Managing AFD
Jan 1, 2009 12:00 PM, By Clarence Hau
Keep image format under your control.
Production scenarios
One of the main benefits of AFD is the ability for program providers to maintain creative control of their content. With proper use of AFD, broadcasters can maintain this creative control by allowing the audience to view content as the content creator originally intended.
Table 1. AFD usage for HD to SD downconversion
Click to enlarge
The most proven use of AFD is applied at the final stage of production — not on individual production elements. Within this model, the proper AFD code is recorded onto the final version of content as it will be aired, whether it be a live show, recorded program or commercial spot.
From an operational standpoint, a predefined downconversion format should be assigned to HD programs or commercial spots through application of the proper AFD code.
For live programming produced from a control room, a device can be used to insert the proper SMPTE 2016-3 AFD code into the main program output. This will help to ensure that the proper AFD code is attached to the final version for air and available to all downstream distribution systems. (See Figure 5.)
Figure 5. AFD insertion in live control room
For preproduced programming, a number of options are available for AFD authoring. If the output of the production session is HD-SDI, the same SMPTE 2016-3 AFD insertion described above can be effectively used.
For finished content that is delivered via tape (e.g. commercial spots) or baseband video (e.g. from edit rooms), the appropriate AFD data can be authored during the ingest process as shown in Figure 6 on page 58. Using this model, a process should be established to allow the content providers to select how their HD content should be downconverted.
Figure 6. AFD insertion during ingest operation
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With file-based transfers from NLEs to playout servers becoming more common, AFD insertion in this domain has also been successfully implemented. This production workflow allows for finished NLE projects to be automatically transcoded and transferred to playout servers without going back to baseband video. As illustrated in Figure 7 on page 58, a completed project can be transcoded and transferred directly to a playout server with the desired AFD code attached.
Figure 7. File-based AFD insertion
Figure 8. Downstream AFD paths
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Many HD servers preserve vertical ancillary data space (VANC) on ingest and playout. However, each manufacturer has a proprietary method of storing VANC information in its internal file formats. For this reason, simple file transfers in and out of playout servers may result in the loss of AFD data. Limiting these types of file transfers between the same manufacturer models will help to protect against AFD data loss. File compatibility and VANC preservation must be tested and established.
Distribution and AFD preservation
As discussed in the previous section, ensuring that AFD data is encoded on final versions of content is the first important step in establishing a successful AFD driven process. Ensuring that the AFD data is preserved through the downstream signal path within the broadcast facility is just as critical.
AFD data in accordance with SMPTE 2016 is carried in the VANC of SDI video signals. Specific attention should be paid toward ensuring all downstream video processing systems (e.g. video routers, switchers, DAs and frame syncs) will reliably pass VANC data uncorrupted. The following recommendations should also be considered:
1. Focus on HD signals.
System designs
should be focused on carriage
of AFD data in the HD-SDI signals
for the purposes of downconversion
to SD. Most legacy SD devices have
been found to strip out any VANC
data present in the video signal, so
system designs should not rely on
AFD data within SD signals.
2. Select video line location.
Select a
specific video line location to carry
AFD data to be followed plant-wide.
This will be essential for quality control
and troubleshooting throughout
the signal path.
3. Avoid multiple AFD codes.
For any
AFD inserting system that is implemented,
specific care should be taken
to avoid the introduction of multiple
AFD codes in a single video stream.
This can cause adverse affects in many
AFD supported systems.
4. Insert AFD on upconversion.
SD
originated content must go through
an upconversion process before being
inserted into a facility’s HD distribution
path. Where possible, the proper
AFD code should be inserted at the
point of upconversion. Many upconversion
devices allow for this and will
help to ensure that the AFD code will
be available throughout the facility.
Despite best efforts to encode program-specific AFD codes, mecha mechanisms should be put into place to ensure that a default AFD code is automatically inserted into video streams when authored AFD data is not present. This is best handled close the final distribution point out of the facility as shown in Figure 8.(with Figure 7 above)
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