How to find Zero Cross PCM...

Hi Guys,

I am new to DSP programming, but I found a good software called BASS to
allow me to do looping for music.

My problem is that when I pick a start loop and end loop point and then
begin looping you can hear a click in there. I was told by someone that
this is because my loop start and end points must be adjusted forward or
backward to ensure the points are at zero cross.

The data I am working with is 16-bit samples signed (-32768 to 32767). If
I am given a loop start point in bytes, how would I find the nearest zero
cross point?

I tried the following, but it is not working. I still get a click in my
loop.

QWORD CLoopingDlg::FindZeroCross(QWORD dwPos)
{
	QWORD dwZeroCrossPos = -1;
	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
BASS_STREAM_DECODE);
	BASS_ChannelSetPosition(hStream, dwPos);

	dwZeroCrossPos = dwPos;

	short buf[50000];
	DWORD count=0,pos;

	int a,b = BASS_ChannelGetData(hStream, buf, 20000);
	b/=2;
	a=0;
	if (a<b) {		
               // move back to a quieter sample (to avoid "click")
		for (;a && abs(buf[a])>500/4;a--,dwZeroCrossPos-=2) ;
	}

	return dwZeroCrossPos;
}

Thanks



_____________________________________
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>Hi Guys,
>
>I am new to DSP programming, but I found a good software called BASS to
>allow me to do looping for music.
>
>My problem is that when I pick a start loop and end loop point and then
>begin looping you can hear a click in there. I was told by someone that
>this is because my loop start and end points must be adjusted forward or
>backward to ensure the points are at zero cross.
>
>The data I am working with is 16-bit samples signed (-32768 to 32767).
If
>I am given a loop start point in bytes, how would I find the nearest
zero
>cross point?
>
>I tried the following, but it is not working. I still get a click in my
>loop.
>
>QWORD CLoopingDlg::FindZeroCross(QWORD dwPos)
>{
>	QWORD dwZeroCrossPos = -1;
>	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
>BASS_STREAM_DECODE);
>	BASS_ChannelSetPosition(hStream, dwPos);
>
>	dwZeroCrossPos = dwPos;
>
>	short buf[50000];
>	DWORD count=0,pos;
>
>	int a,b = BASS_ChannelGetData(hStream, buf, 20000);
>	b/=2;
>	a=0;
>	if (a<b) {		
>               // move back to a quieter sample (to avoid "click")
>		for (;a && abs(buf[a])>500/4;a--,dwZeroCrossPos-=2) ;
>	}
>
>	return dwZeroCrossPos;
>}
>
>Thanks
>
>
>
>_____________________________________
>Do you know a company who employs DSP engineers?  
>Is it already listed at http://dsprelated.com/employers.php ?
>

Ok, I modified my code and I almost have it working. It makes my loops
seamless in some instances, but not in others

QWORD CLoopingDlg::FindZeroCross(QWORD dwPos, bool bEnd)
{
	QWORD dwZeroCrossPos = -1;
	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
BASS_STREAM_DECODE);
	BASS_ChannelSetPosition(hStream, dwPos);

	dwZeroCrossPos = dwPos;

	short buff[50000];
	memset(buff, 0, sizeof(buff)); 
	bool bFoundZeroCross = false;
	int ni=0;
	int dw=0;
	do
	{
		DWORD dwBytes = BASS_ChannelGetData(hStream, buff, 100);
		DWORD dwSamples = dwBytes/2;
		for(int i=0; i<dwSamples; i+=2)
		{

			if(bEnd)
			{
				if( buff[i]>=0 && buff[i+2]<0)
				{
					///TRACE("Found Zero Cross s1:%d s2:%d\n", buff[i], buff[i+2]);
					bFoundZeroCross = true;
				}
			}
			else
			{
				if(buff[i]<0 && buff[i+2]>=0 /*|| buff[i]>=0 && buff[i+2]<0*/)
				{
					///TRACE("Found Zero Cross s1:%d s2:%d\n", buff[i], buff[i+2]);
					bFoundZeroCross = true;
				}
			}
			//TRACE("%d\n", buff[i]);
			dwZeroCrossPos+=2;

			ni = i;
			if(bFoundZeroCross)
				break;
		}

		dw++;
	}while(BASS_ChannelGetPosition(hStream) <= dwPos +
BASS_ChannelSeconds2Bytes(hStream, 0.5f) && !bFoundZeroCross);

	return dwZeroCrossPos;
}

_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?

