diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index c4cf266b80..bfc81db26f 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -1374,7 +1374,7 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
         avctx->audio_service_type = AV_AUDIO_SERVICE_TYPE_KARAOKE;
 
     /* get output buffer */
-    frame->nb_samples = s->num_blocks * 256;
+    frame->nb_samples = s->num_blocks * AC3_BLOCK_SIZE;
     if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
         return ret;
 
@@ -1395,7 +1395,7 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
         }
         if (err)
             for (ch = 0; ch < s->out_channels; ch++)
-                memcpy(((float*)frame->data[ch]) + AC3_BLOCK_SIZE*blk, output[ch], 1024);
+                memcpy(((float*)frame->data[ch]) + AC3_BLOCK_SIZE*blk, output[ch], sizeof(**output) * AC3_BLOCK_SIZE);
         for (ch = 0; ch < s->out_channels; ch++)
             output[ch] = s->outptr[channel_map[ch]];
         for (ch = 0; ch < s->out_channels; ch++) {
@@ -1408,7 +1408,7 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
 
     /* keep last block for error concealment in next frame */
     for (ch = 0; ch < s->out_channels; ch++)
-        memcpy(s->output[ch], output[ch], 1024);
+        memcpy(s->output[ch], output[ch], sizeof(**output) * AC3_BLOCK_SIZE);
 
     *got_frame_ptr = 1;