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Re: bttv driver errors

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Hi Trent,

Le lundi 18 août 2008, Trent Piepho a écrit :
> What's more, yuv420 works by sending one line at 2 bytes/pixel and the next
> line at 1 byte/pixel.  If the bt878 had a large fifo, the bit rate might
> average out, but it doesn't.  It's something microscopic like 128 bytes.

You're right, it's (in practice) 128 bytes.

While reading the BT878 datasheet to try to better understand how
this all happens, I came to wonder how the chip can actually handle
planar formats with vertical subsampling.

To do vertical subsampling, you obviously need to handle several
lines together (2 in the case of yuv420). However, the FIFO is too
small to contain a complete line of data, and there doesn't seem to
be any RISC instruction for fetching chroma information back from
memory either. This suggests that the BT878 is cheating on vertical
subsampling, and instead of averaging on 2 lines (2x2 chroma
subsampling), it averages on 1 line (2x1 chroma subsampling) and just
skips the chroma information of the next line. Can you please confirm
or infirm this? If I'm wrong then I would be grateful if you could
explain how the BT878 achieves vertical subsampling.

Thanks,
-- 
Jean Delvare
Suse L3

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