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Decode MP3 file from NSData

For my application, I need to decode an MP3 file that is stored in an NSData object.

For security reasons, it is not advisable to write the NSData object to disk and reopen it using a link to the system URL, even if it is only stored locally for several seconds.

I would like to use the advanced audio file services (or audio files) to do this, but it’s hard for me to get an NSData view that exists only in the memory that can be read by this Service audio file.

Edit: I want to decode MP3 data so that I can access linear PCM audio samples for manipulation. Playback from an NSData object is not a problem.

My code is as follows:

decryptedData; //an NSData object which has already been initialized const void *dataBytes = decryptedData.bytes; //pointer to the bytes in my NSData object //this creates a CFURLRef from the pointer to the byte data //I have printed out the resulting CFURL and have confirmed that it is indeed reading the bytes correctly CFURLRef audioFileURLFromBytes = CFURLCreateWithBytes (kCFAllocatorDefault, dataBytes, decryptedData.length, kCFStringEncodingASCII, NULL); //attempt to open the the URL using Extended Audio File Services ExtAudioFileRef outExtAudioFile; OSStatus err = 0; err = ExtAudioFileOpenURL(audioFileURLFromBytes, &outExtAudioFile); if (err != noErr) { NSLog(@"ExtAudioFileOpenURL failed with OSStatus Code %i \n", err); } //Attempt to open the URL using Audio File Services AudioFileID audioFile; OSStatus res = 0; res = AudioFileOpenURL(audioFileURLFromBytes, kAudioFileReadPermission, kAudioFileMP3Type, &audioFile); if (res != noErr) { NSLog(@"AudioFileOpenURL failed with OSStatus Code %i \n", res); }

Both attempts to open the URL result in OSStatus 43 code, which is "file not found".

I checked that my pointer points to the correct memory address for NSData and that the bytes can be read correctly.

Are there any restrictions for advanced audio file services that prohibit references to bytes stored in memory?

Thanks for any help you can provide.

Edit: I figured out how to do this using the Sbooth suggestion. Code below: This function accepts an NSData object containing an mp3 representation of the audio file. It decodes it as linear PCM, so you can get samples and then transcode it as AAC. I don’t think MP3 encoding is available in CoreAudio on all platforms (mobile / work). This code was tested on my Mac and did the job.

