Vijayakumar Moorthy
Listen to this articleYou have an audio file and want to generate a video file for the same. You can do so by transcoding the audio file with an image. We can do this by creating a video stream from the image and attaching the audio stream to it. We use ffmpeg for this process.
Conversion process
Step 1:
Create a video directly from poster image & audio stream.
For example we use the following FFMPEG command in mobile conversion:
ffmpeg -loop 1 -i <<<<INPUT_IMAGE_FILE.jpg>>>> -i <<<<INPUT_AUDIO_FILE.mp3>>>> -crf 25 -c:v libx264 -vf scale=266x150,format=yuv420p -acodec aac -ab 32k -ar 22050 -ac 1 -strict experimental -map 0:0 -map 1:1 -shortest -y <<<<OUTPUT_TEMP_FILE.mp4>>>>What are the parameters used for video encoding?
| Parameter | Value | Purpose |
| -loop | 1 | Repeat the stream for infinite |
| -i | {{INPUT_IMAGE_FILE.jpg}} {{INPUT_AUDIO_FILE.mp3}} | Input files (image & audio files) |
| -crf | 25 | Constant Rate Factor (CRF) tells encoder to attempt to achieve a certain output quality (18 - 28 is decent) |
| -c:v | libx264 | Video codec |
| -vf | scale=266x150,format=yuv420p | Video filter |
| -acodec | aac | Audio codec |
| -ab | 32k | Audio bitrate |
| -ar | 22050 | Audio rate |
| -ac | 1 | Audio channel |
| -map | 0:0 1:1 | Map video stream from input file 0 and audio stream from input file 1 |
| -shortest | nothing | Informs the encoder to use shortest input to mark end of output file |
Step 2:
Adjust the audio length to match the video length (audio padding to match video length).
For example we use the following FFMPEG command in mobile conversion:
ffmpeg -i <<<<OUTPUT_TEMP_FILE.mp4>>>> -map 0:0 -map 0:1 -filter_complex "[0:1]apad" -acodec aac -ab 32k -ar 22050 -ac 1 -strict experimental -shortest -y <<<<OUTPUT_FILE.mp4>>>>What are the parameters used for video encoding?
| Parameter | Value | Purpose |
| -filter_complex | [0:1]apad | Pad the audio stream to match video stream |
| -map | 0:0 0:1 | Map video stream from input file 0 and audio stream from input file 0 |
Other parameters are same as in Step 1.
References:
http://ffmpeg.org/ffmpeg-filters.html#format;
https://trac.ffmpeg.org/wiki/Encode/H.264/;
https://trac.ffmpeg.org/ticket/2214;
http://askubuntu.com/questions/352920/fastest-way-to-convert-videos-batch-or-single
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