Nevion UDC-3G-XMUX4+ - Manual (Page 27)

UDC-3G-XMUX4+ Rev. C
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3.6.3 Aspect Ratio Converter block
The aspect ratio converter block may be used to stretch or shrink a picture vertically and/or
horizontally. The picture may also be offset with respect to the original centre of the picture.
The primary difficulty with selecting the conversion is the sheer number of possible
conversions. This can be greatly reduced by specifying the desired output aspect ratio (4:3 or
16:9). We call this setting the output environment. The actual scaling will depend on the
input signal.
The output environment setting actually describes the aspect ratio of the pixels. The fill
factor is a term to describe the degree to which the output picture will fill the output frame, or
the presence of horizontal or vertical curtains or black bars.
After conversion the output signal will have the appropriate AFD, VI, WSS and S352M
embedded. These types of metadata can also be individually disabled.
There are four operational modes for the module:
1. AFD -> Frame fill setting -> default conversion
2. AFD -> default conversion
3. Frame fill setting -> default conversion
4. Default conversion, no auto
The primary assumption for the first three modes is that an input signal with the same aspect
ratio as the output environment will not be scaled. (There are a couple of exceptions if the
picture has both horizontal and vertical curtains.)
In mode 4 the zoom and position settings from the selected scaling will be applied for all
input signals, regardless of the detected input environment. If one of the pre-defined fixed
scalings is used, the output environment will be given by the selected scaling. The only
exception is “No conversion”, which has no inherent output aspect ratio; when this scaling is
selected, the output environment is taken from the general output environment setting. For
the user scalings, the user must specify the output environment. This also provides an
opportunity to change the output environment via GPIs, see chapter 3.6.7.
3.6.4 Automatic scaling modes
The following applies to the first three (automatic) modes of operation:
The scaling performed by the module is determined by the input picture aspect ratio and fill
factor (presence of ‘curtains’) but normal SD video does not natively state of what aspect
ratio the pixels are, or if another conversion has already been applied. There are three
sources of information that may be present in the video that can provide some or all of this
information.
Active Format Descripton (SMPTE 2016 and SMPTE RP186, the latter referred to as Video
Index) describes both the aspect ratio and the fill factor of the picture. However, the fill factor
descriptor may contain a code to indicate that the fill factor of the picture is unknown. In that
case, the selected fill factor setting is used.
SMPTE352M is a data packet that can be used to identify the aspect ratio of the picture, and
thereby the pixels.