Stephen R. van den Berg writes:
> [ If you've seen this submission before, I apologise, but it appears
> that it got discarded because I wasn't subscribed to the list yet. SRB ]
>
> I'm trying to figure out the data format used in the Bt878 for
> the BT848_COLOR_FMT_RAW VBI frame outputmode.
>
> The Conexant PDF files only talk about "RAW 8X Data".
> Yet they don't specify what the exact format is of this data.
> As it seems, indeed 8 bytes per pixel are being recorded. I tried
> using all kinds of YUV permutations on it to get a normal color-display.
> To no avail.
>
> The only thing I can see is that, if I try and use the 8 byte blocks
> as -bgr values, that I get a seemingly normal gray-scale image.
> However, in that image, true grey colours give a steady on-screen image,
> but non-grey colours show de-Moirre interference (something is alternating
> the values presented).
>
> Any pointers?
"Raw 8x data" data is the raw composite signal sampled at the full
sampling frequency of the Bt848 (~35MHz for PAL).
The interference pattern is not surprising since the color information
is encoded at a specific carrier frequency (which depends on the used
color norm).
So, you actually get several samples per actual "pixel". Those oscillate
around the luminance value of the "pixel" with the frequency of the color carrier.
Just write the necessary frequency filters and extract the color information.
The Bt848 data sheet shows the filters which are used in non-raw modes.
But I think the raw data is only really useful if you want to sample
VBI data or maybe for high quality still pictures.
Ralph
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