[FFmpeg-devel] [PATCH] af_hdcd: Improve HDCD detection

[Burt P]

HDCD is only "detected" if a valid code is active in both
channels simultaneously, as described here:
https://hydrogenaud.io/index.php/topic,79427.msg900371.html#msg900371

Signed-off-by: Burt P <pburt0 at gmail.com>
---
 libavfilter/af_hdcd.c | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/libavfilter/af_hdcd.c b/libavfilter/af_hdcd.c
index 6f0db71..4b48967 100644
--- a/libavfilter/af_hdcd.c
+++ b/libavfilter/af_hdcd.c
@@ -818,15 +818,25 @@ static const int32_t gaintab[] = {
 
 typedef struct {
     uint64_t window;
-    unsigned char readahead, arg, control;
-    int running_gain;
-    unsigned sustain, sustain_reset;
-    int code_counterA;
-    int code_counterA_almost; /* looks like an A code, but a bit expected to be 0 is 1 */
-    int code_counterB;
+    unsigned char readahead;
+
+    /* arg is set when a packet prefix is found.
+     * control is the active control code, where
+     * bit 0-3: target_gain, 4-bit (3.1) fixed-point value
+     * bit 4  : peak_extend
+     * bit 5  : transient_filter
+     * bit 6,7: always zero */
+    unsigned char arg, control;
+    unsigned sustain, sustain_reset; /* code detect timer */
+
+    int running_gain; /* 11-bit (3.8) fixed point, extended from target_gain */
+
+    int code_counterA;            /* 8-bit format packet */
+    int code_counterA_almost;     /* looks like an A code, but a bit expected to be 0 is 1 */
+    int code_counterB;            /* 16-bit format packet, 8-bit code, 8-bit XOR of code */
     int code_counterB_checkfails; /* looks like a B code, but doesn't pass the XOR check */
-    int code_counterC;
-    int code_counterC_unmatched; /* told to look for a code, but didn't find one */
+    int code_counterC;            /* packet prefix was found, expect a code */
+    int code_counterC_unmatched;  /* told to look for a code, but didn't find one */
 
     /* For user information/stats, pulled up into HDCDContext
      * by filter_frame() */
@@ -1096,7 +1106,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     AVFrame *out;
     const int16_t *in_data;
     int32_t *out_data;
-    int n, c;
+    int n, c, detect;
 
     out = ff_get_audio_buffer(outlink, in->nb_samples);
     if (!out) {
@@ -1112,6 +1122,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         out_data[n] = in_data[n];
     }
 
+    detect = 0;
     s->det_errors = 0;
     for (c = 0; c < inlink->channels; c++) {
         hdcd_state_t *state = &s->state[c];
@@ -1120,11 +1131,14 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         s->uses_peak_extend |= !!state->count_peak_extend;
         s->uses_transient_filter |= !!state->count_transient_filter;
         s->max_gain_adjustment = FFMIN(s->max_gain_adjustment, GAINTOFLOAT(state->max_gain));
-        s->hdcd_detected |= state->code_counterB || state->code_counterA;
+        if (state->sustain) detect++;
         s->det_errors += state->code_counterA_almost
             + state->code_counterB_checkfails
             + state->code_counterC_unmatched;
     }
+    /* HDCD is detected if a valid packet is active in all (both)
+     * channels at the same time. */
+    if (detect == inlink->channels) s->hdcd_detected = 1;
 
     av_frame_free(&in);
     return ff_filter_frame(outlink, out);
-- 
2.7.4