How to convert sample rate from AV_SAMPLE_FMT_FLTP to AV_SAMPLE_FMT_S16?

When processing audio data, especially when using the FFmpeg library for audio encoding and decoding, we often need to change the sample format. AV_SAMPLE_FMT_FLTP is a constant representing a floating-point planar sample format, while AV_SAMPLE_FMT_S16 represents a 16-bit integer interleaved sample format. Converting the sample format from AV_SAMPLE_FMT_FLTP to AV_SAMPLE_FMT_S16 involves several key steps:

1. Understanding the two formats:

   • AV_SAMPLE_FMT_FLTP (Float Planar): In this format, samples for each channel are stored separately, with each sample being a floating-point number.
   • AV_SAMPLE_FMT_S16 (Signed 16-bit): In this format, sample data is stored interleaved, with each sample being a 16-bit integer.

2. Configuring the resampler: Use FFmpeg's libswresample library or similar libraries to perform the actual format conversion. The primary task is to configure a resampler that can accept FLTP format input and output S16 format data.

3. Initializing the conversion context: Create and initialize a SwrContext for the conversion process. You need to specify input and output audio parameters, including channel count, sample rate, format, etc.

   c
   struct SwrContext *swr = swr_alloc();
   av_opt_set_int(swr, "in_channel_layout",  input channel layout, 0);
   av_opt_set_int(swr, "in_sample_rate", input sample rate, 0);
   av_opt_set_sample_fmt(swr, "in_sample_fmt", AV_SAMPLE_FMT_FLTP, 0);
   
   av_opt_set_int(swr, "out_channel_layout", output channel layout, 0);
   av_opt_set_int(swr, "out_sample_rate", output sample rate, 0);
   av_opt_set_sample_fmt(swr, "out_sample_fmt", AV_SAMPLE_FMT_S16, 0);
   
   swr_init(swr);

4. Performing the conversion: Use the swr_convert() function to convert input buffer data to the output buffer.

   c
   int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,
                                 const uint8_t **in , int in_count);

   Here, in is a pointer to the input data (FLTP format), and out is a pointer to the output data buffer (to be converted to S16 format).

5. Cleaning up resources: After conversion, ensure that the SwrContext and other allocated resources are released to avoid memory leaks.

   c
   swr_free(&swr);

Example: Suppose you have an audio file in FLTP format that you need to convert to S16 format. You will follow the above steps to configure the resampler, read the audio data, perform the conversion, and finally write the converted data to the output file. This process can be implemented by writing corresponding C code using FFmpeg libraries such as libavutil and libswresample to handle the conversion logic.

Through this method, you can effectively convert audio from AV_SAMPLE_FMT_FLTP format to AV_SAMPLE_FMT_S16 format to meet various audio processing and playback requirements.