At Arizona State University's KAET-TV, seamless switching between local programming and incoming PBS multichannel audio soundtracks is being done with the Linear Acoustics LA-5448 Multi-Input AC-3 Synchronizer and Rate Shaper. By switching AC-3 bit streams directly, the station avoids additional audio processing stages that can degrade audio quality. The LA-5448 also ensures that the metadata parameters encoded at PBS Washington headquarters, including dialnorm, are preserved and passed along the broadcast chain to viewers.
The LA-5448 was designed specifically to correct timing and synchronization anomalies in AC-3 digital audio bit streams, and eliminates many problems caused by poorly formatted signals. The alternative is to decode and re-encode audio bit streams, which can cause the loss of metadata and reduced audio quality. The LA-5448 handles frame synch, error correction and rate shaping for up to four separate AC-3 streams at a variety of data rates, ensuring consistent audio flow while working across multiple program sources.
At KAET, an audio stream switch from local to PBS feed happens at least once a day. Operating like a video frame synchronizer, the LA-5448 skips or repeats a frame of audio as needed to maintain synchronization, targeting the upstream audio to the local broadcaster's data rate without any telltale audio artifacts. As a result, multiplexing and transmission of surround sound can be accomplished without sacrificing audio quality.
Another application allows switching between AC-3 sources of different data rates prior to the LA-5448 using a standard AES routing switcher, and results in a single, error-free output at a fixed data rate. This is a useful and cost-effective method for commercial insertions.
For more information, visit www.linearacoustic.com.