NPR One: How do I format my audio to MP4?
We worked with an advisory group of stations to better understand the workflow and products necessary to meet the preferred format of HE-AAC/.MP4. We are providing recommendations below based on what has worked for other stations. Please be advised that we are only providing these examples as recommendations, not requirements.
Note: You may incur additional costs or licensing fees, depending on which product you find works best for your station.
Why HE-AAC/.MP4 Audio QualityHighest quality/sounds the best - This is an app and people are listening on the go.
- Standardization for seamless listening from national to local newscasts
- Smaller file format, which saves space on your hosting server
- Smaller files mean a faster load time to reach the CDN (Content Delivery Network), if it takes too long to load it will be skipped
- Better for mobile devices
- Note: MP4 preferred, MP3 accepted - Using MP3 means that your newscasts will take longer to load and may get skipped due to longer loading/buffering times. And also creates a jarring experience for the listener if the quality isn't high.
Channels: 1 (mono)
Bit Rate: 64 kbps
Sampling Rate: 44.1 Khz
Normalization: -16 LUFS
Methods for Transcoding to MP4
Please find a list of products stations have used to transcode to MP4 successfully:
- Click on the open folder icon under “Files” in the left-hand corner and navigate to the file you wish to open for transcoding.
- Next select the open .wav file and navigate to File->Export
- Select File and a new “Export File” window will open
- In the Format drop down Select “Apple Audio Toolbox”
- Next to Format Settings click the “Change…” button
- Change Format to match the following
- File Type: MPEG-4 Audio
- Format: MPEG-4 HE AAC
- Mode: Constant Bitrate
- Bitrate: 40 kbps
- File will be named newscast.mp4 (so you should see an .mp4 extension)
Under the i-Tunes preferences menu > General > Import Settings (new window will open)
- Import Using: AAC Encoder
- Setting: Custom (new window will open)
- Stereo Bit Rate: 64 kbps
- Sample Rate: 44.100kHz
- Channels: Mono
- Select - Use High efficiency Encoding (HE)
- Select - Optimize for voice
- Click OK in all three windows
- Copy/Drag your original audio files (.wav or .aiff) into iTunes
- Select the audio file you wish to convert
- Go to File > Create New Version > Create AAC Version
- The AAC Version will appear above the original
- Change the new file extension from .mra to .mp4
- Click on the “Music Converter” button (a new window will open)
- Navigate to the folder that contains your file.
- Select your file you wish to convert and click “Open” (a new window will open)
- In the drop-down menu at the top: Converting To select “m4a FDK (AAC)”
- Target: Bit Rate (CBR)
- Bit Rate: move the slider to 64 kbps
- In the drop-down menu under Bit Rate select “HE AAC (below 96 kbps)
- Output Location: Select “Folder” and navigate to the folder you would like your converted audio output
- Scroll down to the bottom and click “Convert >”
- Navigate to your output folder and change the .m4a extension to .mp4
- Upload your audio
Some stations already utilize ffmpeg, along with watched folders on their networks. Below is the command line for transcoding a .wav file to an .mp4 file. Stations will need to also install the libfdk_aac library in order to be able to use the HE-AAC v1 codec, and this may require additional fees for licensing.
AAC-HE version 1 (Currently used)
Here's a bit more from ffmpeg page: http://trac.ffmpeg.org/wiki/AACEncodingGuide#fdk_aac
ffmpeg command line to get .mp4 (AAC-HE v1) ffmpeg -i inputnewscast.wav -c:a libfdk_aac -profile:a aac_he -b:a 64k newscast.mp4
Did you find this article helpful?