Synchronizing RTSP signals with FFMPEG - ffmpeg

Question

I have achieved to merge two RTSP signals in one with FFMPEG but in the resulting stream, the first input has a delay of between 2 and 5 seconds with respect to the second. And it is always the first input that is delayed compared to the second.
The two RTSP signals come from the same camera model, same configuration, same room ...
However, if I put the same RTSP signal (either of the two) as input 1 and input 2, the same thing happens. Despite being the same signal, the first input is delayed compared to the second one.
How could I get them synchronized?
This is the command that I execute:
ffmpeg -rtsp_transport tcp -thread_queue_size 512 -rtbufsize 50M -r 15
-i rtsp://XXXX -rtsp_transport tcp -thread_queue_size 512 -rtbufsize 50M -r 15
-c:a aac -i rtsp://YYYY -filter_complex "[0:v]pad=iw*2:ih,setpts=PTS-STARTPTS[bg];
[1:v]setpts=PTS-STARTPTS[fg]; [bg][fg]overlay=w[out]" -map "[out]" -f hls
-hls_time 2 -hls_list_size 5 -use_localtime 1 -use_localtime_mkdir 1
-hls_segment_filename 'LIVE/file-%s.ts' -map a -ar 16000 -ac 1 -ab 64000 -c:a aac
-y output.m3u8
Here you have the process informationn:
ffmpeg version 3.4.2 Copyright (c) 2000-2018 the FFmpeg developers
built with Apple LLVM version 9.0.0 (clang-900.0.39.2)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.4.2 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-jack --enable-gpl --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --enable-videotoolbox --disable-lzma
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
[rtsp # 0x7fe284000e00] Missing PPS in sprop-parameter-sets, ignoring
Input #0, rtsp, from 'rtsp://XXXX':
Metadata:
title : Media Presentation
Duration: N/A, start: 0.000000, bitrate: N/A
Stream #0:0: Video: h264 (Main), yuv420p(progressive), 1280x720, 15 fps, 15 tbr, 90k tbn, 30 tbc
Stream #0:1: Audio: aac (LC), 16000 Hz, mono, fltp
[rtsp # 0x7fe28484de00] Missing PPS in sprop-parameter-sets, ignoring
Input #1, rtsp, from 'rtsp://XXXX':
Metadata:
title : Media Presentation
Duration: N/A, start: 0.000000, bitrate: N/A
Stream #1:0: Video: h264 (Main), yuv420p(progressive), 1280x720, 15 fps, 15 tbr, 90k tbn, 30 tbc
Stream #1:1: Audio: aac (LC), 16000 Hz, mono, fltp
Stream mapping:
Stream #0:0 (h264) -> pad (graph 0)
Stream #1:0 (h264) -> setpts (graph 0)
overlay (graph 0) -> Stream #0:0 (libx264)
Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 # 0x7fe286802000] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 # 0x7fe286802000] profile High, level 4.0
[libx264 # 0x7fe286802000] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=1 weightp=2 keyint=250 keyint_min=15 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[hls # 0x7fe286800000] Opening 'LIVE/file-1524728763.ts' for writing
Output #0, hls, to 'output.m3u8':
Metadata:
title : Media Presentation
encoder : Lavf57.83.100
Stream #0:0: Video: h264 (libx264), yuv420p, 2560x720, q=-1--1, 15 fps, 90k tbn, 15 tbc (default)
Metadata:
encoder : Lavc57.107.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Stream #0:1: Audio: aac (LC), 16000 Hz, mono, fltp, 64 kb/s
Metadata:
encoder : Lavc57.107.100 aac
[hls # 0x7fe286800000] Opening 'LIVE/file-1524728782.ts' for writinged=1.09x
[hls # 0x7fe286800000] Opening 'output.m3u8.tmp' for writing
[hls # 0x7fe286800000] Opening 'output.m3u8.tmp' for writing=N/A speed=1.07x
frame= 396 fps= 15 q=-1.0 Lsize=N/A time=00:00:27.00 bitrate=N/A speed=1.05x
video:2946kB audio:147kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Answer 1

From the ffmpeg documentation
https://ffmpeg.org/ffmpeg-filters.html#Examples-139:
try setting video pts with
setpts='(RTCTIME - RTCSTART) / (TB * 1000000)'
(some documentation says this is deprecated but I am using the November 27th build and it works.)
Sync the video streams to the first stream i.e
-map "[out]",0:v
and set pts on the audio
-af 'asetpts=N/SR/TB'
YMMV, but it works with my cheap Chinese IP cams