ConvertToYV12(matrix="Rec601") after ColorMatrix(source="Rec601",dest="Rec601"), but verify waveform on green vector. These values stem from standard evolution: analog PAL used simplified ~0.30G coefficients, while digital SD refined to 0.587G (with 0.299R+0.114B=0.413, rest G).