dx_listenecrex( )

Description | Cautions | Example | Errors | See Also

Name:	int dx_listenecrex(chdev, sc_tsinfop, ecrctp)
Inputs:	int chdev	device handle of the voice channel device on which echo cancellation will be performed
	SC_TSINFO *sc_tsinfop	pointer to SCbus time slot information structure
	DX_ECRCT void *ecrctp	pointer to ECR characteristic table
Returns:	0 on success
	-1 on error
Includes:	dxxxlib.h
Category:	Echo Cancellation Resource
Mode:	Synchronous

Description

The dx_listenecrex( ) function performs identically to dx_listenecr( ) and also modifies the characteristics of the echo canceller. The characteristics of the echo canceller can be set using the echo cancellation resource characteristic table DX_ECRCT.

Parameter	Description
chdev	Specifies the voice channel device handle obtained when the channel was opened using dx_open( ).
sc_tsinfop	Specifies a pointer to the data structure SC_TSINFO.
ecrctp	Pointer to DX_ECRCT structure cast to a (void *).

One characteristic of the echo canceller that can be set using dx_listenecrex( ) is the non-linear processor (NLP). When the NLP is activated, the output of the echo canceller is replaced with an estimate of the background noise. The NLP provides full echo suppression as long as the echo-reference signal contains speech signals and the echo-carrying signal does not. In this case, the echo canceller cancels the echo and maintains the full duplex connection.

NOTE:

Disable the NLP when using the echo canceller output for voice recognition algorithms as the NLP may clip the beginning of speech.

The DX_ECRCT structure is declared as follows:

typedef struct dx_ecrct {  
        int ct_length;                      /*size of this structure*/  
        unsigned char ct_NLPflag            /*ECR with NLP requested ECR_CT_ENABLE:ECR_CT_DISABLE */  
} DX_ECRCT;  
#define SIZE_OF_ECR_CT  sizeof (DX_ECRCT)   /*size of DX_ECRCT*/  
#define ECR_CT_ENABLE  0  
#define ECR_CT_DISABLE 1

Setting ct_NLP flag to ECR_CT_ENABLE activates NLP and setting ct_NLP flag to ECR_CT_DISABLE disables NLP.

NOTE:

The application must include the following line in order to handle different sized DX_ECRCT structures without the need for recompiling the application:

ecrct.ct_length=size_of_ecr_ct;

Cautions

This function fails when:

• An invalid channel device handle is specified.
• The ECR feature is not enabled on the board specified.
• The ECR feature is not supported on the board specified.
• The characteristic table contains invalid fields.

Example

#include <stdio.h>  
#include <windows.h>  
#include <srllib.h>  
#include <dxxxlib.h>  
#include <msilib.h>  
main()  
{  
   int msdev1, chdev2;          /* MSI/SC Station, and Voice Channel device handles */  
   SC_TSINFO  sc_tsinfo;        /* Time slot information structure */  
   DX_ECRCT  dx_ecrct;          /* ECR Characteristic Table */  
   long scts;                   /* SCbus time slot */  
   /* Open MSI/SC board 1 station 1 device */  
   if ((msdev1 = ms_open("msiB1C1", 0)) == -1) {  
       /* Perform system error processing */  
       exit(1);  
   }  
   /* Open board 1 channel 2 device */  
   if ((chdev2 = dx_open("dxxxB1C2", 0)) == -1) {  
       /* Perform system error processing */  
       exit(1);  
   }  
   /* Fill in the SCbus time slot information */  
   sc_tsinfo.sc_numts = 1;  
   sc_tsinfo.sc_tsarrayp = &scts;  
   /* Fill in the ECR Characteristic Table : with NLP turned off */  
   dx_ecrct.ct_length = size_of_ecr_ct;  
   dx_ecrct.ct_NLPflag = ECR_CT_DISABLE;  
   /* Get SCbus time slot connected to transmit of MSI/SC station 1 on board 1 */  
   if (ms_getxmitslot(msdev1, &sc_tsinfo) == -1) {  
       printf("Error message = %s", ATDV_ERRMSGP(msdev1));  
       exit(1);  
   }  
   /* Connect the echo-reference receive of voice channel 2 on board 1 to   
      the transmit signal of msdev1 */   
   if (dx_listenecrex(chdev2, &sc_tsinfo, (void *)&dx_ecrct) == -1) {  
       printf("Error message = %s", ATDV_ERRMSGP(chdev2));  
       exit(1);  
   }  
   /* Continue  
   .  
   .  
   .  
   /* Then perform xx_unlisten()s and dx_unlistenecr(), plus all xx_close()s */  
     return(0);  
}

Errors

If the function returns -1, use the SRL Standard Attribute function ATDV_LASTERR( ) to obtain the error code or use ATDV_ERRMSGP( ) to obtain a descriptive error message. One of the following error codes may be returned:

Equate	Returned When
EDX_BADPARM	Parameter error
EDX_SH_BADCMD	Function is not supported in current bus configuration
EDX_SH_BADEXTTS	SCbus time slot is not supported at current clock rate
EDX_SH_BADINDX	Invalid Switch Handler index number
EDX_SH_BADLCLTS	Invalid channel number
EDX_SH_BADMODE	Function not supported in current bus configuration
EDX_SH_BADTYPE	Invalid channel type (voice, analog, etc.)
EDX_SH_CMDBLOCK	Blocking function is in progress
EDX_SH_LCLDSCNCT	Channel already disconnected from SCbus
EDX_SH_LIBBSY	Switch Handler library busy
EDX_SH_LIBNOTINIT	Switch Handler library uninitialized
EDX_SH_MISSING	Switch Handler is not present
EDX_SH_NOCLK	Switch Handler clock fallback failed
EDX_SYSTEM	Error from operating system; use dx_fileerrno( ) to obtain error value

See Also

• dx_listenecr( )

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