re-format usage information output, update README accordingly

README.md

@@ -21,13 +21,32 @@ please read at least the --help information and *understand* what -a does.
 
 ---
 
-How it works (the short version)
---------------------------------
+How it works (precise description)
+-----------------------------------
 
-tcd decodes your audio file, converts it to the frequency domain (like a
-graph showing how much energy exists at each frequency), then measures several
-properties of that frequency graph. Each property is a clue about whether the
-audio was produced by a lossy encoder. Combined, these clues give a verdict.
+tcd decodes the audio file to PCM via FFmpeg, applies a Hann window, and
+computes the FFT (default 4096-point) with 50% overlap for each windowed
+frame. Magnitude spectra from all frames are accumulated and averaged into a
+single power spectrum. From this averaged spectrum, five metrics are computed:
+
+- **Cutoff**: highest frequency bin whose magnitude ≥ peak × 10^(threshold_db/20),
+  controlled by the sensitivity setting (see [Threshold sensitivity](#threshold-sensitivity)).
+- **Steepness**: transition bandwidth between the −20 dB and −60 dB points
+  (relative to peak), measuring how sharply the spectrum rolls off.
+- **Noise floor**: average magnitude in the top quartile of the spectrum
+  (≈16500–22050 Hz at 44.1 kHz sample rate), expressed in dB relative to peak.
+- **Roughness**: coefficient of variation (standard deviation / mean) of
+  magnitude values in the 60%–95% band below cutoff. This quantifies spectral
+  irregularity introduced by quantization noise.
+- **Band ratio**: ratio of mean magnitude in 16–20 kHz to mean magnitude in
+  12–16 kHz. Lossy codecs under-allocate bits above 16 kHz, producing a
+  characteristic dip in this region.
+
+The verdict is determined by evaluating these metrics against empirically
+derived thresholds (which are scaled by the [sensitivity setting](#threshold-sensitivity)),
+first using cutoff + steepness, then secondarily using roughness + band ratio
+for high-cutoff cases. For lossy input formats, cutoff is compared against
+bitrate-specific expectations to detect upscaling.
 
 ---
 
@@ -268,6 +287,77 @@ If neither layer triggers, the file is **GENUINE** (native lossless).
 
 ---
 
+Threshold sensitivity (`-t`)
+-----------------------------
+
+The `-t` parameter (1–99, default 50) controls how aggressively tcd detects
+transcodes and upscaled files. It works at two levels.
+
+### 1. Cutoff detection threshold
+
+The cutoff frequency is defined as the highest bin whose magnitude is at
+least `peak × 10^(threshold_db / 20)`. The percentage is mapped linearly
+to dB:
+
+```
+threshold_db = −(40 + (pct − 1) × 40 / 98)
+```
+
+| `-t` value | threshold_db | Magnitude threshold | Behaviour |
+|-----------|-------------|--------------------|-----------|
+| 1         | −40.0 dB    | 1.0 % of peak      | Least sensitive – only the strongest signal counts |
+| 50        | −60.0 dB    | 0.1 % of peak      | Default – balances sensitivity and specificity |
+| 99        | −80.0 dB    | 0.01 % of peak     | Most sensitive – detects cutoff deep in the noise floor |
+
+A higher `-t` (more negative dB) detects the cutoff at a *higher* frequency
+because it can follow the spectrum deeper into the noise floor. This makes
+the tool stricter: small spectral remnants above a true lossy cutoff are
+still recognised as "signal."
+
+### 2. Verdict-threshold scaling
+
+The same percentage also produces a continuous `sensitivity` factor:
+
+```
+sensitivity = (pct − 1) / 98.0
+```
+
+| `-t` value | sensitivity | Effect on verdict thresholds |
+|-----------|-------------|------------------------------|
+| 1         | 0.00        | Doubles the allowed steepness – very permissive, few false positives |
+| 50        | 0.50        | Base thresholds used as published above |
+| 99        | 1.00        | Steepness limits multiplied by 0.25; roughness thresholds lowered by up to 4× – very strict, catches borderline cases |
+
+The table of steepness thresholds used in the transcode verdict (see
+[How tcd combines these clues](#how-tcd-combines-these-clues-into-a-verdict))
+is scaled by:
+
+```
+bw_factor = max(2.0 × (1.0 − sensitivity), 0.25)
+allowed_steepness = base_max_bw × bw_factor
+```
+
+The roughness thresholds (r1 = 0.40, r2 = 0.30, r3 = 0.20) are likewise
+scaled:
+
+```
+r1 = 0.40 × (1.0 + (0.5 − sensitivity) × 1.5)
+r2 = 0.30 × (1.0 + (0.5 − sensitivity) × 1.5)
+r3 = 0.20 × (1.0 + (0.5 − sensitivity) × 1.5)
+```
+
+### 3. Practical guidance
+
+- **Default (50):** well-tuned for most material. Use this unless you have a
+  specific reason to change it.
+- **Lower values (1–40):** reduce false positives on already-filtered material
+  (e.g. deliberate lowpass during mastering). Rarely needed.
+- **Higher values (60–99):** catch transcodes that leave very little spectral
+  evidence. Useful for batch cleaning of a library, but may increase false
+  positives on quiet or synthetic content.
+
+---
+
 Confidence score
 ----------------
 