 -(void) audioFileReaderWithData: (NSData *) audioData { AudioFileID refAudioFileID; ExtAudioFileRef inputFileID; ExtAudioFileRef outputFileID; OSStatus result = AudioFileOpenWithCallbacks(audioData, readProc, 0, getSizeProc, 0, kAudioFileMP3Type, &refAudioFileID); if(result != noErr){ NSLog(@"problem in theAudioFileReaderWithData function: result code %i \n", result); } result = ExtAudioFileWrapAudioFileID(refAudioFileID, false, &inputFileID); if (result != noErr){ NSLog(@"problem in theAudioFileReaderWithData function Wraping the audio FileID: result code %i \n", result); } // Client Audio Format Description AudioStreamBasicDescription clientFormat; memset(&clientFormat, 0, sizeof(clientFormat)); clientFormat.mFormatID = kAudioFormatLinearPCM; clientFormat.mFramesPerPacket = 1; clientFormat.mChannelsPerFrame = 2; clientFormat.mBitsPerChannel = 32; clientFormat.mBytesPerPacket = clientFormat.mBytesPerFrame = 4 * clientFormat.mChannelsPerFrame; clientFormat.mFormatFlags = kAudioFormatFlagsNativeFloatPacked; clientFormat.mSampleRate = 44100; //Output Audio Format Description AudioStreamBasicDescription outputFormat; memset(&outputFormat, 0, sizeof(outputFormat)); outputFormat.mChannelsPerFrame = 2; outputFormat.mSampleRate = 44100; outputFormat.mFormatID = kAudioFormatMPEG4AAC; outputFormat.mFormatFlags = kMPEG4Object_AAC_Main; outputFormat.mBitsPerChannel = 0; outputFormat.mBytesPerFrame = 0; outputFormat.mBytesPerPacket = 0; outputFormat.mFramesPerPacket = 1024; // create the outputFile that we're writing to here.... UInt32 outputFormatSize = sizeof(outputFormat); result = 0; result = AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &outputFormatSize, &outputFormat); if(result != noErr) NSLog(@"could not set the output format with status code %i \n",result); NSMutableString *outputFilePath = [NSMutableString stringWithCapacity: 100]; [outputFilePath setString:@"/Users/You/Desktop/testAudio.m4a"]; NSURL *sourceURL = [NSURL fileURLWithPath:outputFilePath]; result = 0; result = ExtAudioFileCreateWithURL((CFURLRef)sourceURL, kAudioFileM4AType, &outputFormat, NULL, kAudioFileFlags_EraseFile, &outputFileID); if(result != noErr){ NSLog(@"ExtAudioFileCreateWithURL failed for outputFileID with status %i \n", result); } int size = sizeof(clientFormat); result = 0; result = ExtAudioFileSetProperty(inputFileID, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat); if(result != noErr) NSLog(@"error on ExtAudioFileSetProperty for input File with result code %i \n", result); size = sizeof(clientFormat); result = 0; result = ExtAudioFileSetProperty(outputFileID, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat); if(result != noErr) NSLog(@"error on ExtAudioFileSetProperty for output File with result code %i \n", result); int totalFrames = 0; UInt32 outputFilePacketPosition = 0; //in bytes UInt32 encodedBytes = 0; while (1) { UInt32 bufferByteSize = 22050 * 4 * 2; char srcBuffer[bufferByteSize]; UInt32 numFrames = (bufferByteSize/clientFormat.mBytesPerFrame); AudioBufferList fillBufList; fillBufList.mNumberBuffers = 1; fillBufList.mBuffers[0].mNumberChannels = clientFormat.mChannelsPerFrame; fillBufList.mBuffers[0].mDataByteSize = bufferByteSize; fillBufList.mBuffers[0].mData = srcBuffer; result = 0; result = ExtAudioFileRead(inputFileID, &numFrames, &fillBufList); if (result != noErr) { NSLog(@"Error on ExtAudioFileRead with result code %i \n", result); totalFrames = 0; break; } if (!numFrames) break; totalFrames = totalFrames + numFrames; result = 0; result = ExtAudioFileWrite(outputFileID, numFrames, &fillBufList); if(result!= noErr){ NSLog(@"ExtAudioFileWrite failed with code %i \n", result); } encodedBytes += numFrames * clientFormat.mBytesPerFrame; } //Clean up ExtAudioFileDispose(inputFileID); ExtAudioFileDispose(outputFileID); AudioFileClose(refAudioFileID); }

And you will also need these features ...

 static OSStatus readProc(void* clientData, SInt64 position, UInt32 requestCount, void* buffer, UInt32* actualCount) { NSData *inAudioData = (NSData *) clientData; size_t dataSize = inAudioData.length; size_t bytesToRead = 0; if(position < dataSize) { size_t bytesAvailable = dataSize - position; bytesToRead = requestCount <= bytesAvailable ? requestCount : bytesAvailable; [inAudioData getBytes: buffer range:NSMakeRange(position, bytesToRead)]; } else { NSLog(@"data was not read \n"); bytesToRead = 0; } if(actualCount) *actualCount = bytesToRead; return noErr; } static SInt64 getSizeProc(void* clientData) { NSData *inAudioData = (NSData *) clientData; size_t dataSize = inAudioData.length; return dataSize; }
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2 answers

The problem is that you are trying to create a CFURLRef object from audio bytes (MP3 frames) using ASCII encoding. CFURLCreateWithBytes intended for use with byte strings, not binary data (ie http://www.apple.com "like char * ). To accomplish what you want to use AudioFileOpenWithCallbacks , pass your NSData object as refcon and handle the raw read / search in your custom callbacks working with the passed NSData .

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Use Audio Queue Services or AVPlayer to play audio from a stream or memory.

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Source: https://habr.com/ru/post/948171/

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