Don't force 2.0 downmix with KEEP_DMIX_2CH flag alone.

Previously setting KEEP_DMIX_2CH flag alone forced the 2.0 downmix using
default coefficients if none are embedded, while combining it with
KEEP_DMIX_6CH performed the 2.0 downmix only when custom coefficients
are present.

Inverse the meaning of combination of these flags: make KEEP_DMIX_2CH
flag extract embedded downmix when used alone. Add special
FORCE_DMIX_2CH constant (both flags set) to force downmix.

Forcing downmix with a single KEEP_DMIX_2CH flag is a bad idea as it
might result in clipping for streams that were not prepared for 2.0
downmix. Make this caveat explicit.

README.md

@@ -25,10 +25,13 @@ Help screen of the program is reproduced below.
 Usage: ./dcadec [-26bcfhlmnPqSx] <input.dts> [output.wav]
 dcadec is a free DTS Coherent Acoustics decoder. Supported options:
 
--2  Extract embedded 2.0 downmix.
+-2  Extract embedded 2.0 downmix if present, otherwise extract 5.1 downmix.
 
 -6  Extract embedded 5.1 downmix.
 
+-26 Extract embedded 2.0 downmix if present, otherwise force 2.0 downmix using
+    default coefficients. Might result in clipping.
+
 -b  Force fixed point DTS core interpolation. Developer option, degrades sound
     quality.
 

dcadec.c

@@ -46,10 +46,13 @@ static void print_help(char *name)
 "Usage: %s [-26bcfhlmnPqSx] <input.dts> [output.wav]\n"
 "dcadec is a free DTS Coherent Acoustics decoder. Supported options:\n"
 "\n"
-"-2  Extract embedded 2.0 downmix.\n"
+"-2  Extract embedded 2.0 downmix if present, otherwise extract 5.1 downmix.\n"
 "\n"
 "-6  Extract embedded 5.1 downmix.\n"
 "\n"
+"-26 Extract embedded 2.0 downmix if present, otherwise force 2.0 downmix using\n"
+"    default coefficients. Might result in clipping.\n"
+"\n"
 "-b  Force fixed point DTS core interpolation. Developer option, degrades sound\n"
 "    quality.\n"
 "\n"

libdcadec/dca_context.c

@@ -144,17 +144,17 @@ static void clip_samples(struct dcadec_context *dca, int nchannels)
 static int down_mix_prim_chset(struct dcadec_context *dca, int **samples,
                                int nsamples, int *ch_mask, int *dmix_coeff)
 {
-    // With both KEEP_DMIX flags set, perfrom 2.0 downmix only when custom
-    // matrix is present
-    if (!dmix_coeff && (dca->flags & DCADEC_FLAG_KEEP_DMIX_6CH))
-        return 0;
-
     // No action if already 2.0. Remove LFE channel if 2.1.
     if ((*ch_mask & ~SPEAKER_MASK_LFE1) == (SPEAKER_MASK_L | SPEAKER_MASK_R)) {
         *ch_mask = SPEAKER_MASK_L | SPEAKER_MASK_R;
         return 0;
     }
 
+    // Unless both KEEP_DMIX flags are set, perform 2.0 downmix only when
+    // custom matrix is present
+    if (!dmix_coeff && !(dca->flags & DCADEC_FLAG_KEEP_DMIX_6CH))
+        return 0;
+
     // Reallocate downmix sample buffer
     if (ta_alloc_fast(dca, &dca->dmix_sample_buffer, 2 * nsamples, sizeof(int)) < 0)
         return -DCADEC_ENOMEM;

libdcadec/dca_context.h

@@ -99,12 +99,18 @@
 /* Use FIR filter for floating point DTS core LFE channel interpolation */
 #define DCADEC_FLAG_CORE_LFE_FIR        0x10
 
-/** Extract embedded 2.0 downmix */
+/** Extract embedded 2.0 downmix if present, otherwise extract 5.1 downmix */
 #define DCADEC_FLAG_KEEP_DMIX_2CH       0x20
 
 /** Extract embedded 5.1 downmix */
 #define DCADEC_FLAG_KEEP_DMIX_6CH       0x40
 
+/**
+ * Extract embedded 2.0 downmix if present, otherwise force 2.0 downmix using
+ * default coefficients. Might result in clipping.
+ */
+#define DCADEC_FLAG_FORCE_DMIX_2CH      0x60
+
 /** Output native DTS channel layout, not WAVEFORMATEXTENSIBLE layout */
 #define DCADEC_FLAG_NATIVE_LAYOUT       0x80