On 2007-04-20, DJGuy <greg_ellis@hotmail.com> wrote:
>
> Ok, I modified my code and I almost have it working. It makes my loops
> seamless in some instances, but not in others

Do you ensure the crossings you pair up are both going in the same
direction?

-- 
Ben Jackson AD7GD
<ben@ben.com>
http://www.ben.com/

>On 2007-04-20, DJGuy <greg_ellis@hotmail.com> wrote:
>>
>> Ok, I modified my code and I almost have it working. It makes my loops
>> seamless in some instances, but not in others
>
>Do you ensure the crossings you pair up are both going in the same
>direction?
>
>-- 
>Ben Jackson AD7GD
><ben@ben.com>
>http://www.ben.com/
>

Hi There,

Ya I changed my code again to make sure that the crossings are going in
the same direction now like shown below, but now it never removes the
clicks at all. I am confused since there are 4 bytes per sample in a 16
bit stereo stream so this should definately work I think... but it does
not. I also tried checking the right channel as well as the left channle,
but that made no difference. Anyone know why this isnt working?

QWORD CLoopingDlg::FindZeroCross(QWORD dwPos, bool bEnd)
{
	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
BASS_STREAM_DECODE);
	BASS_ChannelSetPosition(hStream, dwPos);

	int i = 0;
	DWORD dwBytes = BASS_ChannelSeconds2Bytes(hStream, 0.5f);
	short *buff = (short*)malloc(dwBytes);
	dwBytes = BASS_ChannelGetData(hStream, buff, dwBytes);
	for (i=0; i<dwBytes/4; i+=2)
	{
		if ((buff[i]*buff[i+2]<0))
			break;
	}
	free(buff);
	return dwPos+i*4;
}

_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?

For anyone who needs this in the future, I finally got it all working. 

QWORD CLoopingDlg::FindZeroCross(QWORD dwPos)
{
	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
BASS_STREAM_DECODE|BASS_STREAM_PRESCAN);
	BASS_ChannelSetPosition(hStream, dwPos);

	int i = 0;
	DWORD dwBytes = BASS_ChannelSeconds2Bytes(hStream, 0.1f);
	short *buff = (short*)malloc(dwBytes);
	dwBytes = BASS_ChannelGetData(hStream, buff, dwBytes);
	for (i=0; i<dwBytes/2; i+=2)
	{
		if ((buff[i]>0 && buff[i+2]<=0))
			break;
	}

	free(buff);
	BASS_StreamFree(hStream);
	return dwPos+i*2;
}

BOOL CLoopingDlg::FindZeroCross(QWORD* dwLoopStartPos, QWORD*
dwLoopEndPos)
{
	BOOL bSuccess = TRUE;
	HSTREAM hStream = BASS_StreamCreateFile(FALSE, cszPath, 0,0,
BASS_STREAM_DECODE|BASS_STREAM_PRESCAN);
	if(hStream)
	{
		int i = 0;
		DWORD dwBytes = BASS_ChannelSeconds2Bytes(hStream, 0.1f);
		short *buff = (short*)malloc(dwBytes);

		if(BASS_ChannelSetPosition(hStream, *dwLoopStartPos))
		{
			dwBytes = BASS_ChannelGetData(hStream, buff, dwBytes);
			for (i=0; i<dwBytes/2; i+=2)
				if ((buff[i]>0 && buff[i+2]<=0))break;
			*dwLoopStartPos = *dwLoopStartPos+i*2;
		}
		else
			bSuccess = FALSE;

		if(BASS_ChannelSetPosition(hStream, *dwLoopEndPos))
		{
			dwBytes = BASS_ChannelGetData(hStream, buff, dwBytes);
			for (i=0; i<dwBytes/2; i+=2)
				if ((buff[i]>0 && buff[i+2]<=0))break;
			*dwLoopEndPos = *dwLoopEndPos+i*2;
		}
		else
			bSuccess = FALSE;

		free(buff);
		BASS_StreamFree(hStream);
	}
	else
		bSuccess = FALSE;

	return bSuccess;
}

Hope it helps some people.

_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?