MP3 Encoder (Run)
Contents
MP3 Encoder (Run)#
This article explains how you can use Transcoder::run to encode a WAV file to MP3 (MPEG Audio Layer III) format.
The code snippets in this article are from the enc_mp3_file macOS sample.
Source Audio#
For source we use the equinox-48KHz.wav file from the AVBlocks Assets repository. After downloading and unzipping you will find equinox-48KHz.wav in the aud subdirectory.
Code#
This code takes a WAV file and encodes it to compressed MP3 format.
Initialize AVBlocks#
The first step in any AVBlocks application is to initialize the library. This must be done before using any other AVBlocks functionality. The Library::initialize() method sets up the internal state and loads necessary codecs. Always remember to call Library::shutdown() at the end of your program to properly clean up resources and release any allocated memory.
int main(int argc, char* argv[])
{
Options opt;
switch(prepareOptions(opt, argc, argv))
{
case Command: return 0;
case Error: return 1;
case Parsed: break;
}
Library::initialize();
bool result = encode(opt);
Library::shutdown();
return result ? 0 : 1;
}
Configure Output Socket#
The output socket defines where and how the encoded audio data will be written. In this case, we’re configuring it to output an MP3 file. The socket represents the output destination, while the pin represents the specific audio stream within that destination. The AudioStreamInfo object specifies the audio format parameters - you can customize the bitrate, sampling rate, number of channels, or stereo mode by uncommenting the relevant lines.
primo::ref<MediaSocket> createOutputSocket(Options& opt)
{
// create stream info to describe the output audio stream
auto asi = primo::make_ref(Library::createAudioStreamInfo());
asi->setStreamType(StreamType::MPEG_Audio);
asi->setStreamSubType(StreamSubType::MPEG_Audio_Layer3);
// The default bitrate is 128000. You can set it to 192000, 256000, etc.
// asi->setBitrate(192000);
// Optionally set the sampling rate and the number of the channels, e.g. 44.1 Khz, Mono
// asi->setSampleRate(44100);
// asi->setChannels(1);
// create a pin using the stream info
auto pin = primo::make_ref(Library::createMediaPin());
pin->setStreamInfo(asi.get());
// the pin allows you to specify additional parameters for the encoder
// for example, change the stereo mode, e.g. Joint Stereo
// pin->params()->addInt(Param::Encoder::Audio::MPEG1::StereoMode, StereoMode::Joint);
// finally create a socket for the output container format which is MP3 in this case
auto socket = primo::make_ref(Library::createMediaSocket());
socket->setStreamType(StreamType::MPEG_Audio);
socket->setStreamSubType(StreamSubType::MPEG_Audio_Layer3);
socket->pins()->add(pin.get());
// output to a file
auto output_file = primo::ustring(opt.outputFile);
socket->setFile(output_file);
return socket;
}
Configure Transcoder and Encode#
This is the main encoding function that ties everything together. First, we create an input socket that points to the WAV file we want to encode. Then we create the output socket using our helper function. The transcoder is the core component that performs the actual encoding - it takes the uncompressed LPCM data from the input and converts it to compressed MP3 data for the output.
The setAllowDemoMode(true) call allows the transcoder to work even without a valid license (useful for testing, but not recommended for production). We then add our input and output sockets to the transcoder, open it to prepare for processing, run the actual transcoding operation, and finally close it to clean up.
bool encode(Options& opt)
{
// create input socket
primo::ref<MediaSocket> inSocket (Library::createMediaSocket());
inSocket->setFile(primo::ustring(opt.inputFile));
// create output socket
auto outSocket = createOutputSocket(opt);
// create transcoder
auto transcoder = primo::make_ref(Library::createTranscoder());
transcoder->setAllowDemoMode(true);
transcoder->inputs()->add(inSocket.get());
transcoder->outputs()->add(outSocket.get());
// transcoder will fail if output exists (by design)
deleteFile(primo::ustring(opt.outputFile));
if (!transcoder->open())
{
printError("Transcoder open", transcoder->error());
return false;
}
if (!transcoder->run())
{
printError("Transcoder run", transcoder->error());
return false;
}
transcoder->close();
return true;
}
Complete C++ Code#
Here’s the complete working example that demonstrates MP3 encoding using AVBlocks. This code combines all the previous snippets into a functional program that can be compiled and run. The main function handles command-line argument parsing, initializes AVBlocks, performs the encoding operation, and properly shuts down the library before exiting.
