audioconvert: Always apply noise when asked

Rename empty.noise -> dither.noise and always add this amount of noise
when > 0. This also adds the noise to silent sounds, not only when
nothing is connected because that would also be a problem when an amp
needs to be kept alive with an non-0 signal.

Rename noise -> dither because we can use this also for dithering later.

See #705

spa/plugins/audioconvert/dither-ops.c

@@ -0,0 +1,84 @@
+/* Spa
+ *
+ * Copyright © 2022 Wim Taymans
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include <string.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <math.h>
+
+#include <spa/param/audio/format-utils.h>
+#include <spa/support/cpu.h>
+#include <spa/support/log.h>
+#include <spa/utils/defs.h>
+
+#include "dither-ops.h"
+
+typedef void (*dither_func_t) (struct dither *d, void * SPA_RESTRICT dst[],
+		const void * SPA_RESTRICT src[], uint32_t n_samples);
+
+static const struct dither_info {
+	dither_func_t process;
+	uint32_t cpu_flags;
+} dither_table[] =
+{
+	{ dither_f32_c, 0 },
+};
+
+#define MATCH_CPU_FLAGS(a,b)	((a) == 0 || ((a) & (b)) == a)
+
+static const struct dither_info *find_dither_info(uint32_t cpu_flags)
+{
+	size_t i;
+	for (i = 0; i < SPA_N_ELEMENTS(dither_table); i++) {
+		if (!MATCH_CPU_FLAGS(dither_table[i].cpu_flags, cpu_flags))
+			continue;
+		return &dither_table[i];
+	}
+	return NULL;
+}
+
+static void impl_dither_free(struct dither *d)
+{
+	d->process = NULL;
+}
+
+int dither_init(struct dither *d)
+{
+	const struct dither_info *info;
+	size_t i;
+
+	info = find_dither_info(d->cpu_flags);
+	if (info == NULL)
+		return -ENOTSUP;
+
+	if (d->intensity >= 64)
+		return -EINVAL;
+
+	for (i = 0; i < SPA_N_ELEMENTS(d->tab); i++)
+		d->tab[i] = (drand48() - 0.5) / (UINT64_C(1) << d->intensity);
+
+	d->free = impl_dither_free;
+	d->process = info->process;
+	return 0;
+}