ABC NETWORK

To recap, distribution of ABC DTV network audio to affiliates uses two-channel MPEG (LtRt) and high-rate (640 kbps) AC-3 for the 5.1-channel audio, and both are carried via a mezzanine compression system (higher rate than ATSC emission). Local affiliate stations must receive the network signal, then decode the high-rate AC-3 signal to 5.1 PCM channels, mix or switch local audio in, then Dolby Digital (AC-3) encode for emission.

HIGH-RATE AC-3

For drawing, click here.

Just like other approaches, this one delivers baseband audio and video which can appear as another source on the master control switcher. A simplified version of the setup is contained in the drawing above, and shows all of the required and recommended components to get the 5.1 channel audio on the air.

Some ABC affiliates have chosen to install the Linear Acoustic AERO.air to add protection against wrong or missing metadata. The Linear Acoustic AERO.air also adds the capability of simple insertion of local audio for voiceovers. Because it includes upmixing capabilities, the AERO.air fixes the long-standing problem of playing two-channel commercials out of only the Left and Right channels due to the lack of metadata. By using the AERO.air, the problem is cured as local content can now correctly fill all of the channels via a simple contact closure from master control.

KEY POINTS

• Network IRD delivers 640 kbps AC-3 stream which is decoded by affiliate's professional decoder (Dolby DP562 or the newer replacement DP564) into baseband PCM which is In Time with baseband video output of the network IRD
• Baseband audio and video signals can be treated as another HD source
• Currently no capability to pass Professional Metadata to control Dolby Digital (AC-3) encoder
• Final emission Dolby Digital (AC-3) encoding is performed at 384-448 kbps
• Adding a Linear Acoustic AERO.air at the affiliate station can protect audio from incorrect or missing metadata and local content can be selectively upmixed to match network sound sources

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