USB ID Line Microphone Detection Using Current Source
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Solution Overview
Problem
Mini-B or Micro-B USB connectors, as per existing specifications like CEA-936A, cannot support the combination of a microphone and a stereo headset due to limited pins, making accessory detection unreliable and difficult to reuse for other accessories.
Innovation Solution
A low value current source is used to exploit the current versus voltage characteristics of the JFET preamplifier in microphones, allowing the ID line to be used for both accessory identification and microphone operation, enabling the detection of multiple accessories like stereo headsets with internal microphones by multiplexing signals onto the same pins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a comparator is used to check the voltage across the microphone with a pull up resistor, then microphone detection is possible, but the bias voltage varies due to JFET characteristics making accessory identification unreliable
Solution Approach 1:
The patent changes the detection parameter from voltage-based to current-based measurement. By using a current source instead of a voltage comparator with pull-up resistor, the system exploits the current versus voltage characteristics of the JFET preamplifier, providing a more reliable and consistent method for distinguishing microphones from other accessories regardless of JFET bias voltage variations.
2Adaptability or versatility
If the ID line is used for resistor value checking to identify accessories, then accessory identification is possible, but the same connection cannot simultaneously accommodate microphone and stereo headset detection
Solution Approach 1:
The patent makes the ID line multi-functional by using it for both accessory identification (resistor value checking) and microphone detection (current characteristics analysis). The current source method allows the system to distinguish between different accessory types on the same connection, enabling the ID line to serve multiple purposes simultaneously without compromising detection reliability.
Solution Approach 2:
The current source acts as an intermediary element that enables dual functionality on the ID line. By injecting a known current and measuring the resulting voltage drop, the system can identify accessories based on their electrical characteristics while maintaining the line's ability to carry identification signals, thus mediating between conflicting detection requirements.
3Device complexity
If a Mini-B or Micro-B USB connector is used with limited pins, then device compactness is improved, but the connector cannot support multiple accessories like microphone and stereo headset simultaneously
Solution Approach 1:
The patent applies multi-functionality to the ID line, allowing it to serve both as an identification resistor measurement path and as a microphone detection path. This enables a 5-pin Mini-B or Micro-B connector to support accessory types that would otherwise require additional pins, maintaining device compactness while expanding accessory support capability.
Solution Approach 2:
The patent merges the accessory identification function and microphone detection function into a single detection pathway using the ID line. By combining these functions that would traditionally require separate connections, the system achieves the capability to support multiple accessory types through a limited pin connector.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution allows reliable detection and operation of microphones and other accessories, providing a large voltage range for resistor value sensing, enabling the use of Mini-B or Micro-B USB connectors to support multiple accessories, including stereo headsets with internal microphones, by effectively distinguishing microphone connections from other accessories.
Implementation Method 1
A low value current source is used to exploit the current versus voltage characteristics of the JFET preamplifier internal to the microphone
Data Source
AI summary
The coupling of an electret microphone is detected and identifying whether there is monaural or stereo speakers coupled to the USB connector is accomplished by coupling a current source to the USB ID terminal and detecting the voltage on the ID terminal. If the voltage on the ID line is substantially below the voltage associated with any other accessory when an electret microphone is coupled between the ID terminal and the ground terminal, and a microphone bias resistor is coupled to the ID line. On detection of an electret microphone being coupled between the ID terminal and the ground terminal, whether there is a monaural or stereo speakers coupled to the USB connector is determined by detecting the impedance on the one of the D+ and D− terminals to which a monaural speaker would not be coupled. An open circuit indicates a monaural speaker, otherwise stereo speakers are connected. An alternate embodiment is disclosed.