#include <primo/avblocks/avb.h>
#include <primo/platform/reference++.h>
#include <primo/platform/error_facility.h>
#include <primo/platform/ustring.h>
#include "util.h"
#include "options.h"
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
using namespace primo::codecs;
using namespace primo::avblocks;
using namespace std;
primo::ref<MediaSocket> createOutputSocket(Options& opt)
{
// create stream info to describe the output audio stream
auto asi = primo::make_ref(Library::createAudioStreamInfo());
asi->setStreamType(StreamType::MPEG_Audio);
asi->setStreamSubType(StreamSubType::MPEG_Audio_Layer3);
// The default bitrate is 128000. You can set it to 192000, 256000, etc.
// asi->setBitrate(192000);
// Optionally set the sampling rate and the number of the channels, e.g. 44.1 Khz, Mono
// asi->setSampleRate(44100);
// asi->setChannels(1);
// create a pin using the stream info
auto pin = primo::make_ref(Library::createMediaPin());
pin->setStreamInfo(asi.get());
// the pin allows you to specify additional parameters for the encoder
// for example, change the stereo mode, e.g. Joint Stereo
// pin->params()->addInt(Param::Encoder::Audio::MPEG1::StereoMode, StereoMode::Joint);
// finally create a socket for the output container format which is MP3 in this case
auto socket = primo::make_ref(Library::createMediaSocket());
socket->setStreamType(StreamType::MPEG_Audio);
socket->setStreamSubType(StreamSubType::MPEG_Audio_Layer3);
socket->pins()->add(pin.get());
// output to a file
auto output_file = primo::ustring(opt.outputFile);
socket->setFile(output_file);
return socket;
}
bool encode(Options& opt)
{
// create input socket
primo::ref<MediaSocket> inSocket (Library::createMediaSocket());
inSocket->setFile(primo::ustring(opt.inputFile));
// create output socket
auto outSocket = createOutputSocket(opt);
// create transcoder
auto transcoder = primo::make_ref(Library::createTranscoder());
transcoder->setAllowDemoMode(true);
transcoder->inputs()->add(inSocket.get());
transcoder->outputs()->add(outSocket.get());
// transcoder will fail if output exists (by design)
deleteFile(primo::ustring(opt.outputFile));
if (!transcoder->open())
{
printError("Transcoder open", transcoder->error());
return false;
}
if (!transcoder->run())
{
printError("Transcoder run", transcoder->error());
return false;
}
transcoder->close();
return true;
}
int main(int argc, char* argv[])
{
Options opt;
switch(prepareOptions(opt, argc, argv))
{
case Command: return 0;
case Error: return 1;
case Parsed: break;
}
Library::initialize();
bool result = encode(opt);
Library::shutdown();
return result ? 0 : 1;
}
How to Run#
See the build instructions for macOS and the enc_mp3_file example for details.
Command Line#
./enc_mp3_file --input <wav file> --output <mp3 file>
Examples#
List options:
./bin/x64/enc_mp3_file --help
Usage: enc_mp3_file --input <wav file> --output <mp3 file>
-h, --help
-i, --input input WAV file
-o, --output output MP3 file
The following example encodes input file ./assets/aud/equinox-48KHz.wav into output file equinox-48KHz.mp3:
mkdir -p ./output/enc_mp3_file
./bin/x64/enc_mp3_file \
--input ./assets/aud/equinox-48KHz.wav \
--output ./output/enc_mp3_file/equinox-48KHz.mp3