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

There are two types of pulse amplitude modulation:

• Insingle polarity PAM, a suitable fixedDC bias is added to the signal to ensure that all the pulses are positive.
• Indouble polarity PAM, the pulses are both positive and negative.

Pulse-amplitude modulation is widely used inmodulating signal transmission of digital data, with non-baseband applications having been largely replaced bypulse-code modulation, and, more recently, bypulse-position modulation.

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.

Some versions of theEthernet communication standard are an example of PAM usage.

• 100BASE-T4 andBroadR-Reach Ethernet standard use three-level PAM modulation (PAM-3).
• 1000BASE-T Gigabit Ethernet uses five-level PAM-5 modulation.^[1][a]
• 10GBASE-T 10 Gigabit Ethernet uses aTomlinson–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 by7⁄6.^[2]
• 25 Gigabit Ethernet and some copper variants of100 Gigabit Ethernet and200 Gigabit Ethernet use PAM-4 modulation.

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.^[3]Thunderbolt 5 uses the same PHY.^[4]

GDDR6X, developed by Micron^[5] and Nvidia and first used in theNvidia 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.^[6][7]

GDDR7 utilizes PAM-3 signaling to achieve speeds of 36 Gbps/pin. The higher data transmission rate per cycle compared toNRZ/PAM-2-signaling used byGDDR6 and prior generations improves power efficiency and signal integrity.^[8] Compared to PAM-4 (GDDR6X), it is less strict on manufacturing equipment.^[9]

PCI Express 6.0 has introduced PAM-4 usage.^[10]

The North AmericanAdvanced Television Systems Committee standards fordigital television uses a form of PAM to broadcast the data that makes up the television signal. This system, known as8VSB, is based on an eight-level PAM.^[11] It uses additional processing to suppress onesideband and thus make more efficient use of limitedbandwidth. Using a single 6 MHz channel allocation, as defined in the previousNTSC 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.

The concept is also used for the study ofphotosynthesis using a specialized instrument that involves aspectrofluorometric measurement of the kinetics of fluorescence rise and decay in the light-harvesting antenna ofthylakoid membranes, thus querying various aspects of the state of the photosystems under different environmental conditions.^[12] Unlike the traditional dark-adaptedchlorophyll fluorescence measurements, pulse amplitude fluorescence devices allow measuring under ambient light conditions, which made measurements significantly more versatile.^[13]

Pulse-amplitude modulation has also been developed for the control oflight-emitting diodes (LEDs), especially for lighting applications.^[14] LED drivers based on the PAM technique offer improvedenergy efficiency over systems based upon other common driver modulation techniques such aspulse-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.

Wikimedia Commons has media related toPulse amplitude modulation.

• 8VSB
• Amplitude-shift keying
• Carrier-sense multiple access
• Pulse-density modulation
• Pulse-forming network
• Quadrature amplitude modulation (QAM)

• Quantized radio modulation modes

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