Forums

SNR computation in SRC (cntd..)

Started by Govind October 1, 2005
Hi r b-j,

I am trying to compute SNR with the method you've suggested. This time I
am trying upconversion of a signal from 8KHz to 44.1KHz. I have
implemented a four stage SRC code in matlab (with ratios 3/2, 3/2, 7/5
and
7/4 with filter taps 414, 54, 84 and 70 respectively using remez
algorithm). I multiply the output by a factor of 441 (3*3*7*7) in time
domain to compensate for interpolation stage losses. The frequency
response seems to be good. But still I am getting SNR not more than 30-40
DB. I am sure that I am not making a mistake in taking the delay into
account. 

Ray had suggested to check 'Single precision float round-off error'. But
I
am using simple matlab floating point variables, which are by default
double-precision. (Ray, please correct me if I am failing to understand
your point.)

Please help me out, I am very eager to see 100+DB SNR!

Thank you 

Govind

>in article urKdnY7SWfelqaneRVn-gw@giganews.com, Govind at >govind_s@conceptemsys.com wrote on 09/23/2005 12:58: > >> I have written a simple matlab code for converting sampling rate from
8KHz
>> to 16KHz. Now I have to measure it's SNR. For this I converted both
the
>> signals to frequency domain (taking their FFTs) and then computed >> signal-to-noise ratio (sum of squared input spectral samples devided
by
>> sum of squared difference between the output spectral samples and the >> input spectral samples). With this method I am getting SNR not more
than
>> 50-60dB (even for very high filter lengths). Can you please tell me
what's
>> wrong with my method > >not a particular good method to compute SNR. try upsampling a collection
of
>known sine waves from 8 to 16kHz. then generate the exact same sine
waves
>(from the same math) but at the 16 kHz rate. make sure you have your
timing
>lined up because your SRC will have some delay. then subtract to get a >difference (or error or "noise) signal and use that in your SNR
computation.
> >> (I have seen posting mentioning that SNR should go as high as 140dB)? > >how long is your impulse response of your LPF? to get 140, you will
need
>about a 64 tap FIR. > > >-- > >r b-j rbj@audioimagination.com > >"Imagination is more important than knowledge." > > >
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Govind <govind_s@conceptemsys.com> wrote:

> I am trying to compute SNR with the method you've > suggested. This time I am trying upconversion of a signal from > 8KHz to 44.1KHz. I have implemented a four stage SRC code in > matlab (with ratios 3/2, 3/2, 7/5 and 7/4 with filter taps 414, > 54, 84 and 70 respectively using remez algorithm). I multiply the > output by a factor of 441 (3*3*7*7) in time domain to compensate > for interpolation stage losses. The frequency response seems > to be good. But still I am getting SNR not more than 30-40 DB.
You could consider using Lagrangian interpolation instead of this cascade of filters. Steve
>Govind <govind_s@conceptemsys.com> wrote: > >> I am trying to compute SNR with the method you've >> suggested. This time I am trying upconversion of a signal from >> 8KHz to 44.1KHz. I have implemented a four stage SRC code in >> matlab (with ratios 3/2, 3/2, 7/5 and 7/4 with filter taps 414, >> 54, 84 and 70 respectively using remez algorithm). I multiply the >> output by a factor of 441 (3*3*7*7) in time domain to compensate >> for interpolation stage losses. The frequency response seems >> to be good. But still I am getting SNR not more than 30-40 DB. > >You could consider using Lagrangian interpolation instead of >this cascade of filters. > >Steve >
Hi Steve, Thanks for the reply. I have started looking for interpolation methods. Would you like to suggest/give me related docs or book? Govind This message was sent using the Comp.DSP web interface on www.DSPRelated.com
The documentation on the analog devices sample rate conversion chips has some 
good theory info.  Start here and also check out the datasheets for the parts 
mentioned:

http://www.analog.com/en/content/0,2886,765%255F807%255F9690,00.html

By the way, if it were me, I would do this in one stage rather than 4.  It is 
much simpler to implement that way (though probably requires more memory, but 
that's not generally an issue in Matlab).

-- 
Jon Harris
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"Govind" <govind_s@conceptemsys.com> wrote in message 
news:WoqdnUGsSq4auN_eRVn-gQ@giganews.com...
> >Govind <govind_s@conceptemsys.com> wrote: >> >>> I am trying to compute SNR with the method you've >>> suggested. This time I am trying upconversion of a signal from >>> 8KHz to 44.1KHz. I have implemented a four stage SRC code in >>> matlab (with ratios 3/2, 3/2, 7/5 and 7/4 with filter taps 414, >>> 54, 84 and 70 respectively using remez algorithm). I multiply the >>> output by a factor of 441 (3*3*7*7) in time domain to compensate >>> for interpolation stage losses. The frequency response seems >>> to be good. But still I am getting SNR not more than 30-40 DB. >> >>You could consider using Lagrangian interpolation instead of >>this cascade of filters. >> >>Steve >> > > Hi Steve, > > Thanks for the reply. I have started looking for interpolation methods. > Would you like to suggest/give me related docs or book? > > Govind > > This message was sent using the Comp.DSP web interface on > www.DSPRelated.com