science

Pulse-amplitude modulation

Pulse-amplitude modulation 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 Pulse-amplitude modulation rather than just read about it. In short: Pulse-amplitude modulation (PAM) is a form of signal modulation in which the message information is encoded in the amplitude of a pulse train interrupting the carrier frequency. Demodulation is performed by detecting the amplitude level of the carrier at every single period.

Key takeaways

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

Reference excerpt

Pulse-amplitude modulation (PAM) is a form of signal modulation in which the message information is encoded in the amplitude of a pulse train interrupting the carrier frequency. Demodulation is performed by detecting the amplitude level of the carrier at every single period.

Types

Polarities There are two types of pulse amplitude modulation:

In single polarity PAM, a suitable fixed DC bias is added to the signal to ensure that all the pulses are positive. In double polarity PAM, the pulses are both positive and negative. Pulse-amplitude modulation is widely used in modulating signal transmission of digital data, with non-baseband applications having been largely replaced by pulse-code modulation, and, more recently, by pulse-position modulation.

Amplitudes The number of possible pulse amplitudes in analog PAM is theoretically infinite. Digital PAM reduces the number of pulse amplitudes to some natural number not less than 3 (PAM-2 would be a simple binary signal and is usually not considered to be PAM). Common choices for the number of amplitudes are: 3, 4, 5, 8, 16.

Uses

Ethernet Some versions of the Ethernet communication standard are an example of PAM usage.

100BASE-T4 and BroadR-Reach Ethernet standard use three-level PAM modulation (PAM-3). 1000BASE-T Gigabit Ethernet uses five-level PAM-5 modulation. 10GBASE-T 10 Gigabit Ethernet uses a Tomlinson–Harashima precoded (THP) version of pulse-amplitude modulation with 16 discrete levels (PAM-16). The THP precoding provides for noise resistance. Two consecutive PAM-16-encoded symbols are interpreted according to a two-dimensional checkerboard pattern known as DSQ128, where 128 out of 256 possible combinations are picked to maximize their "distance" (again for noise resistance). This provides the same SNR as PAM-8 while increasing the data rate by 7⁄6. 25 Gigabit Ethernet and some copper variants of 100 Gigabit Ethernet and 200 Gigabit Ethernet use PAM-4 modulation. 100 Gigabit Ethernet with single lambda on single-mode optical fiber use PAM-4 modulation.

USB USB4 Version 2.0 uses PAM-3 signaling for USB4 80 Gbps (USB4 Gen 4×2) and USB4 120 Gbps (USB4 Gen 4 Asymmetric) transmitting 3 bits per 2 clock cycles. Thunderbolt 5 uses the same PHY.

Video memory GDDR6X, developed by Micron and Nvidia and first used in the Nvidia RTX 3080 and 3090 graphics cards, uses PAM-4 signaling to transmit 2 bits per clock cycle without having to resort to higher frequencies or two channels or lanes with associated transmitters and receivers, which may increase power or space consumption and cost. Higher frequencies require higher bandwidth, which is a significant problem beyond 28 GHz when trying to transmit through copper. PAM-4 costs more to implement than earlier NRZ (non return to zero, PAM-2) coding partly because it requires more space in integrated circuits, and is more susceptible to SNR (signal to noise ratio) problems. GDDR7 utilizes PAM-3 signaling to achieve speeds of 36 Gbps/pin. The higher data transmission rate per cycle compared to NRZ/PAM-2-signaling used by GDDR6 and prior generations improves power efficiency and signal integrity. Compared to PAM-4 (GDDR6X), it is less strict on manufacturing equipment.

PCI Express PCI Express 6.0 has introduced PAM-4 usage.

Digital television The North American Advanced Television Systems Committee standards for digital television uses a form of PAM to broadcast the data that makes up the television signal. This system, known as 8VSB, is based on an eight-level PAM. It uses additional processing to suppress one sideband and thus make more efficient use of limited bandwidth. Using a single 6 MHz channel allocation, as defined in the previous NTSC analog standard, 8VSB is capable of transmitting 32 Mbit/s. After accounting for error-correcting codes and other overhead, the data rate in the signal is 19.39 Mbit/s.

Photobiology The concept is also used for the study of photosynthesis using a specialized instrument that involves a spectrofluorometric measurement of the kinetics of fluorescence rise and decay in the light-harvesting antenna of thylakoid membranes, thus querying various aspects of the state of the photosystems under different environmental conditions. Unlike the traditional dark-adapted chlorophyll fluorescence measurements, pulse amplitude fluorescence devices allow measuring under ambient light conditions, which made measurements significantly more versatile.

Electronic drivers for LED lighting Pulse-amplitude modulation has also been developed for the control of light-emitting diodes (LEDs), especially for lighting applications. LED drivers based on the PAM technique offer improved energy efficiency over systems based upon other common driver modulation techniques such as pulse-width modulation (PWM) as the forward current passing through an LED is relative to the intensity of the light output and the LED efficiency increases as the forward current is reduced. Pulse-amplitude modulation LED drivers are able to synchronize pulses across multiple LED channels to enable perfect color matching. Due to the inherent nature of PAM in conjunction with the rapid switching speed of LEDs, it is possible to use LED lighting as a means of wireless data transmission at high speed.

Illustrations

Worked examples

Example 1 — a first encounter with Pulse-amplitude modulation

Start with the simplest possible case. Write down what Pulse-amplitude modulation 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 Pulse-amplitude modulation 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 Pulse-amplitude modulation 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 Pulse-amplitude modulation

In research: Pulse-amplitude modulation 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 Pulse-amplitude modulation 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: Pulse-amplitude modulation is common in secondary-school and first-year university syllabi. It links to neighbouring topics Quantized radio modulation modes, so understanding it makes those chapters shorter.
In everyday life: Look for Pulse-amplitude modulation 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Pulse-amplitude modulation in 20 minutes

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

Frequently asked questions

What is Pulse-amplitude modulation in simple terms?

Why does Pulse-amplitude modulation 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 Pulse-amplitude modulation?

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 Pulse-amplitude modulation.