Sampling Problems

AntiSPAM_g9u5dd43@yahoo.com wrote:

> On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
> wrote:
> 
>>I'm writing some material about sampling for the beginner.  I want to 
>>include a bit about non-ideal sampling.  Assuming that sampling is 
>>defined as a process that takes on the value of a continuous-time signal 
>>at the instant that t = (sample time) * n, the only difficulties that I 
>>can attribute to the sampling process itself are:
>>1.  Finite Aperture, i.e. the signal will be filtered a little bit 
>>before sampling.
>>2.  Jitter, i.e. the actual sample time will vary from the assumed 
>>sample time by some amount.
>>What have I missed
>>Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
> 
> 
> How about quantization by the resolution of the converter and the
> process of reconstruction of the original signal.  

I have that covered.

> I suppose the
> differences in zero and first order hold is inappropriate for the
> beginner.
> 
I hope so -- I'm only covering the zero-order hold.

-- 
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Jerry Avins wrote:

> AntiSPAM_g9u5dd43@yahoo.com wrote:
> 
>> On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
>> wrote:
>>
>>> I'm writing some material about sampling for the beginner.  I want to 
>>> include a bit about non-ideal sampling.  Assuming that sampling is 
>>> defined as a process that takes on the value of a continuous-time 
>>> signal at the instant that t = (sample time) * n, the only 
>>> difficulties that I can attribute to the sampling process itself are:
>>> 1.  Finite Aperture, i.e. the signal will be filtered a little bit 
>>> before sampling.
>>> 2.  Jitter, i.e. the actual sample time will vary from the assumed 
>>> sample time by some amount.
>>> What have I missed
>>> Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
>>
>>
>>
>> How about quantization by the resolution of the converter and the
>> process of reconstruction of the original signal.  I suppose the
>> differences in zero and first order hold is inappropriate for the
>> beginner.
> 
> 
> As I understand it, no hold at all is a series of spikes, a zero-order 
> hold creates a stair case, and a first-order hold is either a 
> continuation of the previous section's slope, of linear interpolation. 
> The only first-order hold I've ever seen was one I built myself to 
> smooth the digitally generated carrier of a home-brew 212 modem.
> 
> <quibble> A zero-order hold doesn't attenuate the high frequencies. It 
> greatly amplifies the whole usable spectrum, just a little less at the 
> high end. </quibble>

Well, that depends on how you do your math in the first place.  One way 
or another, however, the high-frequency energy ends up being much less 
than the low.
> 
> Leave out the detail. It's not too deep, but it sidetracks the thought 
> process. Describe the stepwise nature of the output, mention in passing 
> (if you must) the high-end loss, but pass on. High-end attenuation is 
> zilch anyway in servos with their necessary oversampling. MHO.
> 
One of my goals is to explain why, if you're building a servo system and 
the guy next to you is working on a telephony application, he needs 
fancy anti-alias and reconstruction filters and you need "good enough" 
anti-alias and vestigal reconstruction filters.

-- 
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott wrote:
> Jerry Avins wrote:

   ...

>> Leave out the detail. It's not too deep, but it sidetracks the thought 
>> process. Describe the stepwise nature of the output, mention in 
>> passing (if you must) the high-end loss, but pass on. High-end 
>> attenuation is zilch anyway in servos with their necessary 
>> oversampling. MHO.
>>
> One of my goals is to explain why, if you're building a servo system and 
> the guy next to you is working on a telephony application, he needs 
> fancy anti-alias and reconstruction filters and you need "good enough" 
> anti-alias and vestigal reconstruction filters.

Chris Bore used to be a regular contributor to comp.dsp. Explore his 
site, especially if you're putting together a course. It's copyrighted, 
but ideas are free. (Look at the on-line courses http://bores.com/.) He 
made a remark one time that I occasionally use as a sig. See below.

There's more than not _needing_ fancy filters, they hurt. A substantial 
delay when reproducing recorded music goes unnoticed, but even a faint 
artifact in the sound is intrusive. Real-time voice reproduction is more 
tolerant of artifacts, but less tolerant of delay -- more than 5 ms 
becomes uncomfortable. 5 ms will kill a servo, but if a motor doesn't 
like the buzz and whistle from the aliasing, it will never tell you.

Servo filters should be "prompt" (close to minimum phase) and as "light" 
(short) as possible. Servos are practically a different technology: 
higher sample rates for the bandwidth, different filtering goals, flash 
or parallel successive-approximation ADCs, and so on.

Jerry
-- 
... the worst possible design that just meets the specification - almost
a definition of practical engineering.                     .. Chris Bore
������������������������������������������������������������������������

Jerry Avins wrote:

> Tim Wescott wrote:
> 
>> Jerry Avins wrote:
> 
> 
>   ...
> 
>>> Leave out the detail. It's not too deep, but it sidetracks the 
>>> thought process. Describe the stepwise nature of the output, mention 
>>> in passing (if you must) the high-end loss, but pass on. High-end 
>>> attenuation is zilch anyway in servos with their necessary 
>>> oversampling. MHO.
>>>
>> One of my goals is to explain why, if you're building a servo system 
>> and the guy next to you is working on a telephony application, he 
>> needs fancy anti-alias and reconstruction filters and you need "good 
>> enough" anti-alias and vestigal reconstruction filters.
> 
> 
> Chris Bore used to be a regular contributor to comp.dsp. Explore his 
> site, especially if you're putting together a course. It's copyrighted, 
> but ideas are free. (Look at the on-line courses http://bores.com/.) He 
> made a remark one time that I occasionally use as a sig. See below.

Thanks for the link.  The missing 'www' is interesting.
> 
> There's more than not _needing_ fancy filters, they hurt.

Well, that's why I said you need "good enough" filters -- I wanted to be 
stronger than to say you don't need fancy ones.  This is one of the 
points that I intend to make -- while you do sometimes need 
anti-aliasing filters, particularly if you're in an electrically noisy 
environment, you also need to pay careful attention to your filter's 
phase delay, and with almost all plants you _don't_ need to worry about 
a bit of aliasing 'cause it all falls outside of the plant's passband 
anyway.

snip

> 5 ms will kill a servo, but if a motor doesn't 
> like the buzz and whistle from the aliasing, it will never tell you.

I've had to spend significant time convincing folks of this, in the 
past.  On one occasion I ended up just specifying the phase delay of the 
filter and letting the circuit designer go ahead and implement the extra 
3 poles of anti-aliasing.
> 
> Servo filters should be "prompt" (close to minimum phase) and as "light" 
> (short) as possible. Servos are practically a different technology: 
> higher sample rates for the bandwidth, different filtering goals, flash 
> or parallel successive-approximation ADCs, and so on.
> 
> Jerry
Yea verily.

-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott wrote:
> I'm writing some material about sampling for the beginner.  I want to 
> include a bit about non-ideal sampling.  Assuming that sampling is 
> defined as a process that takes on the value of a continuous-time signal 
> at the instant that t = (sample time) * n, the only difficulties that I 
> can attribute to the sampling process itself are:
> 
> 1.  Finite Aperture, i.e. the signal will be filtered a little bit 
> before sampling.
> 
> 2.  Jitter, i.e. the actual sample time will vary from the assumed 
> sample time by some amount.
> 
> What have I missed?
> 
> -------------------------------------------
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com
> 

Tim,
For some content you may want to look at the book "The Art of 
Electronics".  While the book may not go into the "WHY", you can 
probably extract that out. They most likely address some of the issues 
that are experienced in the real-world.

Cheers,
David