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PIXHELL: When Pixels Learn to Scream

Mordechai Guri

TL;DR

The paper addresses the challenge of producing audible sound without traditional audio hardware by exploiting LCD-induced acoustic noise from rapid pixel transitions. It develops a bitmap-driven method to generate controlled acoustic emissions, using PWM-based square-wave patterns tuned to a pixel clock and corrected for LCD timing. The authors implement M-FSK and 2-FSK modulation, establish a simple packet protocol, and demonstrate multi-signal transmission through vertical screen splitting and OFDM concepts, with empirical SNR measurements across distances and displays. The work reveals practical potential for low-power auditory feedback, device pairing, and covert channels in constrained environments, while detailing performance limits and trade-offs in brightness, distance, and segmentation. The study provides a reference framework for LCD-based acoustic communication and highlights both opportunities and security implications for future exploration.

Abstract

This paper presents a technique for generating sound by leveraging the electrical properties of liquid crystal displays (LCDs). The phenomenon occurs due to vibrational noise produced by capacitors within the LCD panel during rapid pixel state transitions. By modulating these transitions through specially crafted bitmap patterns projected onto the screen, we demonstrate how weak yet audible acoustic signals can be generated directly from the display. We designed, implemented, evaluated, and tested a system that repurposes the LCD as a sound-emitting device. Potential applications for this technique include low-power auditory feedback systems, short-range device communication, air-gap covert channels, secure auditory signaling, and innovative approaches to human-computer interaction.

PIXHELL: When Pixels Learn to Scream

TL;DR

The paper addresses the challenge of producing audible sound without traditional audio hardware by exploiting LCD-induced acoustic noise from rapid pixel transitions. It develops a bitmap-driven method to generate controlled acoustic emissions, using PWM-based square-wave patterns tuned to a pixel clock and corrected for LCD timing. The authors implement M-FSK and 2-FSK modulation, establish a simple packet protocol, and demonstrate multi-signal transmission through vertical screen splitting and OFDM concepts, with empirical SNR measurements across distances and displays. The work reveals practical potential for low-power auditory feedback, device pairing, and covert channels in constrained environments, while detailing performance limits and trade-offs in brightness, distance, and segmentation. The study provides a reference framework for LCD-based acoustic communication and highlights both opportunities and security implications for future exploration.

Abstract

This paper presents a technique for generating sound by leveraging the electrical properties of liquid crystal displays (LCDs). The phenomenon occurs due to vibrational noise produced by capacitors within the LCD panel during rapid pixel state transitions. By modulating these transitions through specially crafted bitmap patterns projected onto the screen, we demonstrate how weak yet audible acoustic signals can be generated directly from the display. We designed, implemented, evaluated, and tested a system that repurposes the LCD as a sound-emitting device. Potential applications for this technique include low-power auditory feedback systems, short-range device communication, air-gap covert channels, secure auditory signaling, and innovative approaches to human-computer interaction.

Paper Structure

This paper contains 41 sections, 32 equations, 11 figures, 8 tables, 3 algorithms.

Table of Contents

  1. Introduction
  2. Use Cases for LCD-Based Audio Communication
  3. Related Work
  4. Active Acoustic Side-Channels
  5. Passive Acoustic Side-Channels
  6. Technical Background
  7. LCD Screen Types
  8. Internal Components
  9. Noise Sources in LCD Screens
  10. Signal Generation
  11. The Pixel Clock
  12. PWM and Square Waves
  13. Frequency and Pixel Clock Correlation
  14. Impact on LCD Noise
  15. Bitmap Generation
  16. ...and 26 more sections

Figures (11)

  • Figure 1: The 'Screaming Pixels' Phenomenon. An illustration of the acoustic waves generated by the visual pixel patterns displayed on the LCD screen.
  • Figure 2: Four bitmaps generated by the algorithm with different parameters; resolution, pixel clock, and frequency. Clockwise direction: 1280$\times$720@60Hz 1000 Hz, 1280$\times$720@60Hz 2000 Hz, 1920$\times$1080@60Hz 8000 Hz, and 1920$\times$1080@60Hz 16000 Hz.
  • Figure 3: Generated square wave patterns with parameters: 1920x1080 resolution, 60Hz refresh rate, and 15,000 Hz frequency. The image includes three duty cycles: 25%, 50%, and 75%.
  • Figure 4: Spectrogram illustrating the 2-FSK transmission. The two distinct frequency components, $f_0$ and $f_1$, are visible, corresponding to binary symbols '0' and '1', respectively.
  • Figure 5: Packet structure used for square wave modulation. The packet is divided into four fields: Preamble (8 bits), Optional header (8 bits), Payload (32 bits), and Checksum (8 bits). Each field serves a specific purpose, ensuring reliable communication and synchronization.
  • ...and 6 more figures