Merging video and audio stream, where audio drifts - audio

Question

I want to record audio and video with my raspberry pi b+ 2.
I tried to accomplish this with one ffmpeg command but this is to slow. and i could not get it working correctly
I have a raspberry pi camera module and a Cirrus audio card. On the raspberry i have compiled a new kernel with support for the audio card. I also compiled ffmpeg on the raspberr with alsa support
~$ ffmpeg
ffmpeg version N-71470-g2db24cf Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 4.6 (Debian 4.6.3-14+rpi1)
configuration: --arch=armel --target-os=linux --enable-gpl --extra-libs=-lasound --enable-nonfree
libavutil 54. 22.101 / 54. 22.101
libavcodec 56. 34.100 / 56. 34.100
libavformat 56. 30.100 / 56. 30.100
libavdevice 56. 4.100 / 56. 4.100
libavfilter 5. 14.100 / 5. 14.100
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 1.100 / 1. 1.100
libpostproc 53. 3.100 / 53. 3.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...
Now i try to record an audio stream and a video stream 'at the same time'
I do this my running a shell script
raspivid -t 60000 -vs -w 1280 -h 720 -b 5000000 -fps 25 -o video.h264 &
arecord -Dhw:sndrpiwsp -r 44100 -c 2 -d 60 -f S32_LE audio.aac
i also tried with -r 22050 and -f S16_LE
when running this it sometimes gives an (i think)
overrun!!! (at least 1038.725 ms long)
at the end of the script i have two files. a video and a audio file.
now i want to merge those two together by using ffmpeg
ffmpeg -i video.h264 -i audio.aac -c:v copy -c:a aac -strict experimental output.mp4
this gives the output:
ffmpeg version N-71470-g2db24cf Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 4.6 (Debian 4.6.3-14+rpi1)
configuration: --arch=armel --target-os=linux --enable-gpl --extra-libs=-lasound --enable-nonfree
libavutil 54. 22.101 / 54. 22.101
libavcodec 56. 34.100 / 56. 34.100
libavformat 56. 30.100 / 56. 30.100
libavdevice 56. 4.100 / 56. 4.100
libavfilter 5. 14.100 / 5. 14.100
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 1.100 / 1. 1.100
libpostproc 53. 3.100 / 53. 3.100
Input #0, h264, from 'video_1min_3.h264':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: h264 (High), yuv420p, 1280x720, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Guessed Channel Layout for Input Stream #1.0 : stereo
Input #1, wav, from 'audio_1min_3.aac':
Duration: 00:01:00.00, bitrate: 705 kb/s
Stream #1:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 22050 Hz, 2 channels, s16, 705 kb/s
[mp4 # 0x3230f20] Codec for stream 0 does not use global headers but container format requires global headers
Output #0, mp4, to 'output_1min_3.mp4':
Metadata:
encoder : Lavf56.30.100
Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720, q=2-31, 25 fps, 25 tbr, 1200k tbn, 1200k tbc
Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 22050 Hz, stereo, fltp, 128 kb/s
Metadata:
encoder : Lavc56.34.100 aac
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))
Press [q] to stop, [?] for help
frame= 1822 fps=310 q=-1.0 Lsize= 33269kB time=00:01:12.84 bitrate=3741.7kbits/s
video:32300kB audio:941kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.086073%
so finally i have a file output.mp4 that is a movie with audio that is in sync at the beginning but drifts away to a difference of about 4 seconds. where the audio is ahead of the video.
I hope you can help me trying to solve this issue so the audio does not drift away anymore.
Thanks in advance
( i tried to be as clear as possible )

Answer 1

We can try to use the -async and -vsync options to correct the audio and video time shift.
for example, i have used the below option to reduce the time lag of 2 sec seen in the audio.
./ffmpeg -async 1 -i "weatherinput.mov" -strict -2 -vcodec libx264 -movflags +faststart -vprofile high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -threads 0 -b:a 128k -pix_fmt yuv420p "weatheroutput.mp4"
Also we can use vsync options if required apart from the ioffset.
The link below can also referred for other combination of using th async, vsync and i offset to avoid the drift.