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Discussion Groups | Comp.DSP | Effect of OverSampling


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Effect of OverSampling - pratik13 - 2006-06-21 08:57:00

I'm new to DSP and also this group.
My question is "what can be the negative effect of Oversamling in terms of
Destortion in original signal?". As in undersampling two spectrum are
overlapped and we may not be able to get original signal.. is that any
kind of effect in oversampling?


Re: Effect of OverSampling - steve - 2006-06-21 10:22:00

pratik13 wrote:
> I'm new to DSP and also this group.
> My question is "what can be the negative effect of Oversamling in terms of
> Destortion in original signal?". As in undersampling two spectrum are
> overlapped and we may not be able to get original signal.. is that any
> kind of effect in oversampling?

I don't know in theory if there is, but oversampling requires the
consumption of extra resources, faster a/d, faster processor, more
power, more time, more money, also the performance of a  given a/d
generally degrades at higher sampling rates (becomes more non-linear),
and requires more talent to make the board work (layout becomes more
critical), together these may cancel out any performance increase you
were expecting.


Re: Effect of OverSampling - Jerry Avins - 2006-06-21 10:35:00

pratik13 wrote:
> I'm new to DSP and also this group.
> My question is "what can be the negative effect of Oversamling in terms of
> Destortion in original signal?". As in undersampling two spectrum are
> overlapped and we may not be able to get original signal.. is that any
> kind of effect in oversampling?

The only drawback is the need for extra computation. Up to a point, it 
can make some computations easier.

The original signal is what it is. If it contains distortion, that will 
be reproduced.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯


Re: Effect of OverSampling - Brad Griffis - 2006-06-22 09:17:00

pratik13 wrote:
> I'm new to DSP and also this group.
> My question is "what can be the negative effect of Oversamling in terms of
> Destortion in original signal?". As in undersampling two spectrum are
> overlapped and we may not be able to get original signal.. is that any
> kind of effect in oversampling?
> 
> 

As you probably learned early in your DSP work, you can only sample data 
with the highest frequency equal to half of your sampling rate.  In 
order to achieve/guarantee this you typically need an anti-aliasing 
filter (i.e. an analog filter in front of the A/D).  By oversampling the 
signal you can have a simpler, cheaper anti-aliasing filter and then 
implement a "brick wall filter" digitally which is cheap these days.

On a related note this technique is built into sigma-delta converters. 
They do 1-bit sampling at a very fast rate, typically referred to as the 
modulation frequency.  That data is then put through a digital filter to 
get the "normal" 16- or 24-bit data at the "sampling frequency" you're 
accustomed to seeing.  Generally that digital filter will get rid of any 
frequencies above half the modulation frequency leaving you with a 
alias-free output.  So in the case of sigma-delta converters you 
generally don't need an anti-aliasing filter as long as you don't have 
signal energy above half the modulation frequency.

Brad


Re: Effect of OverSampling - steve - 2006-06-22 09:38:00

Brad Griffis wrote:

> On a related note this technique is built into sigma-delta converters.
> They do 1-bit sampling at a very fast rate, typically referred to as the
> modulation frequency.  That data is then put through a digital filter to
> get the "normal" 16- or 24-bit data at the "sampling frequency" you're
> accustomed to seeing.  Generally that digital filter will get rid of any
> frequencies above half the modulation frequency leaving you with a
> alias-free output.  So in the case of sigma-delta converters you
> generally don't need an anti-aliasing filter as long as you don't have
> signal energy above half the modulation frequency.
>
> Brad

So using a low speed sigma-delta converter (say 1Khz conversion rate
with 64Khz modulation frequency)  would be similar in performance to
using a higher speed SAR converter (say 64Khz) with oversampling/brick
wall filter (assuming equivalent anti-aliasing filter)?

I have read various technical notes concerning sigma-delta converters
and can't really figure them out so I am unable to answer that question
for myself. :(

steve


Re: Effect of OverSampling - Brad Griffis - 2006-06-22 19:10:00

steve wrote:
> Brad Griffis wrote:
> 
>> On a related note this technique is built into sigma-delta converters.
>> They do 1-bit sampling at a very fast rate, typically referred to as the
>> modulation frequency.  That data is then put through a digital filter to
>> get the "normal" 16- or 24-bit data at the "sampling frequency" you're
>> accustomed to seeing.  Generally that digital filter will get rid of any
>> frequencies above half the modulation frequency leaving you with a
>> alias-free output.  So in the case of sigma-delta converters you
>> generally don't need an anti-aliasing filter as long as you don't have
>> signal energy above half the modulation frequency.
>>
>> Brad
> 
> So using a low speed sigma-delta converter (say 1Khz conversion rate
> with 64Khz modulation frequency)  would be similar in performance to
> using a higher speed SAR converter (say 64Khz) with oversampling/brick
> wall filter (assuming equivalent anti-aliasing filter)?
> 
> I have read various technical notes concerning sigma-delta converters
> and can't really figure them out so I am unable to answer that question
> for myself. :(
> 
> steve
> 

The "performance" involves a lot of things, i.e. you'd look at the 
linearity through INL/DNL in addition to things like ENoB.  From a 
signal processing perspective you've got the right idea.  A sigma delta 
converter usually has graphs showing what the frequency response of the 
digital filter looks like.  In SD converters for applications like audio 
that will generally look close to a brick wall filter with the cutoff at 
half the modulation frequency.

Keep in mind that you're only getting 1-bit data at the modulation 
frequency so it takes a lot of oversampling for the filtered data to 
turn into say 16-bits.  Also, because you have a digital filter there is 
some settling time involved.  For applications where you have a bunch of 
channels muxed into one converter you're generally better off with a SAR 
converter because you have to wait the specified settling time each time 
you change inputs to the converter.

Brad


Re: Effect of OverSampling - Jerry Avins - 2006-06-22 20:55:00

Brad Griffis wrote:

> ...   For applications where you have a bunch of
> channels muxed into one converter you're generally better off with a SAR 
> converter because you have to wait the specified settling time each time 
> you change inputs to the converter.

Also for servos and such where delay leads to instability.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯


One possible side effect... - Ikaro - 2006-06-23 10:16:00

Another possible negative side effect of  oversampling not mention so
far is regarding spectral resolution.

If you are dealing with finite storage (ie, memory), you will get
better resolution by sampling at a lower frequency but for a longer
period of time, as opposed to sampling at the highest frequency avaible
for the shortest period of time. Since the spectral resolution is a
function of both the number of samples and the sampling frequency,
sampling at a lower frequency for a given number of points will yield
better spectral resolution.


-Ikaro


pratik13 wrote:
> I'm new to DSP and also this group.
> My question is "what can be the negative effect of Oversamling in terms of
> Destortion in original signal?". As in undersampling two spectrum are
> overlapped and we may not be able to get original signal.. is that any
> kind of effect in oversampling?


Re: One possible side effect... - emre - 2006-06-24 09:20:00

I am afraid this information may be misleading for the novice. It is still
worth emphasizing that as long as the fixed variable is duration (instead
of the number of signal samples), hence number of samples increases
proportional to oversampling, there will be no loss of spectral
resolution.

>Another possible negative side effect of  oversampling not mention so
>far is regarding spectral resolution.
>
>If you are dealing with finite storage (ie, memory), you will get
>better resolution by sampling at a lower frequency but for a longer
>period of time, as opposed to sampling at the highest frequency avaible
>for the shortest period of time. Since the spectral resolution is a
>function of both the number of samples and the sampling frequency,
>sampling at a lower frequency for a given number of points will yield
>better spectral resolution.
>
>
>-Ikaro