Unpacking Active Optical HDMI Cables: A Deep Dive into the Technology

• 0:00 Introduction: The video explores the engineering behind active optical HDMI cables, focusing on cost-effective solutions down to the semiconductor level.
• 0:00:39 HDMI Basics: A typical HDMI cable includes not only video signals, but also synchronization, I²C channels for communication between devices.
• 0:01:11 High-Speed Data: The primary focus is the high data rate signal (18 Gbps for HDMI 2.0), which is primarily sent from the source to the display.
• 0:01:29 Data Encoding: HDMI uses Transition Minimized Differential Signaling (TMDS) with 8b/10b encoding, reducing the number of signal transitions to lower bandwidth requirements, but adding padding.
• 0:03:13 TMDS Encoding: The transition minimization encoding means that for every 8 bits of data, 10 bits are sent with less transitions, lowering bandwidth requirements
• 0:03:41 Simplified Clocking: The clock signal is sent along with the data, avoiding complex clock recovery on the receiver end, but requires additional twisted pair for the clock.
• 0:05:07 Optical Conversion: The optical cable replaces the electrical twisted pairs with optical fibers, requiring electrical-to-optical conversion at the source end, and optical-to-electrical conversion at the display end.
• 0:06:31 Fiber Types:
  • Single-Mode Fiber: Thin (9 micrometer), low dispersion, for long distances, but requires more expensive interfacing.
  • Multimode Fiber: Wider aperture, easier to couple light into, but has higher dispersion, making it suitable for shorter distances. Optical HDMI cables use multimode fiber.
• 0:07:52 Light Source - VCSELs: Vertical-Cavity Surface-Emitting Lasers (VCSELs) are used as light sources, due to their low cost and high data rate support. They feature a distributed Bragg reflector (DBR) array to generate a specific wavelength.
• 0:09:48 Photodetector - PIN Diode: A PIN diode is used for light detection. Photons entering create electron-hole pairs, generating a current which can then be amplified.
• 0:10:40 Additional Components: Driver for the VCSEL, transimpedance amplifier (TIA) for the PIN diode, and optics to couple light to/from the fiber.
• 0:11:11 Unidirectional Nature: Optical HDMI cables are unidirectional, due to the use of VCSELs as transmitters and PIN diodes as receivers.
• 0:12:17 Cable Teardown: The transmitter end has four differential signal pairs converting to optical signals, with four multimode fibers transmitting the optical signals and copper cables still used for power and I2C.
• 0:13:06 Receiver Teardown: The receiver end converts the optical signals back to electrical ones, using similar optical components to the transmitter, and the same copper cables for I²C and power.
• 0:13:48 Optical Engine: Light from the VCSELs is directed through a small plastic piece with focusing lenses, aligning the light to the fibers with a 90-degree bend.
• 0:15:20 Active Alignment: The system requires active alignment during manufacturing, as several glues are applied with tight alignment tolerances of the optical components.
• 0:16:49 Optical Confinement: Precise optical confinement is necessary to minimize crosstalk between the fiber channels. Misalignment can result in visible artifacts in the displayed images.
• 0:17:42 VCSEL Confirmation: VCSEL light is confirmed through video capture. Using an optical spectrum analyzer, stray light is detected at 843 nm, confirming the use of 850nm VCSELs.
• 0:19:45 NRZ Signaling: The simplest Non-Return-to-Zero (NRZ) modulation format is used due to low data rate.
• 0:20:08 Data Rate Calculation: The data rate for 4K 60 fps is ~18 Gbps, split across three lanes; meaning ~6 Gbps per lane, with some 8b/10b encoding overhead.
• 0:21:01 Eye Diagram Measurement: Measured the eye diagram at the receiver end, confirming a ~2.97GHz bit rate with NRZ signaling for a 4K 30fps display signal.
• 0:22:58 Microscopic Analysis: The video then shifts to a microscopic view of the boards, where detailed inspection of the dies and their components are discussed.
• 0:23:29 Transmitter IC Inspection: The IC on the transmitter side has four identical lanes for the VCSEL drivers with bond wires. Inductors are placed at the input. A metal layer is used for power and ground, with openings to allow for signal routing.
• 0:24:46 VCSEL Structure: Individual vixel structures are marked and labeled, showing bond wires on both sides. This highlights a manufacturing strategy to make large quantities of them on a single wafer.
• 0:25:10 Transmitter IC Details: A deeper look is given to the various structures on the transmitter IC, such as inductors, differential lines and transistor structures.
• 0:27:59 Photo-detector Structure: Similar to the VCSEL, photo-detectors are also individually identified and show bond-wires on both of their sides.
• 0:28:55 Layered Structures: Using the microscope, the different layers of the silicon structure are shown highlighting the cross-section of a semiconductor device.
• 0:29:44 Receiver IC: The receiver IC also uses an inductor at the input and has a single ended to differential conversion, with signal lines going through to a limiting amplifier
• 0:30:45 Photo-detector Details: A detailed view of the photo-detectors shows a single signal output with one side all connected to a biasing plane.
• 0:32:26 DIC Microscopy: Differential Interference Contrast (DIC) microscopy is used to highlight the surface roughness and other imperfections in the structures, revealing that even surfaces are not perfectly planer.
• 0:32:52 Summary: The video concludes with the key takeaways of the inner workings of a fiber optic HDMI cable, highlighting the importance of cost-effectiveness, design and signal integrity considerations.