Nevion ARC-SD-XMUX4 - Manual (Page 9)

ARC-SD-XMUX4 Rev. B
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3.3.2 Manual selection mode
If the SDI signal disappears the board will frame freeze indefinitely.
3.4 De-glitcher
The de-glitcher corrects timing errors within a line of video due to source switching. This
allows perfect synchronous switching.
Non-synchronous switching can result in a frame that is split between the old and the new
video, but the output will always be continuous.
3.5 Aspect Ratio Converter block
The aspect ratio converter block is a 13 tap high quality linear resampling scaling engine. It
may be used to stretch or shrink a picture vertically and horizontally. The picture may also be
offset with respect to the centre of the picture.
The block can detect a change in aspect ratio information embedded in the input signal and
change the scaling during the vertical blanking period allowing on-air automatic switching of
aspect ratio conversion.
Externally triggered changes of aspect ratio are also deferred until the next vertical interval to
allow the use of the module in a transmission signal path.
The module is intended to be used primarily to convert SD video between standard aspect
4:3 and widescreen 16:9.
The primary difficulty with the conversion is the sheer number of possible conversions. This
can be greatly reduced by setting the output aspect ratio to be 4:3 or 16:9. We call this the
output environment. The actual scaling will then depend on the input signal.
The output environment setting actually describes the aspect ratio of the pixels. The fill
factor is the term for the amount that the picture fills the output frame, the presence of
horizontal or vertical curtains or black bars.
The output signal will have the appropriate AFD, VI WSS and S352M embedded. All of these
metadata types may also be 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. Fixed default conversion
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.)
The scaling that is set in default conversion will be used for all input signals.
3.5.1 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 what aspect 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 Descriptor (SMPTE 2016, referred to as AFD) and Video Index (SMPTE
RP186 referred to as VI) describe both the aspect ratio and the fill factor of the picture.