Pulse-amplitude modulation
PAM encodes message information in pulse amplitude.
Pulse-amplitude modulation (PAM) encodes information by varying the amplitude of a series of pulses that interrupt a carrier signal. To recover the original signal, a receiver checks the carrier’s amplitude level once per period.
There are two polarity types. In single-polarity PAM, a fixed DC offset keeps all pulses positive. In double-polarity PAM, pulses can be both positive and negative.
The number of amplitude levels differs between analog and digital PAM. Analog PAM theoretically allows an infinite number of amplitudes. Digital PAM uses a finite set of at least three levels—two-level PAM (PAM-2) is essentially a simple binary signal and is usually not classified as PAM. Common choices for digital PAM include 3, 4, 5, 8, and 16 levels.
PAM is widely used for transmitting digital data. Many non-baseband applications have been replaced by pulse-code modulation or, more recently, by pulse-position modulation.
Several Ethernet standards rely on PAM. 100BASE-T4 and BroadR-Reach use three-level PAM (PAM-3). 1000BASE-T Gigabit Ethernet uses five-level PAM-5. 10GBASE-T uses a Tomlinson–Harashima precoded version of PAM with 16 levels (PAM-16); the precoding improves noise resistance, and two consecutive PAM-16 symbols are mapped to a 128-point checkerboard pattern (DSQ128) to maximize signal distance. 25 Gigabit Ethernet and some copper variants of 100 and 200 Gigabit Ethernet use PAM-4. Single-lambda 100 Gigabit Ethernet over single-mode optical fiber also uses PAM-4.
USB4 Version 2.0 uses PAM-3 signaling for 80 Gbps and 120 Gbps modes, transmitting 3 bits per 2 clock cycles; Thunderbolt 5 uses the same physical layer.
In video memory, GDDR6X (used in Nvidia RTX 3080 and 3090) employs PAM-4 to send 2 bits per clock cycle without needing higher frequencies or extra channels, though it costs more to implement than NRZ (PAM-2) and is more sensitive to signal-to-noise issues. GDDR7 uses PAM-3 to reach 36 Gbps per pin, offering better power efficiency and signal integrity than NRZ, and is less demanding on manufacturing equipment than PAM-4.
PCI Express 6.0 introduced PAM-4.
For digital television, the North American ATSC standard uses 8VSB, an eight-level PAM that suppresses one sideband to fit a 32 Mbit/s raw data stream into a 6 MHz channel; after error correction, the usable data rate is 19.39 Mbit/s.
- Types
- Single polarity PAM and double polarity PAM
- Common amplitudes
- 3, 4, 5, 8, 16
- Ethernet examples
- 100BASE-T4 (PAM-3), 1000BASE-T (PAM-5), 10GBASE-T (PAM-16 with THP), 25 Gigabit Ethernet (PAM-4)
- Usb version
- USB4 Version 2.0 uses PAM-3
- Gddr6x speed
- PAM-4 signaling
- Gddr7 speed
- 36 Gbps/pin using PAM-3
- 8vsb data rate
- 19.39 Mbit/s after overhead
Lore & Background
Pulse-amplitude modulation exists in two polarity types: single polarity PAM, where a fixed DC bias ensures all pulses are positive, and double polarity PAM, where pulses are both positive and negative. The number of possible pulse amplitudes in analog PAM is theoretically infinite, while digital PAM reduces the number to a natural number not less than 3 (PAM-2 is usually not considered PAM). Common choices for the number of amplitudes are 3, 4, 5, 8, and 16.
PAM 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. Various Ethernet standards employ PAM: 100BASE-T4 and BroadR-Reach use PAM-3; 1000BASE-T Gigabit Ethernet uses PAM-5; 10GBASE-T uses a Tomlinson–Harashima precoded version of PAM-16; and 25 Gigabit Ethernet and some copper variants of 100 and 200 Gigabit Ethernet use PAM-4. USB4 Version 2.0 uses PAM-3 signaling, and Thunderbolt 5 uses the same PHY.
In video memory, GDDR6X uses PAM-4 signaling to transmit 2 bits per clock cycle, while GDDR7 utilizes PAM-3 signaling to achieve speeds of 36 Gbps/pin. PCI Express 6.0 has introduced PAM-4 usage. The North American Advanced Television Systems Committee standards use 8VSB, based on eight-level PAM, capable of transmitting 32 Mbit/s in a 6 MHz channel, with a data rate of 19.39 Mbit/s after overhead.
Reader's Guide
Pulse-amplitude modulation has significance across multiple domains. In digital communications, it underpins several Ethernet standards from 100BASE-T4 to 10GBASE-T and beyond, with each generation using higher-level PAM to increase data rates. The introduction of PAM-4 in 25, 100, and 200 Gigabit Ethernet, as well as in PCI Express 6.0, demonstrates its continued relevance. In video memory, GDDR6X adopted PAM-4 to transmit 2 bits per clock cycle without resorting to higher frequencies or multiple channels, though it costs more to implement than NRZ (PAM-2) and is more susceptible to SNR problems. GDDR7 later used PAM-3 for improved power efficiency and signal integrity, being less strict on manufacturing equipment than PAM-4. USB4 Version 2.0 and Thunderbolt 5 also employ PAM-3. Beyond data transmission, PAM is used in digital television broadcasting via 8VSB, in photobiology for studying photosynthesis under ambient light, and in LED lighting drivers that offer improved energy efficiency over pulse-width modulation and enable wireless data transmission at high speed.
Did You Know?
- PAM-2 is usually not considered PAM; common digital PAM amplitudes start at 3.
- GDDR6X uses PAM-4 signaling, while GDDR7 uses PAM-3 signaling at 36 Gbps/pin.
- 8VSB, based on eight-level PAM, transmits 19.39 Mbit/s after error correction in a 6 MHz channel.
More in Radio Modulation Modes, Part 2 1-24
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