Reply by Keith Larson November 12, 20032003-11-12
Hello Hamid

What you will find is that the available sample rates and internal 
filters scale with the master clock.  With this board you have the 
option to also select 16/32 bit modes but my advice here is to leave 
that alone.  I hardcoded the VC33 DSK for just this reason (no options = 
easy to support)! You also have the option of using a DSP derived master 
clock.  This will probably not give you exactly 8.000 Khz, but it should 
be 'close enough' for experimentation.

If you now look at the ADC input and DAC output you will notice that the 
ADC input is pretty simple but the DAC output has a 2nd order filter.  A 
good question at this point would be 'why?' The reason is that the DAC 
output contains a substantial amount of HF switching noise and the 
magnitude of that noise increases with lower clock rates.

Interestingly if anything the VC33 DSK spectrum analyzer indicates 
things are getting better with lower sampling rates so why worry?

Well, if you connect the DAC to an *external* device you will eventually 
notice the noise as it moves down in frequency, and possibly into a 
frequency band that might be of interest.  What you will find is that 
the ADC has a pretty good input anti-aliasing filter that does a very 
good job of rejecting this noise.

If this is a problem, look into changing the cut-off frequency of the 
op-amp 2nd order filter.

As a side note: I find it hard to believe that no one has noticed that 
the cutoff point of that filter chops down a 20Khz signal by ~1db.


Hamid wrote:

> Dear All > > Anybody knows how to change the sampling rate of PCM3003(Daughter Card > for C6711 DSK card), it is 48Ksps and I need to work with 8ksps, > apparently it has internally anti aliasing and post processing filter > for 48Ksps. > > Thanks: Hamid
+------------------------------------------+ |Keith Larson | |Member Group Technical Staff | |Texas Instruments Incorporated | | | | 281-274-3288 | | k-larson2@ti.com | |------------------------------------------+ | TMS320C3x/C4x/VC33 Applications | | | | $150 TMS320VC33 DSK's ARE AVAILABLE NOW | | | | TMS320VC33 | | The lowest cost and lowest power | | floating point DSP on the planet! | | 500uw/Mflop | +------------------------------------------+ Download the latest code at http://dspvillage.ti.com/docs/catalog/resources/appnoteabstract.jhtml?familyId=132&abstractName=spra976
Reply by Donald Reay November 12, 20032003-11-12
Jacek Raczko <wymyslony@op.pl> wrote in message news:<s0wbg44n1md7.huhaz7nxnhi$.dlg@40tude.net>...
> On 10 Nov 2003 07:53:55 -0800, Hamid wrote: > > > Anybody knows how to change the sampling rate of PCM3003(Daughter Card > > for C6711 DSK card), it is 48Ksps and I need to work with 8ksps, > > apparently it has internally anti aliasing and post processing filter > > for 48Ksps. > > oh - and about that 8ksps - you can also use the timer to provide the > signal to the SYSCLK pin of the PCM3003, but then I'm not sure if you'll be > able to obtain exactly 8ksps - I think you can't cause I've had similar > problem trying to obtain 16kHz - finally went down the other path of taking > every third sample...
If you set jumper JP5 on the daughterboard to select the clock from the DSK rather than from the 12.288 MHz oscillator on the daughterboard than you can set up timer 0 within a program to give different sampling rates. These are limited to those described by (37500000/512)/n Hz where n is an integer. Hence, as Jacek recalls, 8.138 kHz rather than exactly 8.000 kHz is possible. This is covered, with software examples, in "DSP Applications using C and the TMS320C6x DSK" by Rulph Chassaing (Wiley). Hardware modifications to source a clock from elsewhere might not be too difficult if exactly 8kHz is required. Donald
Reply by Jerry Avins November 11, 20032003-11-11
Jacek Raczko wrote:

> On 10 Nov 2003 07:53:55 -0800, Hamid wrote: > > >>Anybody knows how to change the sampling rate of PCM3003(Daughter Card >>for C6711 DSK card), it is 48Ksps and I need to work with 8ksps, >>apparently it has internally anti aliasing and post processing filter >>for 48Ksps. > > > PCM3003 is all hardware configured... you change the sampling frequency by > changing the signal on SYSCLK (pin 9)... and about that internal filters - > I'm not an expert here, but don't they "tune in" to appropriate frequencies > according to the frequency supplied to SYSCLK? > > oh... and about the daughter card you're refering to - why don't you just > take 48k/6=8k every sixth sample? and if you're worried about aliasing - > apply some anti-aliasing filter to that stream? hint: you need to gather > all samples, but you only need to filter (dotprod) one sample out of 6 - > you'll be droping those other 5 anyway ;-) > > good luck...
The anti-aliasing has to be applied to the 48 kHz stream. After the decimation, it's too late. If an FIR filter is used, only every 6th output needs to be calculated. (One MAC for every 6 samples into the buffer.) Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Reply by November 11, 20032003-11-11
On 10 Nov 2003 07:53:55 -0800, Hamid wrote:

> Anybody knows how to change the sampling rate of PCM3003(Daughter Card > for C6711 DSK card), it is 48Ksps and I need to work with 8ksps, > apparently it has internally anti aliasing and post processing filter > for 48Ksps.
oh - and about that 8ksps - you can also use the timer to provide the signal to the SYSCLK pin of the PCM3003, but then I'm not sure if you'll be able to obtain exactly 8ksps - I think you can't cause I've had similar problem trying to obtain 16kHz - finally went down the other path of taking every third sample...
Reply by November 11, 20032003-11-11
On 10 Nov 2003 07:53:55 -0800, Hamid wrote:

> Anybody knows how to change the sampling rate of PCM3003(Daughter Card > for C6711 DSK card), it is 48Ksps and I need to work with 8ksps, > apparently it has internally anti aliasing and post processing filter > for 48Ksps.
PCM3003 is all hardware configured... you change the sampling frequency by changing the signal on SYSCLK (pin 9)... and about that internal filters - I'm not an expert here, but don't they "tune in" to appropriate frequencies according to the frequency supplied to SYSCLK? oh... and about the daughter card you're refering to - why don't you just take 48k/6=8k every sixth sample? and if you're worried about aliasing - apply some anti-aliasing filter to that stream? hint: you need to gather all samples, but you only need to filter (dotprod) one sample out of 6 - you'll be droping those other 5 anyway ;-) good luck...
Reply by Hamid November 10, 20032003-11-10
Dear All

Anybody knows how to change the sampling rate of PCM3003(Daughter Card
for C6711 DSK card), it is 48Ksps and I need to work with 8ksps,
apparently it has internally anti aliasing and post processing filter
for 48Ksps.

Thanks: Hamid