Wikipedia
Australian company, IRT, came up with the VIMCAS solution which integrated stereo audio into the video waveform of a television signal during the vertical blanking interval. The audio quality was rated up to 15 kHz on each channel, using 3 lines of vertical interval.
IRT also supplied the VISCAS system 'Vertical Interval Single Channel Audio System' which was rated at 7.5 kHz and suitable for talkback.
The systems were used extensively by the Screensport Channel in the early 1990s to handle the multiple audio channels required by a service broadcasting in several languages.
By using 8 lines of the vertical interval they were able to route the video feed from an OB back to the transmission suite with 4 languages of programme and production talkback on a single link.
Outgoing talkback from the transmission suite was carried on a VISCAS channel in the vertical interval of the Astra satellite transmission.