Help for sampling rate converter

glen herrmannsfeldt wrote:


> When DAT tapes first came out the 48kHz sampling rate was supposedly
> chosen to make sample rate conversion from 44.1kHz CDs hard.

I could never understand that fuss over the direct didital to digital 
copying. A naive copy through the analog will result in some 
deterioration still the resultant quality will be pretty good. Same 
applies to the video DRM crazyness.

> It seems, though, that there are 'good enough' algorithms that
> are now implemented in one chip to convert from almost any
> sampling rate to another, commonly used on inputs to digital
> recorders.  As I understand it, they interpolate to some high
> rate such as 1MHz and then either select the nearest sample
> or linear interpolation from that.  It works even if the
> two sources have different clocks, and so the conversion
> is not synchronous.

It is very simple, similar to the delta sigma DAC followed by the delta 
sigma ADC. The signal is upsampled to ~12.288 MHz @ 1 bit, then 
asynchronously resampled and LPF decimated. Basically, the chip is very 
similar to a common audio codec without the analog part.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On Jan 27, 3:48 pm, Michel Rouzic <Michel0...@yahoo.fr> wrote:
> Erratum :
>
> Michel Rouzic wrote:
> > it makes interpolation by a non-integer
> > ratio much *SOUND* harder than it is.
> > weighted_value = original_signal[indexX] * (0.42 - 0.5*cos(2*pi *
> > (indexX-indexA) / sincsize) + 0.08*cos(4*pi * (indexX-indexA) /
> > sincsize)) * sin(2*pi*0.5 * (indexX-indexA)) / (indexX-indexA)
>
> More like :
>
> weighted_value = original_signal[indexX] * (0.42 - 0.5*cos(2*pi *
> ((indexX-indexA) / sincsize + 0.5)) + 0.08*cos(4*pi * ((indexX-
> indexA) / sincsize + 0.5)) * sin(2*pi*0.5 * (indexX-indexA)) / (indexX-
> indexA)
>
> I always forget that the Blackman function doesn't peak at x = 0.0 but
> at half its size.

Thinking in terms of sine evaluations per tap, using a
Blackman window requires 3.  I wonder how this compares
to using Hamming or Von Hann window with a 50% wider
window on the Sinc, but with only two sine evaluations
per tap resulting in the same number per sample?


IMHO. YMMV.
--
rhn A.T nicholson d.0.t C-o-M
  http://www.nicholson.com/rhn/dsp.html

Andor wrote:

> 
> Well, every kernel interpolation can be written as an upfirdn-
> procedure (exercise to the reader

Not really - although most can. Every resampling scheme ends up being of
the form:

For each output data point compute a weighted sum of some neighboring
input data points 

That could be an FIR filter. But many image resampling algorithms don't
follow this method in a linear fashion which means there will be no
equivalent FIR. They often call these resampling schemes something like
"smart" or "adaptive". Look it up if you are interested.

Also the upfirdn (or multi-phase implementation thereof) procedure has
(lower) limits to the extent of the resampling ratios it can handle. To
get around this there may need to be some version of the non upfirdn
procedure included to make it work at all. In those cases you wouldn't be
able to achieve the equivalent with a pure upfirdn.

-jim

jim wrote:
> Andor wrote:
>
> > Well, every kernel interpolation can be written as an upfirdn-
> > procedure (exercise to the reader
>
> Not really - although most can. Every resampling scheme ends up being of
> the form:
>
> For each output data point compute a weighted sum of some neighboring
> input data points
>
> That could be an FIR filter. But many image resampling algorithms don't
> follow this method in a linear fashion which means there will be no
> equivalent FIR. They often call these resampling schemes something like
> "smart" or "adaptive". Look it up if you are interested.

You're right. I should have added the qualifier "linear" in the
sentence you quoted.

Regards,
Andor

>
> Also the upfirdn (or multi-phase implementation thereof) procedure has
> (lower) limits to the extent of the resampling ratios it can handle. To
> get around this there may need to be some version of the non upfirdn
> procedure included to make it work at all. In those cases you wouldn't be
> able to achieve the equivalent with a pure upfirdn.
>
> -jim