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AMS Codec

The AMS (Amplitude Media Stream) codec is Amplitude's custom audio compression format. It provides a good balance between compression ratio, decode speed, and audio quality for game sound effects and music.

Overview

Property Value
Algorithm ADPCM (Adaptive Differential Pulse-Code Modulation)
Compression ~4:1 vs 16-bit PCM
Decode speed Very fast (single multiply-accumulate per sample)
Seeking Sample-accurate
Bit depth 16-bit equivalent

Encoding

Use the amac CLI tool to create AMS files:

# Basic encoding
amac -e input.wav output.ams

# High quality with increased look-ahead
amac -e --look-ahead 5 input.wav output.ams

# Disable noise shaping (faster encode, slightly lower quality)
amac -e --no-noise-shaping input.wav output.ams

# Custom block size
amac -e --block-size-shift 2 input.wav output.ams

Encoding Parameters

Parameter Default Description
look-ahead 3 Number of samples to look ahead for better prediction. Higher values improve quality at the cost of encode time.
noise-shaping Enabled Applies noise shaping to push quantization error into less audible frequencies.
block-size-shift 0 Controls the ADPCM block size as a power-of-two shift. Larger blocks improve compression but reduce seek granularity.
resample Disabled Resamples the input to a target sample rate before encoding.

Decoding

The AMS decoder is included in Amplitude by default. It is automatically selected for files with the .ams extension.

{
  "id": 1,
  "name": "compressed_sfx",
  "path": "sounds/explosion.ams"
}

File Format

The AMS file format consists of:

AMS Header
├── Magic number (4 bytes): "AMS\0"
├── Version (2 bytes)
├── Sample rate (4 bytes)
├── Channels (2 bytes)
├── Bits per sample (2 bytes)
├── Total frames (8 bytes)
├── Block size (4 bytes)
├── Look-ahead (4 bytes)
│
ADPCM Blocks
├── Block 1: predictor state + compressed samples
├── Block 2: predictor state + compressed samples
├── ...

Each block starts with a fresh predictor state, allowing sample-accurate seeking to any block boundary.

Performance

Operation Relative Cost
AMS decode ~1.5× faster than MP3 decode
AMS decode ~10× faster than Vorbis decode
AMS encode ~5× faster than Vorbis encode

The decode speed makes AMS ideal for games with many simultaneous sounds or tight CPU budgets.

Quality

AMS is a lossy format. The quality is comparable to:

  • IMA ADPCM (similar algorithm, but AMS uses improved prediction)
  • Low-bitrate MP3 (~96–128 kbps for music)

AMS is not suitable for:

  • Professional music mastering (use uncompressed WAV)
  • Content requiring transparent quality (use FLAC or WAV)
  • Heavy transient material (drums, impacts may show artifacts)

When to Use AMS

Use Case Recommendation
Sound effects (gunshots, UI) ✅ Excellent
Ambient loops ✅ Good
Music (mobile) ✅ Acceptable
Music (desktop/console) ⚠️ Consider FLAC or uncompressed
Voice/dialogue ✅ Good

Next Steps