Audio Level Meter Correction for True Signal Power
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Solution Overview
Problem
Audio level meters in AC voltage measurement equipment and audio systems provide readings that differ for the same input signal due to conflicting purposes of indicating perceived loudness and headroom, making it difficult to compare readings across different systems, especially between analog and digital systems, and for sinusoidal signals.
Innovation Solution
An apparatus and method to determine the difference between true signal power and audio level meter readings by considering the treatment of the input signal, attack time, and release time of low-pass filters, using equations to calculate the phase angle and output level, and employing look-up tables for efficient correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an audio level meter indicates perceived loudness (signal power), then the reading reflects human auditory perception, but it cannot simultaneously indicate headroom before saturation
Solution Approach 1:
The patent divides the audio level meter functionality into separate detection paths: one for measuring signal power (perceived loudness) and another for detecting peak levels (headroom). This segmentation allows both measurements to be performed independently and displayed separately, resolving the contradiction between indicating loudness and indicating headroom.
Solution Approach 2:
The patent introduces correction terms as intermediary values that adjust the relationship between input signal and meter reading. These correction terms mediate between the raw signal measurement and the final displayed value, enabling the meter to provide accurate signal power indication while also preserving headroom information through appropriate calibration.
2Productivity
If an audio level meter uses fixed attack/release times, then the measurement is simple and fast, but the reading differs from true signal power for sinusoidal signals
Solution Approach 1:
The patent changes the parameters of the audio level meter by introducing variable correction terms that depend on the attack and release time settings. Instead of using fixed meter characteristics, the system dynamically adjusts the correction applied to the reading based on the specific time constants used, thereby maintaining both fast response and accurate signal power indication.
Solution Approach 2:
The patent performs preliminary calculation of correction terms based on the known attack and release time settings before taking the actual measurement. This preliminary action allows the system to pre-compute the necessary adjustments, so that when the measurement is taken, the correction can be immediately applied without compromising measurement speed or accuracy.
3Ease of manufacture
If analog and digital audio level meters use different measurement approaches, then each can be optimized for its system type, but the readings are not easily comparable
Solution Approach 1:
The patent creates a universal correction framework that works for both analog and digital audio level meters. By formulating correction terms that account for the fundamental measurement principles common to both system types (attack/release time behavior), the invention enables readings from different system types to be compared on a common basis while still allowing each system to maintain its optimization.
Data Source
AI summary
A circuit for correcting the output of an audio level meter comprises input means for generating the square or the absolute value of an input signal, a low pass filter having a predetermined attack time and release time, and output means for converting the output signal from a linear scale to a logarithmic scale. The circuit further comprises a correction means to which an information about whether the input signal to the audio level meter was subject to squaring or converting into an absolute value at the input, as well as the attack and release time of the low pass filter, are supplied as input values, and which provides, at its output, a value representing the difference between the output of the audio level meter and the true signal power of the input signal.