Audio and Video Encoding Guidance

Optimize your podcast files for the best listening experience and fastest downloads.

Quick Reference

Supported File Formats

Audio: MP3, M4A, AAC

Video: MP4 (requires Castos Pro plan)

Castos does not modify your files after upload—what you upload is exactly what listeners receive.

Audio Settings Explained

Bitrate: Why 128 kbps?

Bitrate controls the balance between audio quality and file size:

• Higher bitrate = Better quality, larger files, slower downloads
• Lower bitrate = Smaller files, faster downloads, reduced quality

At 128 kbps, a one-hour episode is approximately 60 MB—small enough for easy downloading while maintaining clear audio quality. This is the industry standard for spoken-word podcasts.

Mono vs. Stereo

• Mono: Best for interview and solo podcasts. Smaller file size, sounds the same in both ears.
• Stereo: Use if your podcast includes music, sound effects, or audio that benefits from spatial separation.

Additional Audio Settings

If your editing software offers these options:

• Sample rate: 44.1 kHz
• Bit depth: 24-bit or 32-bit float
• MP3 encoding: Constant Bit Rate (CBR)

Loudness Standards

Loudness measures the perceived volume of your audio. Consistent loudness ensures listeners don't need to adjust their volume when switching between podcasts.

Target loudness:

• Mono: -19 LUFS
• Stereo: -16 LUFS

These values produce the same perceived loudness. Most audio editing software (Audacity, Adobe Audition, Hindenburg, Descript) includes loudness normalization tools.

Video Podcasts

Plan requirement: Video uploads require a Castos Pro subscription.

Format: MP4 only

Important: Video File Size Considerations

Unlike YouTube, podcast apps don't automatically adjust video quality based on internet speed. Listeners download or stream your video at full resolution, which means:

• Large video files cause slow downloads on slower connections
• Listeners with limited data plans may avoid video episodes
• Some podcast apps handle video poorly

Recommendations:

• Keep video file sizes reasonable (under 500 MB when possible)
• Consider offering both audio-only and video versions
• Use efficient encoding settings (H.264 codec, reasonable resolution)

Free video tools:

• HandBrake — The open-source video transcoder

Compressing Large Files

If your audio editor exports files larger than needed (common with WAV files), compress them before uploading:

Free audio tools:

• Online Audio Convert — Browser-based, no software needed
• Audacity — Free desktop software with export options

Export settings: MP3, 128 kbps, appropriate channel setting (mono/stereo)