amrnbdec: set channels, channel_layout, and sample_rate

Only mono 8kHz is supported.

libavcodec/amrnbdec.c

@@ -43,6 +43,7 @@
 #include <string.h>
 #include <math.h>
 
+#include "libavutil/audioconvert.h"
 #include "avcodec.h"
 #include "dsputil.h"
 #include "libavutil/common.h"
@@ -154,6 +155,14 @@ static av_cold int amrnb_decode_init(AVCodecContext *avctx)
     AMRContext *p = avctx->priv_data;
     int i;
 
+    if (avctx->channels > 1) {
+        av_log_missing_feature(avctx, "multi-channel AMR", 0);
+        return AVERROR_PATCHWELCOME;
+    }
+
+    avctx->channels       = 1;
+    avctx->channel_layout = AV_CH_LAYOUT_MONO;
+    avctx->sample_rate    = 8000;
     avctx->sample_fmt     = AV_SAMPLE_FMT_FLT;
 
     // p->excitation always points to the same position in p->excitation_buf