音视频编解码——LAME

一、LAME简介

LAME是目前非常优秀的一种MP3编码引擎，在业界，转码成Mp3格式的音频文件时，最常用的就是LAME库。当达到320Kbit/s时，LAME编码出来的音频质量几乎可以和CD的音质相媲美，并且还能保证整个音频文件的体积非常小，因此若要在移动端平台上编码MP3文件，使用LAME便成为唯一的选择。

二、使用场景

操作系统：Android。

场景：1.录音时保存Mp3格式的文件 2. 将wav无损音频文件转码成mp3这种体积相对较小的音频文件。 3.可以将获取到的音频流进行录制保存为mp3格式。

附：如何录制wav文件，在之前的博客里面我们讲过：Android 音视频开发(二)：使用 AudioRecord 采集音频PCM并保存到文件

三、开发准备

LAME的源码是托管到sourceforge.net上的，我们开发一个基于LAME的项目，就不得不下载其源码用于编译。

LAME主页：http://lame.sourceforge.net/

LAME下载：http://sourceforge.net/projects/lame/files/lame/3.99/

如果需要集成到Android系统上，就需要开发者具备一些NDK开发的能力。

四、开发过程

定义在java类中定义native方法：

    private static native long nInit(int inSampleRate, int inChannels, int outSampleRate, int outBitrate, int model, int quality);

    private static native int nGetVersion(long lamePtr);

    private static native int mGetMp3bufferSize(long lamePtr);

    private static native int mGetMp3bufferSizeWithSamples(long lamePtr, int samples);

    private static native int nEncodeShortInterleaved(long lamePtr, short[] bufLR, int samples, byte[] outMp3buf);

    private static native int nEncodeShort(long lamePtr, short[] bufL, short[] bufR, int samples, byte[] outMp3buf);

    private static native int nFlush(long lamePtr, byte[] outBuf);

    private static native void nClose(long lamePtr);

生成相应的.h的头文件，并实现该头文件，完成整体逻辑的编写。

#include <jni.h>
#include <cwchar>
#include <math.h>
#include "com_renhui_lame_Lame.h"
#include "libmp3lame/lame.h"

extern "C"


JNIEXPORT jlong JNICALL Java_com_renhui_lame_Lame_nInit(JNIEnv *env, jclass type, jint inSampleRate, jint inChannels, jint outSampleRate, jint outBitrate, jint model, jint quality) {

    lame_global_flags *lameFlags;
    lameFlags = lame_init();
    lame_set_in_samplerate(lameFlags, inSampleRate);
    lame_set_num_channels(lameFlags, inChannels);
    lame_set_out_samplerate(lameFlags, outSampleRate);
    lame_set_brate(lameFlags, outBitrate);
    lame_set_mode(lameFlags, (MPEG_mode) model);
    lame_set_quality(lameFlags, quality);
    int code = lame_init_params(lameFlags);
    if (code != 0) {
        lame_close(lameFlags);
        return code;
    }
    return (long) lameFlags;
}

JNIEXPORT jint JNICALL Java_com_renhui_lame_Lame_nGetVersion(JNIEnv *env, jclass type, jlong lamePtr) {
    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;
    return lame_get_version(lameFlags);
}

JNIEXPORT jint JNICALL Java_com_renhui_lame_Lame_mGetMp3bufferSize(JNIEnv *env, jclass type, jlong lamePtr) {
    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;
    return lame_get_size_mp3buffer(lameFlags);
}

JNIEXPORT jint JNICALL Java_com_renhui_lame_Lame_mGetMp3bufferSizeWithSamples(JNIEnv *env, jclass type, jlong lamePtr, jint samples) {

    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;

    int version = lame_get_version(lameFlags);
    int bitrate = lame_get_brate(lameFlags);
    int sampleRate = lame_get_out_samplerate(lameFlags);

    float p = (bitrate / 8.0f) / sampleRate;

    if (version == 0) {
        // MPEG2: num_samples*(bitrate/8)/samplerate + 4*576*(bitrate/8)/samplerate + 256
        return (jint) ceil(samples * p + 4 * 576 * p + 256);
    } else if (version == 1) {
        // MPEG1: num_samples*(bitrate/8)/samplerate + 4*1152*(bitrate/8)/samplerate + 512
        return (jint) ceil(samples * p + 4 * 1152 * p + 512);
    } else {
        return (jint) ceil((1.25 * samples + 7200));
    }
}

JNIEXPORT jint JNICALL Java_com_renhui_lame_Lame_nEncodeShortInterleaved(JNIEnv *env, jclass type, jlong lamePtr,
                                                   jshortArray bufLR_, jint samples,
                                                   jbyteArray outMp3buf_) {

