From: Jerry Avins on
Tim Wescott wrote:
> On Mon, 18 Jan 2010 09:25:50 -0600, Richello wrote:
>
>> Dear Sir,
>>
>> Could you please help me to solve this or give me hints to do so?
>>
>> An analogue signal x(t)=10cos(500πt) is sampled at 0, T,2T, .... with
>> T=1ms.
>>
>> I want to find a cosine y(t), whose frequency is as close as possible to
>> that of x(t), which when sampled with T=1ms yields the same sample
>> values as x(t). how can I get the equation of y(t)? .. then if x(nT)
>> were the input to a D/A converter, followed by a low-pass smoothing
>> filter, why would the output be x(t) and not y(t) ?
>
> 1: The solution as you state the problem is trivial: y(t) = x(t). I
> assume you mean the frequency should be close to but different.
>
> 2: Have you asked your prof?
>
> 3: Read this: http://www.wescottdesign.com/articles/Sampling/
> sampling.html. Skip down to the part about aliasing.

First, fix the link:
http://www.wescottdesign.com/articles/Sampling/sampling.html

I'm puzzled by Tim's reference to aliasing. The frequency is 250 Hz and
the sample rate is 1000 Hz.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
From: Tauno Voipio on
Tim Wescott wrote:
> On Mon, 18 Jan 2010 16:00:18 +0000, invalid wrote:
>
>> "Tim Wescott" <tim(a)seemywebsite.com> wrote in message
>> news:JfmdnWt5A4KS4snWnZ2dnUVZ_uBi4p2d(a)web-ster.com...
>>> 3: Read this: http://www.wescottdesign.com/articles/Sampling/
>>> sampling.html. Skip down to the part about aliasing.
>> Rejects attempts to read with error 403
>
> Your newsreader (or mine) put the last bit on the following line.
>
> Try this one, all one line from the 'http' to the following 'html':
> http://www.wescottdesign.com/articles/Sampling/sampling.html.

Next time, please put the URL in angle brackets:
<http://www.wescottdesign.com/articles/Sampling/sampling.html>.

--

Tauno Voipio

From: Tim Wescott on
On Mon, 18 Jan 2010 13:40:37 -0500, Jerry Avins wrote:

> Tim Wescott wrote:
>> On Mon, 18 Jan 2010 09:25:50 -0600, Richello wrote:
>>
>>> Dear Sir,
>>>
>>> Could you please help me to solve this or give me hints to do so?
>>>
>>> An analogue signal x(t)=10cos(500πt) is sampled at 0, T,2T, .... with
>>> T=1ms.
>>>
>>> I want to find a cosine y(t), whose frequency is as close as possible
>>> to that of x(t), which when sampled with T=1ms yields the same sample
>>> values as x(t). how can I get the equation of y(t)? .. then if x(nT)
>>> were the input to a D/A converter, followed by a low-pass smoothing
>>> filter, why would the output be x(t) and not y(t) ?
>>
>> 1: The solution as you state the problem is trivial: y(t) = x(t). I
>> assume you mean the frequency should be close to but different.
>>
>> 2: Have you asked your prof?
>>
>> 3: Read this: http://www.wescottdesign.com/articles/Sampling/
>> sampling.html. Skip down to the part about aliasing.
>
> First, fix the link:
> http://www.wescottdesign.com/articles/Sampling/sampling.html
>
> I'm puzzled by Tim's reference to aliasing. The frequency is 250 Hz and
> the sample rate is 1000 Hz.
>
> Jerry

He's asking for a continuous-time signal (presumably not identical to the
given one) that gives the same discrete-time signal after sampling. That
sounds like aliasing to me.

It sounds like a _homework problem_ about aliasing. One I might write
were I teaching a signal processing course, I might add.

--
www.wescottdesign.com
From: Jerry Avins on
Tauno Voipio wrote:
> Tim Wescott wrote:
>> On Mon, 18 Jan 2010 16:00:18 +0000, invalid wrote:
>>
>>> "Tim Wescott" <tim(a)seemywebsite.com> wrote in message
>>> news:JfmdnWt5A4KS4snWnZ2dnUVZ_uBi4p2d(a)web-ster.com...
>>>> 3: Read this: http://www.wescottdesign.com/articles/Sampling/
>>>> sampling.html. Skip down to the part about aliasing.
>>> Rejects attempts to read with error 403
>>
>> Your newsreader (or mine) put the last bit on the following line.
>>
>> Try this one, all one line from the 'http' to the following 'html':
>> http://www.wescottdesign.com/articles/Sampling/sampling.html.
>
> Next time, please put the URL in angle brackets:
> <http://www.wescottdesign.com/articles/Sampling/sampling.html>.

Whether that helps or not depends on the sending newsreader. Thunderbird
doesn't break any line without delimiter. (Space and dash are
delimiters.) Here's a URL:
http://www.google.com/search?q=http%3A%2F%2Fwww.cartalk.com%2Fcontent%2Fpuzzler%2Ftranscripts%2F201002%2Fanswer.html&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
and here's a jumk line:
########################################################################################################

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
From: Jerry Avins on
Tim Wescott wrote:
> On Mon, 18 Jan 2010 13:40:37 -0500, Jerry Avins wrote:
>
>> Tim Wescott wrote:
>>> On Mon, 18 Jan 2010 09:25:50 -0600, Richello wrote:
>>>
>>>> Dear Sir,
>>>>
>>>> Could you please help me to solve this or give me hints to do so?
>>>>
>>>> An analogue signal x(t)=10cos(500πt) is sampled at 0, T,2T, .... with
>>>> T=1ms.
>>>>
>>>> I want to find a cosine y(t), whose frequency is as close as possible
>>>> to that of x(t), which when sampled with T=1ms yields the same sample
>>>> values as x(t). how can I get the equation of y(t)? .. then if x(nT)
>>>> were the input to a D/A converter, followed by a low-pass smoothing
>>>> filter, why would the output be x(t) and not y(t) ?
>>> 1: The solution as you state the problem is trivial: y(t) = x(t). I
>>> assume you mean the frequency should be close to but different.
>>>
>>> 2: Have you asked your prof?
>>>
>>> 3: Read this: http://www.wescottdesign.com/articles/Sampling/
>>> sampling.html. Skip down to the part about aliasing.
>> First, fix the link:
>> http://www.wescottdesign.com/articles/Sampling/sampling.html
>>
>> I'm puzzled by Tim's reference to aliasing. The frequency is 250 Hz and
>> the sample rate is 1000 Hz.
>>
>> Jerry
>
> He's asking for a continuous-time signal (presumably not identical to the
> given one) that gives the same discrete-time signal after sampling. That
> sounds like aliasing to me.
>
> It sounds like a _homework problem_ about aliasing. One I might write
> were I teaching a signal processing course, I might add.

Got it. I would have worded it differently, though.

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