Change sampling rate and buffer size,packet size of the usb audio code

Page last updated 22 Oct 2011, by Philips Philips. 0 replies

Increase the sampling frequency from 32kHz to 48kHz. And then, you'll get transfer size of 960 bytes.

Set the sampling frequency - tSamFreq field of the Type I Format descriptor - DATA_FREQ macro (demo.h)

As the packet size increases, you have to touch to these parameters, too. - wMaxPacketSize of the endpoint descriptor - P_S macro (demo.h)

usbdesc.c

/* Audio Type I Format */
  AUDIO_FORMAT_TYPE_I_DESC_SZ(1),       /* bLength */
  AUDIO_INTERFACE_DESCRIPTOR_TYPE,      /* bDescriptorType */
  AUDIO_STREAMING_FORMAT_TYPE,          /* bDescriptorSubtype */
  AUDIO_FORMAT_TYPE_I,                  /* bFormatType */
  0x01,                                 /* bNrChannels */
  0x02,                                 /* bSubFrameSize */
  16,                                   /* bBitResolution */
  0x01,                                 /* bSamFreqType */
  B3VAL(32000),                         /* tSamFreq */       // <----- B3VAL(48000)
/* Endpoint - Standard Descriptor */
  AUDIO_STANDARD_ENDPOINT_DESC_SIZE,    /* bLength */
  USB_ENDPOINT_DESCRIPTOR_TYPE,         /* bDescriptorType */
  USB_ENDPOINT_OUT(3),                  /* bEndpointAddress */
  //USB_ENDPOINT_TYPE_ISOCHRONOUS,        /* bmAttributes */
  USB_ENDPOINT_TYPE_ISOCHRONOUS | USB_ENDPOINT_SYNC_ASYNCHRONOUS,
  WBVAL(66),                            /* wMaxPacketSize */ // <----- WBVAL(98)
  0x01,                                 /* bInterval */
  0x00,                                 /* bRefresh */
  0x83,                                 /* bSynchAddress */


demo.h
/* Audio Definitions */
#define DATA_FREQ 32000                 /* Audio Data Frequency */ // <--- 48000
#define P_S       32                    /* Packet Size */          // <--- 48

The buffering part of the code expects the buffer size, B_S (ie. P_S), as a number of power of 2, like 2, 4, 8, 16, 32, 64, ... The code abuses "mask" method to keep the buffer indices (DataIn/DataOut) within the array range. For example,

  DataIn += P_S;                          /* Update Data In Index */
    DataIn &= B_S - 1;                      /* Adjust Data In Index */

As we sets P_S to 48, not a number of power of 2, this method failed. Rewrite the buffering code, so that it accept ANY number.

usbuser.c

void USB_SOF_Event (void) {
  DWORD cnt;
  ...
  ...
#if USB_DMA == 0
  if (USB_ReadEP(0x03, (BYTE *)&DataBuf[DataIn])) {
    /* Data Available */
    DataIn += P_S;                          /* Update Data In Index */
//    DataIn &= B_S - 1;                      /* Adjust Data In Index */
    if ( DataIn >= B_S ) {
      DataIn -= B_S;
    }
//    if (((DataIn - DataOut) & (B_S - 1)) == (B_S/2)) {
    if ( DataIn >= DataOut ) {
      cnt = DataIn - DataOut;
    } else {
      cnt = B_S + DataIn - DataOut;
    }
    if ( cnt == (B_S/2)) {
      DataRun = 1;                          /* Data Stream running */
    }
  } else {
    /* No Data */
    DataRun  = 0;                           /* Data Stream not running */
    DataOut  = DataIn;                      /* Initialize Data Indexes */
  }
#endif
}
#endif


void USB_EndPoint3 (DWORD event) {
#if USB_DMA
  USB_DMA_DESCRIPTOR DD;
  DWORD cnt;

  if (event & USB_EVT_OUT_DMA_EOT) {
    /* End of Transfer */
    if (USB_DMA_BufAdr(0x03) != ((DWORD)DataBuf + 2*DataIn)) {
      /* Data Available */
      DataIn += P_C*P_S;                    /* Update Data In Index */
//      DataIn &= B_S - 1;                    /* Adjust Data In Index */
      if ( DataIn >= B_S ) {
        DataIn -= B_S;
      }
//      if (((DataIn - DataOut) & (B_S - 1)) == (B_S/2)) {
    if ( DataIn >= DataOut ) {
      cnt = DataIn - DataOut;
    } else {
      cnt = B_S + DataIn - DataOut;
    }
    if ( cnt == (B_S/2)) {
        DataRun = 1;                        /* Data Stream running */
      }
    } else {
      /* No Data */
      DataRun = 0;                          /* Data Stream not running */
      DataOut = DataIn;                     /* Initialize Data Indexes */
    }
  }


demo.c




void tc0_isr (void) __irq {
  long  val;
  DWORD cnt;




  if (DataRun) {                            /* Data Stream is running */
    val = DataBuf[DataOut];                 /* Get Audio Sample */
//    cnt = (DataIn - DataOut) & (B_S - 1);   /* Buffer Data Count */
    if ( DataIn >= DataOut ) {              /* Buffer Data Count */
      cnt = DataIn - DataOut;
    } else {
      cnt = B_S + DataIn - DataOut;
    }
    if (cnt == (B_S - P_C*P_S)) {           /* Too much Data in Buffer */
      DataOut++;                            /* Skip one Sample */
    }
    if (cnt > (P_C*P_S)) {                  /* Still enough Data in Buffer */
      DataOut++;                            /* Update Data Out Index */
    }
//    DataOut &= B_S - 1;                     /* Adjust Buffer Out Index */
    if ( DataOut >= B_S )                   /* Adjust Buffer Out Index */
      DataOut -= B_S;

Errors where in the originally code, so we have to edit out these lines, and then it works:)

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