Hans Verkuil wrote:
On Sunday 04 December 2005 00:58, Kevin Atkinson wrote:
When playing back an mpeg with encoded VBI data the closed caption
are displaying 1-2 seconds too early. This time seams to correspond
with the encoding delay. Also the pack headers before each VBI
packet are bogus: the System Clock Reference is always 36036 and the
mux rate is not the same as the other pack headers.
From reading the ivtv-devel this seams like a known problem which
isn't fixed because the developer didn't have an NTSC system. Thus,
I am more than willing to work with anyone to fix this problem. I am
a competent programmer myself so if someone can point me in the right
direction I am willing to take a stab at fixing it myself.
Great! It is indeed something that I cannot test myself as PAL doesn't
have CC. The delay can be caused by the encoding, the decoding or both.
I have not studied the code in detail, but here is my guess on what is going
wrong.
My guess is that it is due to the encoder. You start getting VBI data
immenetly but there is a short (1-2 seconds) delay before you start getting
MPEG data, so you drop the some VBI packets. This causes the VBI data to be
displayed 1-2 seconds early since the VBI data is real time while the MPEG
data is delayed. The correct action would be to delay the VBI packets instead
of dropping them. Does this make sense, and are my assumptions correct?
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