@@ -341,30 +431,46 @@ positives on legitimate encodes that simply use a conservative lowpass.
 Usage
 -----
 
-```
     tcd [options] <audio-file>
 
- -t, --threshold PCT  Overall detection sensitivity (1-99). Controls all
-            decision thresholds: transition bandwidth, roughness,
-            and band ratio. Maps to -40 dB cutoff level (1, least
-            sensitive) through -80 dB (99, most sensitive).
-            [default: 50]. 50 is neutral; lower = fewer detections,
-            higher = more detections. Adjust in small steps.
- -f, --fft-size N   FFT size (power of 2) [default: 4096]
- -d, --duration SEC  Max seconds to analyze [default: 60]
- -r, --recursive    Recurse into subdirectories
- -F, --full      Analyze entire file (overrides --duration)
- -v, --verbose     Verbose output
- -s, --visual     Graphical spectrum visualization (TUI)
- -V          Alias for -s
- -a, --auto-remove   Automatically remove non-native files
- -h, --help      Show this help
+When run, tcd prints an analysis summary. Example:
+
 ```
+File:         ./08 You Got Me.mp3
+Format:       mp3
+Bitrate:      320 kbps
+Sample rate:  44100 Hz
+Channels:     2
+Windows:      13550
+Peak:         47.5 dBFS
+Cutoff:       20790 / 22050 Hz = 94.3%
+Steepness:    20209 Hz
+Roughness:    0.323
+Band ratio:   0.555
+Noise floor:  -56.4 dB
+Verdict:      NATIVE
+Verdict Info: bandwidth used=94.3% (20790/22050 Hz), expected≥90% for 320 kbps
+```
+
+Each field is explained in the [What each number means](#what-tcd-displays-and-what-each-number-means) section.
+
+Options:
+
+    -t, --threshold PCT     Detection sensitivity 1-99 [50]
+    -f, --fft-size N        FFT size, power of 2 [4096]
+    -d, --duration SEC      Seconds to analyze [120]
+    -r, --recursive         Recurse into subdirectories
+    -F, --full              Analyze entire file (no duration limit)
+    -v, --verbose           Verbose output with decision log
+    -s, --visual            Graphical spectrum TUI visualization
+    -V                      Alias for -s
+    -a, --auto-remove       Automatically delete detected transcodes
+    -h, --help              Show this help screen
 
 Exit codes:
 
 | Code | Meaning                             |
-|------|-----------------------------------|
+|------|-------------------------------------|
 | 0    | NATIVE or GENUINE (file is clean)   |
 | 1    | UPSCALED or TRANSCODE detected      |
 | 2    | SILENT (no detectable content)      |

tcd.c

@@ -939,8 +939,7 @@ static void print_help(const char *prog)
     {
         char buf[256];
         snprintf(buf, sizeof(buf), ANSI_BOLD ANSI_CYAN "tcd" ANSI_RESET
-                 " \xe2\x80\x94 Transcode Detector   "
-                 "Psychoacoustic audio authenticity analysis");
+                 " (Transcode Detector / Psychoacoustic audio authenticity analysis tool)");
         int v = vis_len(buf);
         printf("  %s", buf);
         int pad = tw - 2 - v;
@@ -967,8 +966,13 @@ static void print_help(const char *prog)
     printf("  " ANSI_BOLD "Options:" ANSI_RESET);
     printf("\n");
 
+#define OPT_COL 28
+
 #define OPT(fmt, desc) do { \
-    printf("    " ANSI_GREEN fmt ANSI_RESET "  %s", desc); \
+    int flen = vis_len(fmt); \
+    int pad = OPT_COL - flen; \
+    if (pad < 1) pad = 1; \
+    printf("    " ANSI_GREEN "%s" ANSI_RESET "%*s%s", fmt, pad, "", desc); \
     printf("\n"); \
 } while (0)
 
@@ -977,12 +981,6 @@ static void print_help(const char *prog)
         snprintf(buf, sizeof(buf), "Detection sensitivity 1-99 [" ANSI_CYAN "%d" ANSI_RESET "]", THRESHOLD_DEFAULT);
         OPT("-t, --threshold PCT", buf);
     }
-    {
-        char buf[80];
-        snprintf(buf, sizeof(buf), ANSI_DIM "  Lower = fewer, higher = more" ANSI_RESET);
-        printf("    %s", buf);
-        printf("\n");
-    }
     {
         char buf[80];
         snprintf(buf, sizeof(buf), "FFT size, power of 2 [" ANSI_CYAN "%d" ANSI_RESET "]", FFT_SIZE_DEFAULT);