CALCRATIO

Table of Contents

• Introduction
• Key Features
• Algorithm Overview
  • API Sequence
• API

Introduction

The CalcRatio algorithm is a real-time sample rate ratio estimator designed to track clock mismatch between two asynchronous audio domains. It computes the ratio of output (Tx) to input (Rx) sample counters and applies a Two-stage moving average to deliver a smooth, stable actual ratio for Asynchronous Sample Rate Conversion (ASRC).

Key Features

• DMA-Counter-Based Ratio Calculation
• Two-Stage Moving Average
• One-Shot Noise Rejection
• High Accuracy Ratio Tracking

Algorithm Overview

The CalcRatio algorithm works as follows:

1. Initialization (calcRatio_init()) :
   • Initializes the CALC_RATIO module with input and output sample rates.
   • Computes the target ratio (sampleRateOut / sampleRateIn).
   • Resets all internal states:
     • Moving-average buffers
     • Window pointers
     • Accumulated sums
     • Counters and the one-shot flag
   • Prepares the module for accurate ratio estimation.
2. First Stage Moving Average (MA1) :
   • Applies a 1024-point moving average to the instantaneous ratio.
   • Removes high-frequency noise and sudden spikes.
   • Produces a smoothed ratio that still responds to short-term changes.
3. Second Stage Filtering (MA2) :
   • Applies a 128-point moving average to the output of MA1.
   • Reduces long-term drift and stabilizes the ratio.
   • Generates a clean, low-noise ratio used by the resampling stage.
4. Ratio Calculation (calcRatio_exec()) :
   • Reads instantaneous ratio from the TX/RX DMA counters.
   • Uses a one-shot skip to ignore the first noisy measurement.
   • Updates MA1 and MA2 buffers and their accumulated sums.
   • Computes the actual ratio using both filtering stages.
5. Output (calcRatio_getActualratio())
   • Provides two key ratio values:
     • Target Ratio: Calculated at initialization.
     • Actual Ratio: Filtered, stable ratio for SW_ASRC operation.
   • Thread-safe access using a global semaphore to avoid race conditions.

API Sequence

To use CALC_RATIO, the application should call the following APIs:

• calcRatio_init():Initialize the CALC_RATIO module .
• calcRatio_exec():Processes the instantaneous ratio input and computes the filtered, stable actual ratio.
• calcRatio_getActualratio(): Returns the latest calculated actual ratio for use by the SW_ASRC

API

APIs for CALCRATIO