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 Quality

Highest 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.

MP4 Specifications:

  • Codec: HE-AACv1

  • Channels: 1 (mono)

  • Bit Rate: 64 kbps

  • Sampling Rate: 44.1 Khz

  • Normalization: -16 LUFS

  • Extension: .mp4

Methods for Transcoding to MP4

Please find a list of products stations have used to transcode to MP4 successfully:

Using Adobe Audition on Mac

Please find instructions for updating format using Adobe Audition below:
  1. Click on the open folder icon under “Files” in the left-hand corner and navigate to the file you wish to open for transcoding.
  2. Next select the open .wav file and navigate to File->Export
  3. Select File and a new “Export File” window will open
  4. In the Format drop down Select “Apple Audio Toolbox” 
  5. Next to Format Settings click the “Change…” button 
  6. Change Format to match the following
  7. File Type: MPEG-4 Audio
  8. Format: MPEG-4 HE AAC
  9. Mode: Constant Bitrate
  10. Bitrate: 40 kbps
  11. File will be named newscast.mp4 (so you should see an .mp4 extension)

Using iTunes

  1. Under the i-Tunes preferences menu > General > Import Settings (new window will open)

  2. Import Using: AAC Encoder
  3. Setting: Custom (new window will open) 
  4. Stereo Bit Rate: 64 kbps
  5. Sample Rate: 44.100kHz
  6. Channels: Mono
  7. Select - Use High efficiency Encoding (HE)
  8. Select - Optimize for voice
  9. Click OK in all three windows 
  10. Copy/Drag your original audio files (.wav or .aiff)  into iTunes 
  11. Select the audio file you wish to convert
  12. Go to File > Create New Version > Create AAC Version
  13. The AAC Version will appear above the original
  14. Change the new file extension from .mra to .mp4


  1. Launch DBpoweramp

  2. Click on the “Music Converter” button (a new window will open) 
  3. Navigate to the folder that contains your file.
  4. Select your file you wish to convert and click “Open” (a new window will open)
  5. In the drop-down menu at the top: Converting To select “m4a FDK (AAC)”
  6. Target: Bit Rate (CBR)
  7. Bit Rate: move the slider to 64 kbps
  8. In the drop-down menu under Bit Rate select “HE AAC (below 96 kbps)
  9. Output Location: Select “Folder” and navigate to the folder you would like your converted audio output
  10. Scroll down to the bottom and click “Convert >”
  11. Navigate to your output folder and change the .m4a extension to .mp4
  12. Upload your audio

Using ffmpeg

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:

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

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