27-09-2017, 06:37 PM
Its easy to understand how a 2 line interpolater works and in a 3 line interpolater it is easy to imagine how the output line can straddle 3 input lines as the output line is wider than a input line. From a 4 line interpolater up it gets difficult to understand how the extra input lines have a positive effect on the output line. I thought it would be interesting to compare interpolaters with different numbers of input lines.
In a BBC Research Department document titled "DIGITAL LINE STORE CONVERSION: determination of the interpolation aperture function" there is a graph showing what was considered the optimum aperture for 2,3,4,6 and 8 lines. I took measurements from the graph and made coefficients for the interpolater as accurately as I could.
I have rewrote the interpolater so can now be switched between 2,3,4,6 and 8 lines.
What I have observed so far is
On Test card C I cannot not see any difference between 2 and 3 lines.
There is what looks like an improvement between 2 and 4 lines, looking at the diagonal lines on TCC they get sharper with the whites getting whiter, similar to the contrast being turned up.
There is little difference between 4,6 and 8 lines.
6 and 8 lines display slight ghosting above and below certain areas. a caption is pictured below displaying it. I don't think it would be very visible on a moving picture.
It is easier to compare the 4,6 and 8 lines to 2 lines as the majority of the information is coming from the same two input lines with the additional lines just adding to that.
With the 3 line the majority of the information is coming from different lines to the 2 line. This has an effect that is very noticeable when switching between 2 and 3 lines if viewing captions, as the letters change in height, they either get taller or shorter depending on what lines they are on.
A photo below shows the same section of a caption viewed on 2 and 3 lines. The thickness of the horizontal bar of the "T" varies quite a bit between the two.
Frank
In a BBC Research Department document titled "DIGITAL LINE STORE CONVERSION: determination of the interpolation aperture function" there is a graph showing what was considered the optimum aperture for 2,3,4,6 and 8 lines. I took measurements from the graph and made coefficients for the interpolater as accurately as I could.
I have rewrote the interpolater so can now be switched between 2,3,4,6 and 8 lines.
What I have observed so far is
On Test card C I cannot not see any difference between 2 and 3 lines.
There is what looks like an improvement between 2 and 4 lines, looking at the diagonal lines on TCC they get sharper with the whites getting whiter, similar to the contrast being turned up.
There is little difference between 4,6 and 8 lines.
6 and 8 lines display slight ghosting above and below certain areas. a caption is pictured below displaying it. I don't think it would be very visible on a moving picture.
It is easier to compare the 4,6 and 8 lines to 2 lines as the majority of the information is coming from the same two input lines with the additional lines just adding to that.
With the 3 line the majority of the information is coming from different lines to the 2 line. This has an effect that is very noticeable when switching between 2 and 3 lines if viewing captions, as the letters change in height, they either get taller or shorter depending on what lines they are on.
A photo below shows the same section of a caption viewed on 2 and 3 lines. The thickness of the horizontal bar of the "T" varies quite a bit between the two.
Frank
