RK3568 Android 11 蓝牙BluetoothA2dpSink 获取用于生成频谱的PCM

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Android 中的 A2DP Sink

A2DP Sink 在 Android 系统中主要用于 接收 其他蓝牙设备（如手机、平板、电脑等）发送过来的 高质量的立体声音频。简单来说，它让你的 Android 设备可以充当一个 蓝牙音箱 或 耳机 的角色。

核心功能：

• 接收音频流： 通过蓝牙协议接收来自其他设备的音频数据。
• 解码音频： 将接收到的音频数据解码成可播放的音频格式。
• 播放音频： 通过设备的扬声器或耳机输出解码后的音频。

应用场景：

• 无线音箱： 将 Android 设备连接到蓝牙音箱，实现无线音乐播放。
• 车载蓝牙： 将手机连接到车载蓝牙系统，通过车载音响播放音乐。
• 蓝牙耳机： 将 Android 设备连接到蓝牙耳机，进行通话或听音乐。

技术实现：

• BluetoothA2dpSink： Android 提供了 BluetoothA2dpSink 类来实现 A2DP Sink 功能。开发者可以通过这个类来管理 A2DP 连接、控制音频播放等。
• 蓝牙配置文件： A2DP（Advanced Audio Distribution Profile）是一种蓝牙配置文件，专门用于高质量立体声音频的无线传输。

如何获取音频数据并生成音频频谱?

什么是音乐频谱?
音乐频谱是声音频率的分布图。声音是由不同频率的声波组成的，这些声波的振幅（强度）不同，就形成了不同的音色。频谱图就是将这些频率和振幅的关系用图形表示出来。

频谱图的组成

• 横轴： 表示频率，通常以赫兹（Hz）为单位。频率越高，音调越高。
• 纵轴： 表示振幅，也就是声音的强度。振幅越大，声音越响。
• 颜色或灰度： 表示不同频率的振幅大小。颜色越深或灰度越高，表示该频率的振幅越大。

频谱图的种类

• 线性频谱图： 频率轴按线性比例分布，适用于分析整个音频频段。
• 对数频谱图： 频率轴按对数比例分布，更适合显示低频部分的细节，常用于音频分析。
• 时频图： 显示声音频率随时间的变化情况，可以直观地看到声音的动态变化。
  

总的来说

音乐频谱是了解声音的重要工具，它不仅能帮助我们更好地理解声音的本质，还能在音乐创作、音频处理等领域发挥重要作用。

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在蓝牙音箱的模式下, 如何生成音频频谱?

    在打上RK提供的A2dpSink补丁后, 手机等设备可以通过蓝牙连接播放音乐, RK3568充当蓝牙音箱的角色. 在这种状态下, 系统播放音频并不是采用android上层的MediaPlayer 或 AudioTrack, 所以无法采用常规的方式来生成, 若需要获取播放器的音频频谱, 首先, 需要获得音频的PCM数据.