    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;

    jshort *bufLR = env->GetShortArrayElements(bufLR_, NULL);
    jbyte *outMp3buf = env->GetByteArrayElements(outMp3buf_, NULL);

    const jsize outMp3bufSize = env->GetArrayLength(outMp3buf_);
    int result = lame_encode_buffer_interleaved(lameFlags, bufLR, samples,
                                                (u_char *) outMp3buf, outMp3bufSize);

    env->ReleaseShortArrayElements(bufLR_, bufLR, 0);
    env->ReleaseByteArrayElements(outMp3buf_, outMp3buf, 0);

    return result;
}

JNIEXPORT jint JNICALL
Java_com_renhui_lame_Lame_nEncodeShort(JNIEnv *env, jclass type, jlong lamePtr, jshortArray bufL_,
                                        jshortArray bufR_, jint samples, jbyteArray outMp3buf_) {
    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;

    jshort *bufL = env->GetShortArrayElements(bufL_, NULL);
    jshort *bufR = env->GetShortArrayElements(bufR_, NULL);
    jbyte *outMp3buf = env->GetByteArrayElements(outMp3buf_, NULL);

    const jsize outMp3bufSize = env->GetArrayLength(outMp3buf_);
    int result = lame_encode_buffer(lameFlags, bufL, bufR, samples,
                                    (u_char *) outMp3buf, outMp3bufSize);

    env->ReleaseShortArrayElements(bufL_, bufL, 0);
    env->ReleaseShortArrayElements(bufR_, bufR, 0);
    env->ReleaseByteArrayElements(outMp3buf_, outMp3buf, 0);

    return result;
}

JNIEXPORT jint JNICALL
Java_com_renhui_lame_Lame_nFlush(JNIEnv *env, jclass type, jlong lamePtr, jbyteArray outBuf_) {

    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;

    jbyte *outBuf = env->GetByteArrayElements(outBuf_, NULL);

    const jsize outBufSize = env->GetArrayLength(outBuf_);
    int result = lame_encode_flush(lameFlags, (u_char *) outBuf, outBufSize);

    env->ReleaseByteArrayElements(outBuf_, outBuf, 0);

    return result;
}

JNIEXPORT void JNICALL
Java_com_renhui_lame_Lame_nClose(JNIEnv *env, jclass type, jlong lamePtr) {
    lame_global_flags *lameFlags;
    lameFlags = (lame_global_flags *) lamePtr;
    lame_close(lameFlags);
}

编写Android.mk和Application.mk，为ndk-build打包做准备。

Android.mk：

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE :=mp3lame
LAME_LIBMP3_DIR :=libmp3lame
LOCAL_SRC_FILES :=
$(LAME_LIBMP3_DIR)/bitstream.c 
$(LAME_LIBMP3_DIR)/fft.c 
$(LAME_LIBMP3_DIR)/id3tag.c 
$(LAME_LIBMP3_DIR)/mpglib_interface.c 
$(LAME_LIBMP3_DIR)/presets.c 
$(LAME_LIBMP3_DIR)/quantize.c 
$(LAME_LIBMP3_DIR)/reservoir.c 
$(LAME_LIBMP3_DIR)/tables.c  
$(LAME_LIBMP3_DIR)/util.c 
$(LAME_LIBMP3_DIR)/VbrTag.c 
$(LAME_LIBMP3_DIR)/encoder.c 
$(LAME_LIBMP3_DIR)/gain_analysis.c 
$(LAME_LIBMP3_DIR)/lame.c 
$(LAME_LIBMP3_DIR)/newmdct.c 
$(LAME_LIBMP3_DIR)/psymodel.c 
$(LAME_LIBMP3_DIR)/quantize_pvt.c 
$(LAME_LIBMP3_DIR)/set_get.c 
$(LAME_LIBMP3_DIR)/takehiro.c 
$(LAME_LIBMP3_DIR)/vbrquantize.c 
$(LAME_LIBMP3_DIR)/version.c 
com_renhui_lame_Lame.cpp
LOCAL_C_INCLUDES += $(LOCAL_PATH)/mp3lame
LOCAL_LDLIBS := -llog -lz
include $(BUILD_SHARED_LIBRARY)

Application.mk：

APP_ABI := all
#APP_ABI := armeabi armeabi-v7a x86

# APP_ABI :=armeabi
APP_PLATFORM := android-14

附：有关编译语法，整理了一篇文章，供大家参考：Android NDK学习（二）：编译脚本语法Android.mk和Application.mk。

五、思维拓展

1. 录音为Mp3格式

https://github.com/renhui/LameAndroid-master