Lesson plan
VIMCAS
VIMCAS, standing for Vertical Interval Multiple Channel Audio System, is a dual-channel Sound-in-Syncs mechanism for transmitting digitally encoded audio in a composite video analogue television signal. Invented by Australian company IRT in the 1980s, the basic concept of VIMCAS is to transmit two channels of PCM-encoded (i.e. digital) audio during the vertical blanking interval of a composite video signal. The encoded audio was transmitted over 6 horizontal scan lines during that interval, the digitally encoded signal being placed onto a series of mid-grey pedestals, in much the same way that the colour subcarrier is placed on top of the monochrome signal. As with the colour subcarrier, there is 4.7kHz bandwidth, so six lines would provide 28kHz of bandwidth (actually slightly less, there being a deliberate redundancy between the final packet of encoded audio on one line and the first packet of encoded audio on the next line to avoid signal corruption). This could be used as a pair of 14kHz channels for stereo audio, or as separate channels to carry dual-language transmissions. In outside broadcast (OB) work, where VIMCAS was used from the OB site back to the studio, it could be used for separate audio channels where one would be effects (i.e. the ambient sound of a sports match) and the other would be the main audio (e.g.…
Article summary
VIMCAS, standing for Vertical Interval Multiple Channel Audio System, is a dual-channel Sound-in-Syncs mechanism for transmitting digitally encoded audio in a composite video analogue television signal. Invented by Australian company IRT in the 1980s, the basic concept of VIMCAS is to transmit two channels of PCM-encoded (i.e. digital) audio during the vertical blanking interval of a composite video signal. The encoded audio was transmitted over 6 horizontal scan lines during that interval, the digitally encoded signal being placed onto a series of mid-grey pedestals, in much the same way that the colour subcarrier is placed on top of the monochrome signal. As with the colour subcarrier, there is 4.7kHz bandwidth, so six lines would provide 28kHz of bandwidth (actually slightly less, there being a deliberate redundancy between the final packet of encoded audio on one line and the first packet of encoded audio on the next line to avoid signal corruption). This could be used as a pair of 14kHz channels for stereo audio, or as separate channels to carry dual-language transmissions. In outside broadcast (OB) work, where VIMCAS was used from the OB site back to the studio, it could be used for separate audio channels where one would be effects (i.e. the ambient sound of a sports match) and the other would be the main audio (e.g.…
Key claims
No claims recorded.
Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
No sources recorded.
Key claims and bibliography are generated from the same tracked claims and sources as the article page.