【Android】【NDK】解決Lame庫編碼MP3聲音速度變快的問題

這個問題可能是由於把單聲道PCM作爲雙聲道PCM來編碼造成的
這樣就有一半的數據從左聲道被編到了右聲道，於是播放速度快了一倍，總時長短了一半
我看網上大多Lame庫的Demo都默認是雙聲道，沒有考慮到單聲道的問題

解決方法：單聲道和雙聲道分開進行編碼

	//轉碼單聲道PCM
	void encodeSingleChannelPcm() {
	    int bufferSize = 1024 * 256;
	    short *buffer = new short[bufferSize / 2];
	    unsigned char *mp3Buffer = new unsigned char[bufferSize];
	    size_t readBufferSize = 0;
	    while ((readBufferSize = fread(buffer, 2, bufferSize / 2, pcmFile)) > 0) {
	        size_t wroteSize = lame_encode_buffer(lame, buffer, nullptr, readBufferSize, mp3Buffer, bufferSize);
	        fwrite(mp3Buffer, 1, wroteSize, mp3File);
	    }
	    delete[] buffer;
	    delete[] mp3Buffer;
	}
	
	//轉碼雙聲道PCM
	void encodeDoubleChannelPcm() {
	    int bufferSize = 1024 * 256;
	    short *buffer = new short[bufferSize / 2];
	    short *leftBuffer = new short[bufferSize / 4];
	    short *rightBuffer = new short[bufferSize / 4];
	    unsigned char *mp3Buffer = new unsigned char[bufferSize];
	    size_t readBufferSize = 0;
	    while ((readBufferSize = fread(buffer, 2, bufferSize / 2, pcmFile)) > 0) {
	        for (int i = 0; i < readBufferSize; i++) {
	            if (i % 2 == 0)
	                leftBuffer[i / 2] = buffer[i];
	            else
	                rightBuffer[i / 2] = buffer[i];
	        }
	        size_t wroteSize = lame_encode_buffer(lame, leftBuffer, rightBuffer, readBufferSize / 2, mp3Buffer, bufferSize);
	        fwrite(mp3Buffer, 1, wroteSize, mp3File);
	    }
	    delete[] buffer;
	    delete[] leftBuffer;
	    delete[] rightBuffer;
	    delete[] mp3Buffer;
	}
	
	//轉碼
	extern "C" void encode() {
	    if (singlechannel)
	        encodeSingleChannelPcm();
	    else
	        encodeDoubleChannelPcm();
	}