set gop size to 20

@@ -234,6 +234,7 @@ int CAVTranscoder::open_output_file(const char *filename)
 				/* video time_base can be set to whatever is handy and supported by encoder */
 				enc_ctx->time_base.num = 1;
 				enc_ctx->time_base.den = 20;
+				enc_ctx->gop_size = 20;
 
 				enc_ctx->me_range = 16;
 				enc_ctx->max_qdiff = 4;