Signal Processing for Digital Vinyl Latency Reduction
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Solution Overview
Problem
Conventional digital vinyl systems experience latency in determining the speed and position of a rotating recording medium from time code signals, which affects the accuracy and responsiveness of audio playback, as they require aggressive noise reduction and smoothing, introducing delays in the audible response.
Innovation Solution
A method and apparatus that filter a time code signal to recover a sinusoidal pilot signal and a binary signal, allowing for accurate determination of the recording medium's speed and position with minimal smoothing, thereby reducing latency and improving responsiveness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If heavy noise reduction or smoothing is applied to the PLL output signal to accurately determine the frequency of an AM-modulated time code signal, then measurement precision is improved, but latency increases due to the weighted moving average calculation over multiple samples
Solution Approach 1:
The patent applies preliminary action by performing low-pass filtering on the input time code signal before it enters the PLL circuit. This pre-filtering removes high-frequency noise components in advance, allowing the PLL to lock onto the carrier frequency more quickly and accurately without requiring aggressive post-processing smoothing that would introduce latency. The filtering is performed on the raw input signal rather than on the PLL output, enabling faster frequency acquisition.
Solution Approach 2:
The patent introduces an intermediary low-pass filter between the input signal source and the PLL circuit. This intermediary component processes the signal in a way that benefits the PLL's frequency detection - by attenuating high-frequency noise before it reaches the phase detector, the filter acts as a mediator that improves measurement precision without requiring the PLL itself to perform time-consuming smoothing operations on its output.
2Reliability
If aggressive smoothing is applied to the PLL output signal to remove high frequency noise, then reliability of frequency detection is improved, but device complexity increases due to additional filtering stages and processing
Solution Approach 1:
By performing the filtering action before the PLL rather than after, the patent simplifies the overall system architecture. The pre-filter is a simple passive or active low-pass filter that can be implemented with basic components, avoiding the need for complex digital signal processing or multiple feedback loops that would be required if aggressive smoothing were applied to the PLL output to achieve the same reliability.
3Measurement precision
If a weighted moving average is calculated over multiple samples to smooth the PLL output signal, then measurement precision is improved, but loss of time increases due to the computational delay inherent in processing multiple samples
Solution Approach 1:
The patent performs the noise reduction action in advance, before the frequency measurement process begins. By filtering the input signal beforehand, the measurement can be made more quickly and accurately from the cleaned signal, eliminating the need to collect and process multiple delayed samples after the PLL output is obtained. This shifts the time investment to a preliminary stage rather than extending the measurement delay.
Data Source
AI summary
Method and apparatus for signal processing for controlling a digital audio source may include retrieving a time code signal from a rotating recording medium. The time code signal may be a sum of a binary signal and a sinusoidal pilot signal in the audible range, and the binary signal has a symbol frequency less than a frequency of the pilot signal. The time code signal may be filtered to recover the pilot signal and the binary signal. An estimated speed of rotation of the recording medium may be determined from a recovered pilot signal, and an estimated position may be determined from a recovered binary signal, for controlling the digital audio source.