在蓝牙音箱模式下, 音频的播放器的位置处于android 源码的 system目录下

system/bt/btif/src/btif_avrcp_audio_track.cc

/** Copyright 2015 The Android Open Source Project** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at**      http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/#define LOG_NDEBUG 1
#define LOG_TAG "bt_btif_avrcp_audio_track"#include "btif_avrcp_audio_track.h"#include <aaudio/AAudio.h>
#include <base/logging.h>
#include <utils/StrongPointer.h>#include "bt_target.h"
#include "osi/include/log.h"using namespace android;typedef struct {AAudioStream* stream;int bitsPerSample;int channelCount;float* buffer;size_t bufferLength;
} BtifAvrcpAudioTrack;#if (DUMP_PCM_DATA == TRUE)
FILE* outputPcmSampleFile;
char outputFilename[50] = "/data/misc/bluedroid/output_sample.pcm";
#endifvoid* BtifAvrcpAudioTrackCreate(int trackFreq, int bitsPerSample,int channelCount) {LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btCreateTrack freq %d bps %d channel %d ",__func__, trackFreq, bitsPerSample, channelCount);AAudioStreamBuilder* builder;AAudioStream* stream;aaudio_result_t result = AAudio_createStreamBuilder(&builder);AAudioStreamBuilder_setSampleRate(builder, trackFreq);AAudioStreamBuilder_setFormat(builder, AAUDIO_FORMAT_PCM_FLOAT);AAudioStreamBuilder_setChannelCount(builder, channelCount);AAudioStreamBuilder_setSessionId(builder, AAUDIO_SESSION_ID_ALLOCATE);AAudioStreamBuilder_setPerformanceMode(builder,AAUDIO_PERFORMANCE_MODE_LOW_LATENCY);result = AAudioStreamBuilder_openStream(builder, &stream);CHECK(result == AAUDIO_OK);AAudioStreamBuilder_delete(builder);BtifAvrcpAudioTrack* trackHolder = new BtifAvrcpAudioTrack;CHECK(trackHolder != NULL);trackHolder->stream = stream;trackHolder->bitsPerSample = bitsPerSample;trackHolder->channelCount = channelCount;trackHolder->bufferLength =trackHolder->channelCount * AAudioStream_getBufferSizeInFrames(stream);trackHolder->buffer = new float[trackHolder->bufferLength]();#if (DUMP_PCM_DATA == TRUE)outputPcmSampleFile = fopen(outputFilename, "ab");
#endifreturn (void*)trackHolder;
}void BtifAvrcpAudioTrackStart(void* handle) {if (handle == NULL) {LOG_ERROR(LOG_TAG, "%s: handle is null!", __func__);return;}BtifAvrcpAudioTrack* trackHolder = static_cast<BtifAvrcpAudioTrack*>(handle);CHECK(trackHolder != NULL);CHECK(trackHolder->stream != NULL);LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btStartTrack", __func__);AAudioStream_requestStart(trackHolder->stream);
}void BtifAvrcpAudioTrackStop(void* handle) {if (handle == NULL) {LOG_DEBUG(LOG_TAG, "%s handle is null.", __func__);return;}BtifAvrcpAudioTrack* trackHolder = static_cast<BtifAvrcpAudioTrack*>(handle);if (trackHolder != NULL && trackHolder->stream != NULL) {LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btStartTrack", __func__);AAudioStream_requestStop(trackHolder->stream);}
}void BtifAvrcpAudioTrackDelete(void* handle) {if (handle == NULL) {LOG_DEBUG(LOG_TAG, "%s handle is null.", __func__);return;}BtifAvrcpAudioTrack* trackHolder = static_cast<BtifAvrcpAudioTrack*>(handle);if (trackHolder != NULL && trackHolder->stream != NULL) {LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btStartTrack", __func__);AAudioStream_close(trackHolder->stream);delete trackHolder->buffer;delete trackHolder;}#if (DUMP_PCM_DATA == TRUE)if (outputPcmSampleFile) {fclose(outputPcmSampleFile);}outputPcmSampleFile = NULL;
#endif
