Audio Transcoding

This guide covers audio quality presets, encoder options, and file size estimates. For an overview of how podkit decides what to transcode, see Transcoding Methodology.

Quality Presets

Preset	Type	Target	Description
`lossless`	Lossless	N/A	Apple Lossless (only from lossless sources)
`max`	VBR	~320 kbps	Highest VBR quality level
`max-cbr`	CBR	320 kbps	Guaranteed 320 kbps
`high`	VBR	~256 kbps	Transparent quality (default)
`high-cbr`	CBR	256 kbps	Predictable file sizes
`medium`	VBR	~192 kbps	Excellent quality
`medium-cbr`	CBR	192 kbps
`low`	VBR	~128 kbps	Good quality, space-efficient
`low-cbr`	CBR	128 kbps

Default: high (VBR ~256 kbps)

Configuration

CLI Usage

# Default: VBR ~256 kbps
podkit sync

# Lossless (ALAC) with lossy quality fallback
podkit sync --audio-quality lossless --lossy-quality max

# Guaranteed 320 kbps CBR
podkit sync --audio-quality max-cbr

# Space-efficient
podkit sync --quality low

# Set unified quality, but override audio specifically
podkit sync --quality medium --audio-quality high

Config File

# Top-level settings in config.toml
quality = "high"          # Unified quality for audio and video
audioQuality = "high"     # Audio-specific override: lossless | max | max-cbr | high | high-cbr | medium | medium-cbr | low | low-cbr
lossyQuality = "max"     # Quality for lossy sources when audioQuality = "lossless"

Example Scenarios

Scenario 1: Audiophile with mixed collection

audioQuality = "lossless"
lossyQuality = "max"

Source	Result
FLAC	ALAC (lossless preserved)
MP3 320	Copy as-is
OGG 192	AAC ~320 VBR (lossy quality) + warning

Scenario 2: Space-conscious user

quality = "medium"

Source	Result
FLAC	AAC ~192 kbps VBR
MP3 128	Copy as-is
Opus 128	AAC ~192 kbps VBR + warning

Scenario 3: Predictable file sizes

quality = "high-cbr"

Source	Result
FLAC	AAC 256 kbps CBR
MP3 320	Copy as-is
OGG 192	AAC 256 kbps CBR + warning

VBR vs CBR

Mode	Pros	Cons
VBR	Better quality-per-MB, adapts to content	Less predictable file sizes
CBR	Predictable file sizes	May waste bits on simple passages

Note: VBR AAC works correctly for seeking on iPods (unlike VBR MP3). podkit defaults to VBR for better quality efficiency.

File Size Guidelines

Preset	Mode	Approx Bitrate	File Size (4 min song)
lossless	Lossless	~900 kbps	~26 MB
max	VBR	~320 kbps	~9.4 MB
high	VBR	~256 kbps	~7.5 MB
medium	VBR	~192 kbps	~5.6 MB
low	VBR	~128 kbps	~3.8 MB

VBR file sizes vary based on content complexity. CBR sizes are exact.

AAC Encoders

FFmpeg can use several AAC encoders. podkit automatically selects the best available:

Encoder	Quality	Platform	Availability
aac_at	Excellent	macOS only	AudioToolbox
libfdk_aac	Excellent	Custom build	Fraunhofer reference
aac (native)	Very Good	All platforms	Always available

Platform Availability

Platform	Encoders Available	Best Available
macOS (Homebrew)	`aac`, `aac_at`	`aac_at`
Linux (apt/dnf)	`aac` only	`aac`
Linux (custom build)	`aac`, `libfdk_aac`	`libfdk_aac`

The native FFmpeg AAC encoder is very good and sufficient for most uses. macOS users get Apple’s encoder (aac_at) automatically.

Check Your Encoders

ffmpeg -encoders 2>/dev/null | grep aac

Troubleshooting

Common Issues

Issue	Cause	Solution
”Encoder not found”	Missing codec	Install FFmpeg properly
Metadata not preserved	FFmpeg issue	Check FFmpeg version
Low quality output	Wrong preset	Check quality configuration
”Invalid data” errors	Corrupt source file	Validate input files

Debugging

# Check FFmpeg encoders
ffmpeg -encoders 2>/dev/null | grep aac

# Check a specific file
ffprobe -v error -show_streams input.flac

# Run sync with debug output
podkit sync -vvv --dry-run