Attempt to allow up to 31 channels of output, up from a limit of 8. Modify the mixdown setting so that "auto" only mixes downwards, e.g. 7.1 to 5.1, and never upwards, e.g. stereo to 5.1. Upmixing is not functional anyway, as the upmixing matrices are all such that the input channels are copied to their equivalent output channels and other output channels remain silent.

audio.c

@@ -458,7 +458,7 @@ uint32_t get_channel_settings(const char *stanza_name, const char *setting_name)
           for (i = 0; i < channel_counts_count; i++) {
             debug(3, "channel count setting %d: %d.", i, channel_counts[i]);
 
-            if ((channel_counts[i] >= 1) && (channel_counts[i] <= 8)) {
+            if ((channel_counts[i] >= 1) && (channel_counts[i] <= SPS_GREATEST_CHANNEL_COUNT)) {
 #ifdef CONFIG_FFMPEG
               channel_set |= (1 << channel_counts[i]);
 #else
@@ -492,7 +492,7 @@ uint32_t get_channel_settings(const char *stanza_name, const char *setting_name)
   char numbuf[32];
   unsigned int c;
   uint32_t t_channel_set = channel_set;
-  for (c = 0; c <= 8; c++) {
+  for (c = 0; c <= SPS_GREATEST_CHANNEL_COUNT; c++) {
     if ((t_channel_set & (1 << c)) != 0) {
       snprintf(numbuf, sizeof(numbuf) - 1, "%u", c);
       snprintf(p, sizeof(buf) - (p - buf) - 1, "%s", numbuf);

common.h

@@ -146,7 +146,13 @@ typedef enum {
 // these sets omit the _UNKNOWN, _AUTO and _ILLEGAL values
 #define SPS_FORMAT_SET (((1 << (SPS_FORMAT_HIGHEST_NATIVE + 1)) - 1) - (1 << SPS_FORMAT_UNKNOWN))
 #define SPS_RATE_SET (((1 << (SPS_RATE_HIGHEST + 1)) - 1) - (1 << SPS_RATE_UNKNOWN))
-#define SPS_CHANNEL_SET (((1 << (8 + 1)) - 1) - (1 << 0)) // channels 1 to 8, not 0-based!
+
+// in SPS_CHANNEL_SET, bit 0 set means a channel set of no channels, bit 1 set means a channel set of 1 channel
+// and so on to bit 31 meaning a channel set of 31 channels. We want to consider all possible channel sets
+// apart from channel set 0.
+#define SPS_GREATEST_CHANNEL_COUNT 31 // should be 32 to be fully in line with ALSA limits
+#define SPS_CHANNEL_SET 0xFFFFFFFE // channel sets 31 to 1, but no channel set 0
+// #define SPS_CHANNEL_SET (((1 << (SPS_GREATEST_CHANNEL_COUNT + 1)) - 1) - (1 << 0)) // channels 1 to 31, not 0-based!
 
 #ifndef CONFIG_AIRPLAY_2
 #define SPS_FORMAT_NON_FFMPEG_SET SPS_FORMAT_SET

player.c

@@ -656,24 +656,43 @@ int setup_software_resampler(rtsp_conn_info *conn, ssrc_t ssrc) {
     // set the resampler's channel layout either automatically or use the
     // setting that has been given
 
+    // We impose the rule that, if the mixdown setting is "auto",
+    // we will not upmix if the number of output channels available
+    // is greater than the number of channels needed.
+
+    // To do an upmix, you will have to specify the upmix target, e.g. "7.1"
+    // in the mixdown setting.
+    
+    // NOTE: upmixing at present, by default, simply copies the input channels to their
+    // equivalents in the output channel set. All the other channels are
+    // left alone.
+
 #if LIBAVUTIL_VERSION_MAJOR >= 57
     {
       AVChannelLayout input_channel_layout;
       av_channel_layout_from_mask(&input_channel_layout, input_layout);
+      int input_channel_count = input_channel_layout.nb_channels;
       av_opt_set_chlayout(swr, "in_chlayout", &input_channel_layout, 0);
       av_channel_layout_uninit(&input_channel_layout);
 
       AVChannelLayout output_channel_layout;
       if (config.mixdown_enable != 0) {
         if (config.mixdown_channel_layout == 0) {
-          av_channel_layout_default(&output_channel_layout,
+          if (CHANNELS_FROM_ENCODED_FORMAT(output_configuration) > (unsigned) input_channel_count) {
+            av_channel_layout_from_mask(&output_channel_layout, input_layout);
+          } else {
+            av_channel_layout_default(&output_channel_layout,
                                     CHANNELS_FROM_ENCODED_FORMAT(output_configuration));
+          }
         } else {
           av_channel_layout_from_mask(&output_channel_layout, config.mixdown_channel_layout);
         }
       } else {
         av_channel_layout_from_mask(&output_channel_layout, output_layout);
       }
+      char layout_desc[2048];
+      av_channel_layout_describe(&output_channel_layout, layout_desc, sizeof(layout_desc));
+      debug(1,"output channel layout: \"%s\"", layout_desc);
       av_opt_set_chlayout(swr, "out_chlayout", &output_channel_layout, 0);
       av_channel_layout_uninit(&output_channel_layout);
     }
@@ -681,8 +700,10 @@ int setup_software_resampler(rtsp_conn_info *conn, ssrc_t ssrc) {
     av_opt_set_int(swr, "in_channel_layout", input_layout, 0);
     if (config.mixdown_enable != 0) {
       if (config.mixdown_channel_layout == 0) {
-        output_layout =
-            av_get_default_channel_layout(CHANNELS_FROM_ENCODED_FORMAT(output_configuration));
+        if (CHANNELS_FROM_ENCODED_FORMAT(output_configuration) < av_get_channel_layout_nb_channels(input_layout)) {
+          output_layout =
+              av_get_default_channel_layout(CHANNELS_FROM_ENCODED_FORMAT(output_configuration));
+        } // else leave output_layout as it was: the sames as the input_layout.
       } else {
         output_layout = config.mixdown_channel_layout;
       }
@@ -962,7 +983,7 @@ int setup_software_resampler(rtsp_conn_info *conn, ssrc_t ssrc) {
 
       if (output_configuration_changed != 0) {
         char channel_mapping_list[256] = "";
-        for (c = 0; c < 8; c++) {
+        for (c = 0; c < SPS_GREATEST_CHANNEL_COUNT; c++) {
           if ((output_channels[c].allocated != 0) &&
               (conn->output_channel_to_resampler_channel_map[c] != silent_channel_index)) {
             char channel_mapping[32] = "";