}void BtifAvrcpAudioTrackPause(void* handle) {if (handle == NULL) {LOG_DEBUG(LOG_TAG, "%s handle is null.", __func__);return;}BtifAvrcpAudioTrack* trackHolder = static_cast<BtifAvrcpAudioTrack*>(handle);if (trackHolder != NULL && trackHolder->stream != NULL) {LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btPauseTrack", __func__);AAudioStream_requestPause(trackHolder->stream);AAudioStream_requestFlush(trackHolder->stream);}
}void BtifAvrcpSetAudioTrackGain(void* handle, float gain) {if (handle == NULL) {LOG_DEBUG(LOG_TAG, "%s handle is null.", __func__);return;}// Does nothing right now
}constexpr float kScaleQ15ToFloat = 1.0f / 32768.0f;
constexpr float kScaleQ23ToFloat = 1.0f / 8388608.0f;
constexpr float kScaleQ31ToFloat = 1.0f / 2147483648.0f;static size_t sampleSizeFor(BtifAvrcpAudioTrack* trackHolder) {return trackHolder->bitsPerSample / 8;
}static size_t transcodeQ15ToFloat(uint8_t* buffer, size_t length,BtifAvrcpAudioTrack* trackHolder) {size_t sampleSize = sampleSizeFor(trackHolder);size_t i = 0;for (; i <= length / sampleSize; i++) {trackHolder->buffer[i] = ((int16_t*)buffer)[i] * kScaleQ15ToFloat;}return i * sampleSize;
}static size_t transcodeQ23ToFloat(uint8_t* buffer, size_t length,BtifAvrcpAudioTrack* trackHolder) {size_t sampleSize = sampleSizeFor(trackHolder);size_t i = 0;for (; i <= length / sampleSize; i++) {size_t offset = i * sampleSize;int32_t sample = *((int32_t*)(buffer + offset - 1)) & 0x00FFFFFF;trackHolder->buffer[i] = sample * kScaleQ23ToFloat;}return i * sampleSize;
}static size_t transcodeQ31ToFloat(uint8_t* buffer, size_t length,BtifAvrcpAudioTrack* trackHolder) {size_t sampleSize = sampleSizeFor(trackHolder);size_t i = 0;for (; i <= length / sampleSize; i++) {trackHolder->buffer[i] = ((int32_t*)buffer)[i] * kScaleQ31ToFloat;}return i * sampleSize;
}static size_t transcodeToPcmFloat(uint8_t* buffer, size_t length,BtifAvrcpAudioTrack* trackHolder) {switch (trackHolder->bitsPerSample) {case 16:return transcodeQ15ToFloat(buffer, length, trackHolder);case 24:return transcodeQ23ToFloat(buffer, length, trackHolder);case 32:return transcodeQ31ToFloat(buffer, length, trackHolder);}return -1;
}constexpr int64_t kTimeoutNanos = 100 * 1000 * 1000;  // 100 msint BtifAvrcpAudioTrackWriteData(void* handle, void* audioBuffer,int bufferLength) {BtifAvrcpAudioTrack* trackHolder = static_cast<BtifAvrcpAudioTrack*>(handle);CHECK(trackHolder != NULL);CHECK(trackHolder->stream != NULL);aaudio_result_t retval = -1;//return 0;
#if (DUMP_PCM_DATA == TRUE)if (outputPcmSampleFile) {fwrite((audioBuffer), 1, (size_t)bufferLength, outputPcmSampleFile);}
#endifsize_t sampleSize = sampleSizeFor(trackHolder);int transcodedCount = 0;do {transcodedCount +=transcodeToPcmFloat(((uint8_t*)audioBuffer) + transcodedCount,bufferLength - transcodedCount, trackHolder);retval = AAudioStream_write(trackHolder->stream, trackHolder->buffer,transcodedCount / (sampleSize * trackHolder->channelCount),kTimeoutNanos);LOG_VERBOSE(LOG_TAG, "%s Track.cpp: btWriteData len = %d ret = %d",__func__, bufferLength, retval);} while (transcodedCount < bufferLength);return transcodedCount;
}

BtifAvrcpAudioTrackWriteData 函数中可以把PCM数据取出来用, 可以打开 DUMP_PCM_DATA 把蓝牙音频播放的PCM内容保存到本地文件char outputFilename[50] = "/data/misc/bluedroid/output_sample.pcm";中, 把文件拿出来用工具打包成WAV格式, 测试音频数据的正确性!

拿到PCM数据后, 通过算法, 便可以轻松实现音频频谱功能.

参考

• Android 音频可视化：频谱特效的探索与实践
• android获取和展示音乐的频谱
• [RK3566-Android11] 关于 a2dpsink -蓝牙支持接收播放/无PIN码连接
• Android 音频可视化

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