Subcarrier

Key takeaways

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

Reference excerpt

A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.

FM stereo Stereo broadcasting is made possible by using a subcarrier on FM radio stations, which takes the left channel and "subtracts" the right channel from it — essentially by hooking up the right-channel wires backward (reversing polarity) and then joining left and reversed-right. The result is modulated with suppressed carrier AM, more correctly called sum and difference modulation or SDM, at 38 kHz in the FM signal, which is joined at 2% modulation with the mono left+right audio (which ranges 50 Hz ~ 15 kHz). A 19 kHz pilot tone is also added at a 9% modulation to trigger radios to decode the stereo subcarrier, making FM stereo fully compatible with mono. Once the receiver demodulates the L+R and L−R signals, it adds the two signals ([L+R] + [L−R] = 2L) to get the left channel and subtracts ([L+R] − [L−R] = 2R) to get the right channel. Rather than having a local oscillator, the 19 kHz pilot tone provides an in-phase reference signal used to reconstruct the missing carrier wave from the 38 kHz signal. For AM broadcasting, different analog (AM stereo) and digital (HD Radio) methods are used to produce stereophonic audio. Modulated subcarriers of the type used in FM broadcasting are impractical for AM broadcast due to the relatively narrow signal bandwidth allocated for a given AM signal. On standard AM broadcast radios, the entire 9 kHz to 10 kHz allocated bandwidth of the AM signal may be used for audio.

Television Likewise, analog TV signals are transmitted with the black and white luminance part as the main signal, and the color chrominance as the subcarriers. A black and white TV simply ignores the extra information, as it has no decoder for it. To reduce the bandwidth of the color subcarriers, they are filtered to remove higher frequencies. This is made possible by the fact that the human eye sees much more detail in contrast than in color. In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking the remainder. (See: YIQ, YCbCr, YPbPr) Various broadcast television systems use different subcarrier frequencies, in addition to differences in encoding. For the audio part, MTS uses subcarriers on the video that can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another for second audio programs (such as descriptive video service for the vision-impaired, and bilingual programs), and yet a third hidden one for the studio to communicate with reporters or technicians in the field (or for a technician or broadcast engineer at a remote transmitter site to talk back to the studio), or any other use a TV station might see fit. (See also NICAM, A2 Stereo.) In RF-transmitted composite video, subcarriers remain in the baseband signal after main carrier demodulation to be separated in the receiver. The mono audio component of the transmitted signal is in a separate carrier and not integral to the video component. In wired video connections, composite video retains the integrated subcarrier signal structure found in the transmitted baseband signal, while S-Video places the chrominance and luminance signals on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture sharpness and brightness).

Private audio Before satellite, Muzak and similar services were transmitted to department stores on FM subcarriers. The fidelity of the subcarrier audio was limited compared to the primary FM radio audio channel. The United States Federal Communications Commission (FCC) also allowed betting parlors in New York state to get horse racing results from the state gaming commission via the same technology. Many non-commercial educational FM stations in the US (especially public radio stations affiliated with NPR) broadcast a radio reading service for the blind, which reads articles in local newspapers and sometimes magazines. The vision-impaired can request a special radio, permanently tuned to receive audio on a particular subcarrier frequency (usually 67 kHz or 92 kHz), from a particular FM station. Services like these and others on broadcast FM subcarriers are referred to as a Subsidiary Communications Authority (SCA) service by the FCC in the United States, and as Subsidiary Communications Multiplex Operations (SCMO) by the Canadian Radio-television and Telecommunications Commission (CRTC) in Canada.

Datacasting

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Subcarrier

Start with the simplest possible case. Write down what Subcarrier claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Subcarrier 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 Subcarrier 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 Subcarrier

In research: Subcarrier appears in engineering 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 Subcarrier 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: Subcarrier is common in secondary-school and first-year university syllabi. It links to neighbouring topics Broadcast engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Subcarrier 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.

How to study Subcarrier in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Subcarrier 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 Subcarrier out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Subcarrier in simple terms?

A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast.

Why does Subcarrier matter?

Because it connects several engineering 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 Subcarrier?

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 Subcarrier.