Merge pull request #2 from PRX/feat/replace-sox-with-ffmpeg

Replace sox with ffmpeg for audio conversion and filtering

Author: kookster

.github/workflows/node.js.yml

@@ -20,10 +20,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
-    - name: Install sox and multimon-ng
+    - name: Install ffmpeg and multimon-ng
       run: |
         sudo apt-get update
-        sudo apt-get install -y sox multimon-ng
+        sudo apt-get install -y ffmpeg multimon-ng
     - name: Use Node.js ${{ matrix.node-version }}
       uses: actions/setup-node@v4
       with:

README.md

@@ -9,8 +9,8 @@ Supports full and **partial** match detection — even if only the attention ton
 ## Requirements
 
 - Node.js >= 18
-- [SoX](https://sox.sourceforge.net/)
-- [multimon-ng](https://github.com/EliasOenal/multimon-ng)
+- [FFmpeg](https://ffmpeg.org/) (`brew install ffmpeg`)
+- [multimon-ng](https://github.com/EliasOenal/multimon-ng) (build from source or `apt install multimon-ng`)
 
 ## Install
 
@@ -20,7 +20,7 @@ npm install @prx.org/eas-detect
 
 ## CLI Usage
 
-Call on any audio file, it will use sox to analyze the file:
+Call on any audio file, it will use ffmpeg to analyze the file:
 
 ```bash
 npx eas-detect recording.wav
@@ -177,7 +177,7 @@ Three detection methods run on the audio:
 
 1. **Attention tone detection** — [Goertzel algorithm](https://en.wikipedia.org/wiki/Goertzel_algorithm) tuned to 853 Hz and 960 Hz (the EAS dual-tone attention signal), using energy-ratio analysis. Also runs on bandpass-filtered audio (800-1000 Hz) to catch tones buried under speech or music.
 2. **FSK detection** — Detects the FSK modulation pattern (mark at 2083.3 Hz, space at 1562.5 Hz). The default mode uses Goertzel energy ratios with mark/space alternation validation. The sensitive mode applies narrow bandpass filters around each frequency and detects anti-correlation between the mark and space energy envelopes using Pearson correlation.
-3. **SAME header decoding** — Pipes audio through [SoX](https://sox.sourceforge.net/) into [multimon-ng](https://github.com/EliasOenal/multimon-ng) for full SAME protocol decoding with error correction.
+3. **SAME header decoding** — Converts audio via [FFmpeg](https://ffmpeg.org/) and pipes into [multimon-ng](https://github.com/EliasOenal/multimon-ng) for full SAME protocol decoding with error correction.
 
 Decoded SAME headers are parsed and enriched with human-readable lookups from 3,235 FIPS county codes and all standard EAS originator/event codes.
 

src/audio.js

@@ -7,9 +7,9 @@ const SAMPLE_RATE = 22050;
 
 /**
  * Read an audio file and return mono PCM samples as a Float64Array at 22050 Hz.
- * Uses sox for format conversion. Also accepts raw s16le PCM if `raw` option is set.
+ * Uses ffmpeg for format conversion. Also accepts raw s16le PCM if `raw` option is set.
  *
- * For non-raw input, sox writes to a temp raw file allowing files of any length
+ * For non-raw input, ffmpeg writes to a temp raw file allowing files of any length
  * to be processed. The temp file path is returned as `rawPath` so downstream
  * consumers (e.g., multimon-ng) can reuse it without re-converting.
  *
@@ -29,20 +29,20 @@ export function readAudio(filePath, { raw = false, sampleRate = SAMPLE_RATE } =
     const dir = mkdtempSync(join(tmpdir(), "eas-"));
     rawPath = join(dir, "audio.raw");
 
-    execFileSync("sox", [
+    execFileSync("ffmpeg", [
+      "-i",
       filePath,
-      "-t",
-      "raw",
-      "-e",
-      "signed-integer",
-      "-b",
-      "16",
-      "-r",
+      "-f",
+      "s16le",
+      "-acodec",
+      "pcm_s16le",
+      "-ar",
       String(sampleRate),
-      "-c",
+      "-ac",
       "1",
+      "-y",
       rawPath,
-    ]);
+    ], { stdio: ["pipe", "pipe", "pipe"] });
     buf = readFileSync(rawPath);
   }
 
@@ -57,9 +57,9 @@ export function readAudio(filePath, { raw = false, sampleRate = SAMPLE_RATE } =
 }
 
 /**
- * Apply a sox bandpass filter to a raw PCM file and return filtered samples.
- * Used to isolate EAS frequency bands before energy-ratio analysis, improving
- * detection of tones buried under speech or music.
+ * Apply a bandpass filter to a raw PCM file and return filtered samples.
+ * Uses ffmpeg's sinc FIR filter generator with afir convolution to produce
+ * a sharp bandpass response equivalent to sox's sinc filter.
  *
  * @param {string} rawPath - Path to s16le 22050 Hz mono raw PCM file
  * @param {number} lowFreq - Lower edge of the bandpass filter in Hz
@@ -71,33 +71,28 @@ export function bandpassFilter(rawPath, lowFreq, highFreq, sampleRate = SAMPLE_R
   const dir = mkdtempSync(join(tmpdir(), "eas-bp-"));
   const filteredPath = join(dir, "filtered.raw");
 
+  // Generate a FIR bandpass kernel with ffmpeg's sinc filter (hp + lp cutoffs),
+  // then apply it to the audio with afir. This matches sox's sinc filter quality:
+  // steep rolloff, flat passband, linear phase.
   try {
-    execFileSync("sox", [
-      "-t",
-      "raw",
-      "-e",
-      "signed-integer",
-      "-b",
-      "16",
-      "-r",
+    execFileSync("ffmpeg", [
+      "-f",
+      "s16le",
+      "-ar",
       String(sampleRate),
-      "-c",
+      "-ac",
       "1",
+      "-i",
       rawPath,
-      "-t",
-      "raw",
-      "-e",
-      "signed-integer",
-      "-b",
-      "16",
-      "-r",
-      String(sampleRate),
-      "-c",
-      "1",
+      "-filter_complex",
+      `sinc=hp=${lowFreq}:lp=${highFreq}:r=${sampleRate}[ir];[0:a][ir]afir`,
+      "-f",
+      "s16le",
+      "-acodec",
+      "pcm_s16le",
+      "-y",
       filteredPath,
-      "sinc",
-      `${lowFreq}-${highFreq}`,
-    ]);
+    ], { stdio: ["pipe", "pipe", "pipe"] });
 
     const buf = readFileSync(filteredPath);
     const sampleCount = buf.length / 2;

src/fsk-detect-sensitive.js

@@ -7,7 +7,7 @@
  * alternate at the baud rate. Music harmonics tend to rise and fall together
  * (positive correlation).
  *
- * Uses narrow bandpass filters (±50 Hz) via sox to isolate each FSK frequency,
+ * Uses narrow bandpass filters (±50 Hz) via ffmpeg to isolate each FSK frequency,
  * then computes energy envelopes and Pearson correlation over sliding regions.
  * The bandpass improves sensitivity by removing competing frequencies that
  * would dilute the correlation signal.