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Old 03-26-2009, 11:06 PM   #1
lalittle
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Default Thin horizontal lines "smear together" when encoded to mpeg2.

I've been doing some encoding test in order to dial in the best settings for DVD creation from various types of Avid projects. For one test I created a project in Avid MC that consisted of a solid black screen with colored lines for the top and bottom 5 lines.

When I encode this in PC3 using high bitrates and the "mastering" setting, the lines become completely smeared together in the resulting encode. I found similar results when using CCE Basic to encode the same material.

Is this a limitation of encoders in general -- i.e. do they not maintain the single-line detail that would be needed to resolve the colored lines in my test image? I would have thought that the resulting encode would look virtually identical to the source, but this is not the case with the single-line detail in this test.

Thanks,

Larry
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Old 03-26-2009, 11:26 PM   #2
GrassValley_BH
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Your result is comes from the encoding format (way MPEG-2 works), and the color sampling. MPEG-2 for DVD uses 4:2:0 chroma subsampling.
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Old 03-26-2009, 11:36 PM   #3
lalittle
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Quote:
Originally Posted by GrassValley_BH View Post
Your result is comes from the encoding format (way MPEG-2 works), and the color sampling. MPEG-2 for DVD uses 4:2:0 chroma subsampling.
I could understand the color sampling causing colors to display a bit odd, but does this explain smearing of the luminance as well? The actual group of individual lines is turned into a single, thick "fuzzy" line. The detail of the lines -- both luminance AND chroma -- is smeared and "fuzzed" together. Can this entire effect be caused by just the chroma subsampling of mpeg2?

Thanks,

Larry
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Old 03-26-2009, 11:48 PM   #4
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To remove chroma from the equation, try encoding a grayscale version.
But the chroma subsampling has a definite effect on the visual sharpness.

What bitrate is the encoding? Lower bitrates will also cause loss of detail.
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Old 03-27-2009, 04:21 AM   #5
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Quote:
Originally Posted by GrassValley_BH View Post
To remove chroma from the equation, try encoding a grayscale version.
But the chroma subsampling has a definite effect on the visual sharpness.

What bitrate is the encoding? Lower bitrates will also cause loss of detail.
My min bitrate was up around 7 to 8K, so it was fairly high, and I was using the "mastering" setting, so the result was most likely as good as it could be. On top of this, the vast majority of the frame as pure black, so most of the frame would take very little data to encode.

I'll play with testing a grayscale image, but does the result I'm describing sound "normal"? i.e. is it expected that single lines will not fully resolve in an mpeg2 encode?

Thanks,

Larry
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Old 03-27-2009, 06:33 AM   #6
jaegersing
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Hi Larry. In addition to the colour sub-sampling issue, I thought you would also get interlacing flicker when you encode single lines of graphics to DVD format MPEG2. If you then view this MPEG2 file on a progressive screen it will probably look like mush in the areas where the single lines were.

Richard
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Old 03-27-2009, 07:49 AM   #7
SoundFreak
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Quote:
Originally Posted by lalittle View Post
I was using the "mastering" setting, so the result was most likely as good as it could be.
Try, just as a test, to encode the same footage, using the same bit rate settings, but now using "Highest Quality" instead of "Master".

I find that "Mastering" often makes the image a bit soft, which can result in "smearing" like effects.

Coming back to a Single Line in Mpeg2, this will always be mission impossible, you could try to play with Sharpness filter etc, but Mpeg2 compression is limited in SD resolution, facts of life
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Old 03-27-2009, 08:16 PM   #8
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While ProCoder doesn't add extra blur, some other encoders do, as blurring helps remove fine detail which is the bane of MPEG compression.

If you really need to preserve single-line detail, you're going to need to increase the bitrate to the 10-25 Mbps range.
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