science

VIMCAS

VIMCAS is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand VIMCAS rather than just read about it. In short: 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.

Key takeaways

VIMCAS belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect VIMCAS to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of VIMCAS from memory before moving on to harder problems.

Reference excerpt

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. the voice of the commentator), or alternatively with the effects audio carried by VIMCAS and the main audio carried as NICAM 728. To fit into the available bandwidth, the audio signal would first be companded and limited before being sampled for PCM encoding. The encoded signal would be transmitted in the six scanlines in time compressed form, i.e. much faster than its actual speed. Decoding was simply the reverse process, with 100ms of audio (at a time) stored in the transmitted digital form into a digital memory and played out from that memory at original speed through a digital-to-analogue converter, with appropriate timing circuits to synchronize this playout with the accompanying video. A reduced version, using just one scan line instead of six and thus providing narrower bandwidth, was called VISCAS (Vertical Interval Single Channel Audio System), which was good enough for talkback between the studio and the OB or foldback.

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Worked examples

Example 1 — a first encounter with VIMCAS

Start with the simplest possible case. Write down what VIMCAS claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to VIMCAS before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about VIMCAS ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of VIMCAS

In research: VIMCAS appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses VIMCAS in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: VIMCAS is common in secondary-school and first-year university syllabi. It links to neighbouring topics Australian inventions, Television technology, so understanding it makes those chapters shorter.
In everyday life: Look for VIMCAS outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study VIMCAS in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what VIMCAS means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain VIMCAS out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is VIMCAS in simple terms?

Why does VIMCAS matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study VIMCAS?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on VIMCAS.