One ,PCM Definition of documents  

  PCM file : Analog audio signal is converted into analog to digital (A/D Transformation ) Directly formed binary sequence
, The file has no additional header and end of file flag .Windows Of Convert The tool can make PCM Convert audio format files into Microsoft Of WAV File format . Digitize audio , In fact, it's about digitizing sound . The most common way is through pulse code modulation PCM(Pulse
Code Modulation) .

 
The above paragraph is excerpted by bloggers on the Internet PCM Definition of documents . actually PCM In a nutshell , Is an uncompressed original audio format , And what we know MP3 Audio files of the same format are compressed ( The result of no compression is that it takes up a lot of space , Will greatly increase the cost of storage and transmission , So what is stored on the computer is usually MP3 Files in the same compressed format ). The player wants to play MP3,AAC,APE,FLAC Audio files in the same format , First, you have to decode them into PCM data ( After decoding, it is usually FLT Format of , Also through resampling to the sound card support S16 format ), Then it can be played .

 

Two , adopt FFmpeg Command conversion PCM file

  As shown in the figure below , There are audio1.mp3 this MP3 file , Next, we introduce how to convert it into PCM Method of documentation .



 

At the command prompt cmd.exe Enter the directory where the current project is located (ffmpeg.exe and audio1.mp3 Directory ), Then enter the command :
ffmpeg -i audio1.mp3 -f s16le audio1.pcm
The function of this command is to set the audio1.mp3 File to s16( The sound card supports this format ), Small end format PCM file :audio1.pcm.

notes :
-i Represents the set input stream
-f It means to set the output format

After the command is executed , We found that the project directory will generate audio1.pcm file , As shown in the figure below :



 

Three ,PCM Document analysis

 
We can see the generated audio1.pcm The file size is 49.5MB, than audio1.mp3 Document's 11.2MB Several times bigger , As shown in the figure below , That proves it PCM File occupancy ratio MP3 The file should be large .





 


  as for audio1.pcm The file size is 49.5MB, this 49.5MB, It can be calculated according to the formula . Use another blog post from the blogger 《FFmpeg Unpacking ( Demultiplexing ) And get the information of local media files 》
https://blog.csdn.net/u014552102/article/details/81709929
<https://blog.csdn.net/u014552102/article/details/81709929>
You can see audio1.mp3 Sampling frequency of ( Audio sampling rate ) by 44100Hz, The sampling bits are 16 position ( This is from the audio sampling format :AV_SAMPLE_FMT_S16P It can be seen that ), Vocal tract ( Number of audio channels ) by 2. time ( Total audio duration ) by 294 second , As shown in the figure below :( We use the same one in both blogs audio1.mp3 file , The following is an excerpt from the above blog )



 

according to PCM The formula of file capacity : Storage capacity = ( sampling frequency * Sampling bits * Vocal tract )* time /8( Company : Number of bytes ).

Calculated audio1.pcm Size of = (44100 * 16 * 2) * 294 / 8 =  51861600( byte ) = 49.45MB

This is what we saw on the computer audio1.pcm The file size is 49.5MB It's consistent . This proves that PCM The size of the file can be calculated .

 

Four , use ffplay.exe play PCM file

  PCM The file is the original uncompressed audio format file , With popular players on the market, we can't play them directly . But we can go through FFmpeg Provided ffplay.exe play PCM file .

  At the command prompt cmd.exe Enter the directory where the current project is located (ffplay.exe and audio1.pcm Directory ), Then enter the command :
ffplay -ar 44100 -ac 2 -f s16le -i audio1.pcm
The purpose of this command is to use the ffplay Playback frequency is 44100Hz, Dual channel ,16 position , Small end audio file audio1.pcm.

notes :
-i Represents the specified input file
-f Represents a format that is mandatory
-ar Represents the sampling rate of played audio data
-ac Represents the number of channels for playing audio data

As shown in the figure below , The following pictures appear and music can be heard , It means that the playback is successful .



 

Five , Problems encountered :

If used ffplay.exe Error playing media file :WASAPI can't initialize audio client. As shown in the figure below :



 

Execute the command at the command prompt first :
set SDL_AUDIODRIVER=directsound
And then use it ffplay.exe Play media files .

 

Six , Resource download

Download resources at https://download.csdn.net/download/u014552102/10671629
<https://download.csdn.net/download/u014552102/10671629>
, Contains all the resources for the above demo . Including the above ffmpeg.exe,ffplay.exe,mp3 file ,pcm Documents, etc .