DC Balanced Control Characters for AC-Coupled Display Interfaces
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Solution Overview
Problem
Existing methods for driving DVI and HDMI display devices over an AC-coupled interface face challenges due to prohibitively large DC drift caused by unbalanced control characters using Transmission Minimized Differential Signaling (TMDS), leading to errors at the display device receiver.
Innovation Solution
A transmitter with logic that constructs rebalancing control characters to achieve DC balance by ensuring an equal number of '1' and '0' bits, which are recognized as control characters but ignored by the receiver, thereby correcting the DC imbalance and allowing transmission over the PCI Express physical layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If control characters are transmitted using TMDS over an AC-coupled interface, then the interface can be simplified and dedicated PHYs are avoided, but DC drift occurs due to unbalanced control characters
Solution Approach 1:
The patent modifies the bit composition of control characters to achieve DC balance. Specifically, it ensures that control characters have an equal number of 0s and 1s by selecting from multiple possible bit patterns representing the same control function. This parameter change in the control character structure eliminates DC drift while maintaining AC-coupled interface simplicity.
Solution Approach 2:
The patent introduces an intermediary processing layer between the control character generation and the TMDS transmission. This intermediary analyzes the bit composition of control characters and selectively applies bit inversion or pattern selection to ensure DC balance, acting as a mediator that preserves both interface simplicity and signal integrity.
2Reliability
If dedicated PHYs for DVI or HDMI displays are used, then reliable transmission is ensured, but chip area and cost increase
Solution Approach 1:
The patent enables the PCI Express PHY to perform multiple functions by making it compatible with both PCI Express data transmission and DVI/HDMI display output. Through the DC-balanced control character mechanism, the same physical layer infrastructure serves dual purposes, eliminating the need for dedicated display PHYs while maintaining transmission reliability.
Solution Approach 2:
The patent makes the existing PCI Express infrastructure self-sufficient for display output by enabling it to generate and transmit DC-balanced control characters natively. This eliminates dependency on external DVI/HDMI encoder chips or dedicated PHYs, as the PCI Express layer itself provides all necessary functions for reliable display transmission.
3Adaptability or versatility
If external DVI or HDMI encoder chips are used, then display device compatibility is ensured, but system cost and complexity increase
Solution Approach 1:
The patent introduces an intermediary processing function within the PCI Express logic that converts internal control characters into DC-balanced formats compatible with DVI/HDMI standards. This intermediary layer ensures broad display device compatibility while keeping the rest of the system architecture simple and integrated.
Solution Approach 2:
The patent replaces the mechanical solution of adding external encoder chips with a software/logic-based solution implemented within the PCI Express controller. The DC-balancing functionality is achieved through logical operations on control character bits rather than through separate hardware encoding circuits, reducing system complexity.
Data Source
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AI summary
According to the present disclosure, a transmitter (110) for transmitting control characters to a display device (112) over an interface (118) includes a transmitter portion configured to transmit a control character having a plurality of bits to the display device. The transmitter also includes logic (120) configured to analyze the values of the bits in the control character and construct a corresponding rebalancing control character. The values of the plurality of bits in the rebalancing control character are selected such that the combination of the control character and the rebalancing control character is DC balanced. As such, the transmitter provides DC balance correction to non- DC balanced control characters in such a way as to allow DVI and HDMI to operate properly on an AC-coupled connection.