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Indicator |
Description |
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[MPG2] |
Indicates the file is "parseable" as an MPEG-2 file, to the
extent required to enable the remaining indicators. If this is
not the case, all the indicators described below remain gray. |
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The remaining indicators provide technical detail about whether
or not the video is interlaced, and if so, how. Terminology
note: In interlaced video, each frame is composed of two
fields, the "top", or 'A' field (the one containing line
#1), and the "bottom", or 'B' field. In MPEG, the decoder
converts compressed data into a picture. That
picture can represent a single field or it can represent the
pair (as well as other combinations). |
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These two indicators indicate "interlaced" or "progressive scan"
(non-interlaced), respectively. Inclusion of the pair is a
convenience, albeit a bit redundant, as the
two are complementary: any time one is active the other is
not. They may, however, both be grayed; as with the other
indicators, this simply means the information
is not available. |
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[TFF] |
Top field first. Frames consist of two fields from the same
picture - first the top field is extracted, then the bottom. |
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[BFF] |
Bottom field
first. As above, but blah blah blah... |
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[PPF] |
Picture-per-field: Each field is encoded separately with its own
picture (requires two pictures per frame). Not too common. |
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[3:2] |
Player telecine: On playback, 24 FPS material in the file is
converted to 30 FPS (60 fields / sec) using "3:2 pulldown". In
other words, the pictures encoded in the file are the original
24 FPS material, and the player or decoder converts each of the
pictures to either two or three fields, "on the fly". |
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Tech notes |
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Notes on the algorithms for the more common indicators |
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[TFF] and/or [BFF] indicators are active if the one or more
occurrences of the corresponding MPEG bit of the same name is
seen in the first ~1MB of the file, unless 3:2 pulldown is
detected, in which case they are both forced off. This means: |
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You may find them both active; this
will occur if the
file switches from one mode to the other during the first 1MB. |
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Even if they are not both active, you are not guaranteed
the file doesn't change someplace after the first 1MB.
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Like the others, the
[TFF] and [BFF] indicators are simply a quick denotation of what is seen near the start of the file. |
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By contrast, the [3:2] indicator does not correspond to
any MPEG bit; it's derived from five or six other bits.
Interpreting these other bits allows GSpot to count up displayed
fields and compare that to count to the count of encoded
pictures. If the ratio, as sampled over the first 1MB, is 2.50
(give or take a bit), the [3:2] pulldown indicator is
activated. At the same time, TFF & BFF, regardless of whether
they were used (and they almost certainly were), are forced off
for clarity. |
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Another, inferior pulldown method that exists is where the pulldown is done first,
externally,
and that result is simply encoded straight like any other video.
GSpot does not attempt to identify or distinguish film files
encoded this way; the [3:2] indicator is specific to the method
as described above. |
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