Today’s TV facilities use the HD-SDI serial interface standard to transport audio and video signals. It was not designed to carry data channels. During the transition to HDTV decades ago, several schemes were used to send auxiliary data, such as time code signals, audio metadata, or compressed audio bitstreams, in spare sections of the serial-digital-interface (SDI) bitstream.
The most straightforward is to use a pair of PCM audio channels as a 1.5-MB/s data channel. However, transmission equipment may change the gain of the channels to correct loudness or resample them to correct time-base errors. This is transparent to audio signals, but destroys the data. It’s possible to remove most sources of gain changes or resampling, but all equipment in the signal path must be programmed so that it doesn’t perform such operations on those channels only. Furthermore, it must be operationally verified and maintained forever.
The MPEG-HAA system (see "MPEG-H Audio Brings New Features to TV and Streaming Sound") avoids this by encoding the metadata as an audio signal, so it may be resampled or scaled without destroying the data payload. The metadata signal can even be edited along with the content’s audio tracks in an audio- or video-editing program without damaging the metadata.