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	Target	Description
`max`	Lossless or ~256 kbps	ALAC if device supports it and source is lossless; otherwise same as `high`
`high`	~256 kbps	Transparent quality (default)
`medium`	~192 kbps	Excellent quality
`low`	~128 kbps	Good quality, space-efficient

Default: high (VBR ~256 kbps)

The max preset is device-aware. On devices that support Apple Lossless (iPod Classic, Video 5G/5.5G, Nano 3G-5G), it produces ALAC from lossless sources. On other devices, it falls back to the same high-quality AAC as the high preset.

Configuration

CLI Usage

# Default: VBR ~256 kbps
podkit sync

# Best quality — ALAC on supported devices, high AAC on others
podkit sync --audio-quality max

# CBR encoding for predictable file sizes
podkit sync --encoding 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: max | high | medium | low
encoding = "vbr"          # Encoding mode: vbr (default) or cbr

Example Scenarios

Scenario 1: Audiophile with iPod Classic

audioQuality = "max"

Source	Result
FLAC	ALAC (Classic supports lossless)
MP3 320	Copy as-is
OGG 192	AAC ~192 kbps VBR (capped at source bitrate) + warning

Scenario 2: Space-conscious user

quality = "medium"

Source	Result
FLAC	AAC ~192 kbps VBR
MP3 128	Copy as-is
Opus 128	AAC ~128 kbps VBR (capped at source bitrate) + warning

Scenario 3: Predictable file sizes

quality = "high"
encoding = "cbr"

Source	Result
FLAC	AAC 256 kbps CBR
MP3 320	Copy as-is
OGG 192	AAC 192 kbps CBR (capped at source bitrate) + warning

VBR vs CBR

VBR is the default encoding mode. You can switch to CBR globally or per device with the encoding option:

encoding = "cbr"  # global

[devices.nano]
encoding = "cbr"  # per device

Mode	Pros	Cons
VBR (default)	Better quality-per-MB, adapts to content	Less predictable file sizes; adjacent preset changes may not always be detected
CBR	Predictable file sizes, reliable preset change detection	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.

Incompatible Lossy Bitrate Capping

When transcoding incompatible lossy sources (OGG, Opus), the effective bitrate is capped at the source file’s bitrate to avoid creating a larger file with no quality benefit. For example, a 128 kbps OGG file transcoded with the high preset (256 kbps target) will be transcoded at 128 kbps, not 256 kbps.

File Size Guidelines

Preset	Approx Bitrate	File Size (4 min song)
max (ALAC)	~900 kbps	~26 MB
max (AAC fallback)	~256 kbps	~7.5 MB
high	~256 kbps	~7.5 MB
medium	~192 kbps	~5.6 MB
low	~128 kbps	~3.8 MB

VBR file sizes vary based on content complexity. CBR sizes are exact. The max preset produces ALAC (lossless) on devices that support it, otherwise AAC at the same quality as high.

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