FAAD AAC Decoder Guide & C API Reference

FAAD (Freeware Advanced Audio Decoder) is an open-source MPEG-2 and MPEG-4 AAC audio decoder.

The released FAAD2 engine is licensed under GPL v2+ (written in C99). The next-generation FAAD3 engine is written in clean C11 under LGPL v2.1+, providing lightweight decoding for embedded systems and cross-platform applications.

See the decoder performance comparison for measured throughput, footprint, and memory use alongside Helix, FDK-AAC, FFmpeg, and Apple.

Hardware fit: FAAD3 targets modern processors with floating-point units (FPUs). Fixed-point LibHelix is a better option to evaluate for legacy hardware without an efficient FPU, provided its profile support meets your needs.

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Choose a decoder version

The installation commands and NeAACDec* examples below target released FAAD2. FAAD3 is a separate development engine; consult its selected build for API and profile support. FAAD2 and FAAD3 have different software licenses.

Technical Highlights

Feature	Details
Licensing	FAAD2: GPL v2+ (C99) | FAAD3: LGPL v2.1+ (C11)
Supported Profiles	MPEG-4 AAC-LC, HE-AAC v1 (SBR), HE-AAC v2 (PS), Main, LTP
Surround Sound	Up to 7.1 channel surround sound with optional 2-channel downmix (-d)
Containers	Raw ADTS .aac, MP4/M4A containers, ADIF streams
Installation	See Installation Guide
Full CLI Options	See FAAD CLI Manual Page

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Quick Start CLI Examples

# 1. Decode AAC or M4A to uncompressed WAV
faad -o decoded_output.wav input.m4a

# 2. Inspect bitstream parameters and audio metadata
faad -i audio_file.aac

# 3. Downmix 5.1 surround sound stream to 2-channel stereo WAV
faad -d -o stereo_downmix.wav surround_51.m4a

For complete options and flags, view the dedicated FAAD Command-Line Manual.

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C API Reference (libfaad)

For the FAAD2 API shown here, link against -lfaad and include <neaacdec.h>.

Integration Lifecycle

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <neaacdec.h>

int main(void) {
    // 1. Allocate decoder handle
    NeAACDecHandle hDecoder = NeAACDecOpen();
    if (!hDecoder) return 1;

    // 2. Configure decoder (16-bit PCM output)
    NeAACDecConfigurationPtr config = NeAACDecGetCurrentConfiguration(hDecoder);
    config->outputFormat = FAAD_FMT_16BIT;
    NeAACDecSetConfiguration(hDecoder, config);

    // 3. Initialize decoder with bitstream header
    unsigned char buffer[2048] = { /* AAC header or first frame bytes */ };
    unsigned long sampleRate;
    unsigned char channels;

    if (NeAACDecInit(hDecoder, buffer, sizeof(buffer), &sampleRate, &channels) < 0) {
        NeAACDecClose(hDecoder);
        return 1;
    }

    // 4. Decode frame
    NeAACDecFrameInfo frameInfo;
    void *pcmSamples = NeAACDecDecode(hDecoder, &frameInfo, buffer, sizeof(buffer));

    // 5. Cleanup
    NeAACDecClose(hDecoder);
    return 0;
}

Core API Functions

NeAACDecOpen

NeAACDecHandle NeAACDecOpen(void);

Initializes a new FAAD decoder instance.

NeAACDecGetCurrentConfiguration / NeAACDecSetConfiguration

NeAACDecConfigurationPtr NeAACDecGetCurrentConfiguration(NeAACDecHandle hDecoder);
unsigned char NeAACDecSetConfiguration(NeAACDecHandle hDecoder, NeAACDecConfigurationPtr config);

Gets and sets decoder settings (PCM sample format, downmixing).

NeAACDecInit

long NeAACDecInit(NeAACDecHandle hDecoder, unsigned char *buffer,
                  unsigned long bufferSize, unsigned long *sampleRate,
                  unsigned char *channels);

Parses stream header / AudioSpecificConfig bytes to set sample rate and channels.

NeAACDecDecode

void* NeAACDecDecode(NeAACDecHandle hDecoder, NeAACDecFrameInfo *frameInfo,
                     unsigned char *buffer, unsigned long bufferSize);

Decodes a single AAC frame into raw PCM audio samples and updates frameInfo.

NeAACDecClose

void NeAACDecClose(NeAACDecHandle hDecoder);

Frees decoder handle memory and resources.