On 6 Oct 2003, Florin Andrei wrote:
> "this procedure forces blocks of a type that don't carry much
> information but are expensive to encode to be simply skipped"
>
> If they are skipped at encoding, with what do they get replaced when
> viewing?
Oops - misunderstanding alert ;) It probably wasn't worded
exactly right.
After going thru the quanization process you have a matrix of
coefficients. Rows which have only a single non-zero value
are quite expensive to encode (they require more bits in the
output stream). On the other hand rows which consist solely
of zero need very few bits in the output stream.
The elimination logic examines the matrix of coefficients
and if it finds a single value over the specified threshold
that one entry is replaced with zero. Now you have an entire
row of zero values that will be sent into the next step of the
encoding process.
It is not a matter of "skipping" the blocks but rather of turning
an almost complete row of zero entries into a row that is all
zero.
The savings vary according to the quality of the source data.
Good clean (not noisy) data benefits the most. I had one
capture from a laser disc via S-Video cable into a Canopus
ADVC100) that saw a a bitrate reduction of 23.2%. Then too
I had a junky (very bad quality) VHS capture that only saw a
2% reduction in bitrate.
-E -10 is a conservative (middle range) setting. Values of 20
or more _may_ artifact - your mileage will vary of course ;)
Cheers,
Steven Schultz
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