+void register_writer_node(struct writer_node *wn, struct btr_node *parent);
+int setup_writer_node(const char *arg, struct btr_node *parent,
+ struct writer_node *wn);
+int get_btr_samplerate(struct btr_node *btrn, int32_t *result);
+int get_btr_channels(struct btr_node *btrn, int32_t *result);