Hi guys,
I have a setup to receive the microphone data from the Thingy 52 on an nrf52840 evaluation board. Currently I recive the 131 audio packages ADPCM encodet. In the second step I implemented the iOS Swift example (https://github.com/NordicSemiconductor/IOS-Nordic-Thingy/blob/develop/IOSThingyLibrary/Classes/Services/SoundService/ThingySoundService.swift) to C to decode to PCM:
/** Intel ADPCM step variation table */ int8_t indexTable[] = { -1, -1, -1, -1, 2, 4, 6, 8, -1, -1, -1, -1, 2, 4, 6, 8 }; /** ADPCM step size table */ int16_t stepTable[] = { 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 19, 21, 23, 25, 28, 31, 34, 37, 41, 45, 50, 55, 60, 66, 73, 80, 88, 97, 107, 118, 130, 143, 157, 173, 190, 209, 230, 253, 279, 307, 337, 371, 408, 449, 494, 544, 598, 658, 724, 796, 876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899, 15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767 }; #define min(a,b) (((a)<(b))?(a):(b)) #define max(a,b) (((a)>(b))?(a):(b)) static void decodeAdpcm(uint8_t* currentFrame, int16_t* output, uint16_t sample) { //Achtung, man muss sich hier sicher sein, dass man auch die vollen 131 byte bekommt // A ADPCM frame on Thingy ia 131 bytes long: // 2 bytes - predicted value // 1 byte - index // 128 bytes - 256 4-bit samples convertable to 16-bit PCM // Read 16-bit predicted value //var valuePredicted: Int32 = Int32(Int16(currentFrame[1]) | Int16(currentFrame[0]) << 8) int16_t val_0 = ((int16_t)currentFrame[0]) & 0x00FF; int16_t val_1 = ((int16_t)(currentFrame[1]))& 0x00FF; int32_t valuePredicted = 0; valuePredicted = ((int32_t)(val_1 | ((val_0 << 8)& 0xFF00))) & 0x0000FFFF; if(valuePredicted > 32767 || valuePredicted < -32767){ NRF_LOG_ERROR("Start value out of range"); } // Read the first index //var index = Int(currentFrame[2]) int8_t index = (int8_t)(currentFrame[2]); if(index > 80){ NRF_LOG_ERROR("Something seems to be wrong with ADPCM Data ( To high)");} if(index < 0){ NRF_LOG_ERROR("Something seems to be wrong with ADPCM Data( Less Zeros)");} uint8_t nextValue = 0;// value to be read from the frame uint8_t bufferStep = 0; //false // should the first f second touple be read from nextValue as index delta uint8_t delta = 0; // index delta; each following frame is calculated based on the previous using an index uint8_t sign = 0; int16_t step = stepTable[index]; int16_t value; int32_t diff; for(int i = 0; i < 128 * 2; i++) { // 3 bytes have already been eaten if (bufferStep == 1) { delta = nextValue & 0x0F; } else { nextValue = currentFrame[3 + i / 2]; delta = (nextValue >> 4) & 0x0F; } bufferStep = !bufferStep; index += indexTable[delta]; index = min(max(index, 0), 88); // index must be <0, 88> sign = delta & 8; // the first bit of delta is the sign delta = delta & 7; // the rest is a value diff = ((int32_t)(step >> 3))& 0x0000FFFF; if ((delta & 4) > 0) { diff += ((int32_t)(step))& 0x0000FFFF; } if ((delta & 2) > 0) { diff += ((int32_t)(step >> 1))& 0x0000FFFF; } if ((delta & 1) > 0) { diff += ((int32_t)(step >> 2))& 0x0000FFFF; } if (sign > 0) { valuePredicted -= diff; } else { valuePredicted += diff; } //let value: Int16 = Int16(min( Int32 (Int16.max), max( Int32 (Int16.min), valuePredicted))) value = (int16_t)(min((int32_t)32767 , max((int32_t)-32767 , valuePredicted))); step = stepTable[index]; output[sample] = value; sample++; } }
When I play the File with Matlab I can hear a very distorted version of the sound. Some of the packages are decoded properly and the other packages contain only 32767.
If I zomm in you can see the pattern. The red 'x' shows the start of a new package:
I dont get the clou why the one package is decoded prpperly and the next one only contains 32767. Maybe I did something wrong in the translation from Swift to C?
Thanks a lot for your help!
Peter