To satisfy my mild old man curiosity:
Grok AI says
How many wires actually carry signal in an HDMI cable?
An HDMI cable has 19 wires in total, but not all of them carry the primary audio and video signals. Here's a breakdown:12 wires (4 shielded twisted pairs, each with 3 wires: signal, signal return, and shield) carry the high-speed digital signals for video and audio data using TMDS (Transition-Minimized Differential Signaling). These are the primary wires for transmitting the core HDMI signal.
7 additional wires handle auxiliary functions:1 wire for the Hot Plug Detect (HPD) signal.
1 wire for the Consumer Electronics Control (CEC) line.
1 wire for the Display Data Channel (DDC) clock (SCL).
1 wire for the DDC data (SDA).
1 wire for +5V power.
1 wire for ground.
1 wire for the Utility/HEAC (HDMI Ethernet and Audio Return Channel, if supported).
So, 12 wires directly carry the audio/video signal, while the remaining 7 support control, power, and auxiliary features.
How many fibers does a cheap optical HDMI cable have?
A typical cheap optical HDMI cable, often referred to as an Active Optical Cable (AOC), usually contains 4 optical fibers for high-speed data transmission, alongside 7 to 9 copper wires for low-speed signals and power. The optical fibers handle the primary audio and video signals (using TMDS channels), while the copper wires manage auxiliary functions like CEC (Consumer Electronics Control), ARC/eARC (Audio Return Channel), DDC (Display Data Channel), and power delivery. This configuration is standard in most budget-friendly optical HDMI cables, as noted in industry sources, balancing cost and performance for applications like 4K or 8K transmission over long distances. Higher-end or specialized cables might use more fibers (e.g., 6 or 12) for enhanced bandwidth or redundancy, but these are less common in cheaper models.
What is Transition-Minimized Differential Signaling?
Transition-Minimized Differential Signaling (TMDS) is a high-speed digital signaling technology used in HDMI (and earlier in DVI) to transmit audio, video, and control data over a cable. It’s designed to deliver high-bandwidth, high-quality digital signals with minimal electromagnetic interference (EMI) and signal degradation. Below is a concise explanation of TMDS and its role in HDMI cables, tailored to the context of your previous questions about HDMI and optical HDMI cables.Key Features of TMDS:
- Purpose:
- TMDS transmits serialized digital data (video, audio, and control signals) over twisted-pair wires (in traditional HDMI cables) or optical fibers (in active optical HDMI cables).
- It ensures reliable, high-speed transmission for resolutions like 1080p, 4K, or 8K, with minimal errors.
- How It Works:
- Differential Signaling: TMDS uses differential pairs (two wires per channel: one for the signal and one for its inverse) to transmit data. This reduces EMI by canceling out noise, as the receiver compares the difference between the two signals.
- Transition Minimization: TMDS encodes data to minimize the number of signal transitions (changes from 0 to 1 or 1 to 0). Fewer transitions reduce power consumption, EMI, and signal errors, especially at high frequencies.
- 8b/10b Encoding: TMDS converts 8-bit data into 10-bit code words. This encoding ensures DC balance (equal numbers of 1s and 0s over time) and provides enough transitions for clock recovery at the receiver.
- Structure in HDMI:
- TMDS uses four channels in HDMI cables:
- Three Data Channels: Each carries video, audio, and auxiliary data (e.g., packetized audio or metadata). These are transmitted over three twisted pairs in copper HDMI cables or four optical fibers in cheap optical HDMI cables (one fiber per channel, with the clock often embedded).
- One Clock Channel: Provides timing information to synchronize data at the receiver.
- Each channel in a standard HDMI cable consists of a shielded twisted pair (signal, signal return, and shield), making up 12 wires for the three data channels and clock in a copper HDMI cable.
- Role in Optical HDMI Cables:
- In cheap optical HDMI cables (AOCs), TMDS signals are converted from electrical to optical form at the source end using VCSEL drivers (lasers) and back to electrical at the display end using photodetectors and transimpedance amplifiers (TIAs).
- The four optical fibers typically carry the three TMDS data channels and the clock channel, while copper wires handle low-speed signals like CEC, DDC, and ARC.
- Performance:
- TMDS supports high data rates: up to 18 Gbps for HDMI 2.0 (4K@60Hz) and 48 Gbps for HDMI 2.1 (8K@60Hz or 4K@120Hz).
- It’s robust for short distances in copper cables but can suffer signal loss over long runs, which is why optical HDMI cables use fibers to maintain signal integrity over distances up to 100–300 meters.
- Advantages:
- Reduces EMI through differential signaling and shielding.
- Supports high-resolution video and multi-channel audio in a single cable.
- Reliable for consumer electronics due to error-resistant encoding.
- Limitations:
- In copper cables, signal degradation occurs over long distances (>15 meters for HDMI 2.0/2.1), necessitating active cables or optical solutions.
- Requires precise chipsets (e.g., re-timers, VCSEL drivers) in active/optical cables to maintain signal quality, adding cost to cheap AOCs.
