Forums

software anti-aliasing filter.

Started by Unknown September 13, 2014
On Mon, 15 Sep 2014 17:13:00 -0400, rickman wrote:

> On 9/15/2014 1:42 PM, Richard Owlett wrote: >> Tim Wescott wrote: >>> On Sun, 14 Sep 2014 22:09:43 -0400, robert bristow-johnson wrote: >>> >>>> On 9/14/14 9:53 PM, Rick Lyons wrote: >>>>> On Sat, 13 Sep 2014 11:57:26 -0500, Tim Wescott >>>>> <seemywebsite@myfooter.really> wrote: >>>>> >>>>>> On Sat, 13 Sep 2014 03:24:22 -0700, zoulzubazz wrote: >>>>>> >>>>>>> I designed a system to acquire certain physiological signals and >>>>>>> recorded these signals using a data acquisition card (agilent >>>>>>> u2531a specifically). the problem is when designing the circuit i >>>>>>> was unable to place an anti-aliasing filter before feeding the >>>>>>> signal to the data acquisition device. The card does not have a >>>>>>> software selectable filter at the input stage either. is there a >>>>>>> way to implement this anti-aliasing filter in software (the >>>>>>> signals are being processed offline)? all ideas welcome. Thanks >>>>>>> very much. >>>>>> >>>>>> Do this simple experiment: >>>>>> >>>>>> Take any 10-page document. Copy all of the pages of the document >>>>>> onto just one sheet, right on top of each other. >>>>>> >>>>>> Is there a way to reconstruct the original document? >>>>>> >>>>>> That's what you're trying to do by anti-aliasing after the fact. >>>>> >>>>> [Snipped by Lyons] >>>>> >>>>> That "paper copy example" is one terrific analogy >>>>> to what happens with aliasing. GOOD JOB! >>>> >>>> or (a similar analogy) is, with one of those cellulose >>>> transparencies, drawing a continuous-time frequency response out and >>>> folding it over, like an accordion, and looking through the multiple >>>> layer. in both the model and reality, "folding frequency" or >>>> "folding over" would have have as literal meaning as doing the wave >>>> equation on the physics of the water's surface. >>>> >>>> i dunno. >>> >>> I was specifically trying to model the futility of reconstructing the >>> original signal. >>> >>> Your "fold the transparency" notion is probably better to educate >>> someone on what's _really_ going on, although it's really "fold and >>> add", which gets beyond what's easy to do graphically. >> >> Careful, use of absolutes cause trouble. >> Consider spectrograms like >> https://en.wikipedia.org/wiki/File:Spectrogram-19thC.png . >> [especially if amplitude is greyscale ;] > > What about them???
Richard was probably referring to the fact that there are no absolutes. None. Absolutely. (Oops) -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Tim Wescott <seemywebsite@myfooter.really> wrote:
> On Mon, 15 Sep 2014 17:13:00 -0400, rickman wrote: >> On 9/15/2014 1:42 PM, Richard Owlett wrote:
(snip)
>>> Careful, use of absolutes cause trouble. >>> Consider spectrograms like >>> https://en.wikipedia.org/wiki/File:Spectrogram-19thC.png . >>> [especially if amplitude is greyscale ;]
>> What about them???
> Richard was probably referring to the fact that there are no absolutes.
I wonder sometimes how FM radio ever got invented. Since the signal has infinite bandwidth, you should only have one station. Conveniently, it seems to fall off fast enough that we have more, and not spaced all that far apart. (But 20 times the AM spacing.)
> None. Absolutely. (Oops)
-- glen
On 9/17/14 2:15 PM, glen herrmannsfeldt wrote:
> > I wonder sometimes how FM radio ever got invented. Since the signal > has infinite bandwidth, you should only have one station.
on top of that, it would fold-over 0 Hz and interfere with itself. doesn't sound too practical to me. what was Armstrong thinking?
> Conveniently, it seems to fall off fast enough that we have more, > and not spaced all that far apart. (But 20 times the AM spacing.)
hence a quite-a-bit better sound than from AM. maybe 20 times better. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
On 9/17/2014 2:15 PM, glen herrmannsfeldt wrote:
> Tim Wescott <seemywebsite@myfooter.really> wrote: >> On Mon, 15 Sep 2014 17:13:00 -0400, rickman wrote: >>> On 9/15/2014 1:42 PM, Richard Owlett wrote: > > (snip) >>>> Careful, use of absolutes cause trouble. >>>> Consider spectrograms like >>>> https://en.wikipedia.org/wiki/File:Spectrogram-19thC.png . >>>> [especially if amplitude is greyscale ;] > >>> What about them??? > >> Richard was probably referring to the fact that there are no absolutes. > > I wonder sometimes how FM radio ever got invented. Since the signal > has infinite bandwidth, you should only have one station. > Conveniently, it seems to fall off fast enough that we have more, > and not spaced all that far apart. (But 20 times the AM spacing.)
20 times the spacing at frequencies more than 20 times higher... -- Rick
rickman <gnuarm@gmail.com> wrote:

(snip, I wrote)

>> I wonder sometimes how FM radio ever got invented. Since the signal >> has infinite bandwidth, you should only have one station. >> Conveniently, it seems to fall off fast enough that we have more, >> and not spaced all that far apart. (But 20 times the AM spacing.)
> 20 times the spacing at frequencies more than 20 times higher...
It was originally 44-50MHz, before RCA convinced the FCC to move it up to 88-108MHz. Still more that 20 times, though. https://en.wikipedia.org/wiki/Edwin_Howard_Armstrong#FM_radio All the already sold radios then became obsolete. -- glen
robert bristow-johnson wrote:
> On 9/17/14 2:15 PM, glen herrmannsfeldt wrote: >> >> I wonder sometimes how FM radio ever got invented. Since the signal >> has infinite bandwidth, you should only have one station. > > on top of that, it would fold-over 0 Hz and interfere with itself. > doesn't sound too practical to me. what was Armstrong thinking? >
SO THAT'S WHERE ALL THE GLOBAL WARMING IS COMING FROM!!!! I just knew it.
>> Conveniently, it seems to fall off fast enough that we have more, >> and not spaced all that far apart. (But 20 times the AM spacing.) > > hence a quite-a-bit better sound than from AM. maybe 20 times better. > >
I've had a couple of AM tuners from the 1960s and it'd shock you how pleasant AM could be. Bad things happen to audio in radio stations and outside the transmission towers/systems. -- Les Cargill
On 9/17/2014 11:43 PM, Les Cargill wrote:
> robert bristow-johnson wrote: >> On 9/17/14 2:15 PM, glen herrmannsfeldt wrote: >>> >>> I wonder sometimes how FM radio ever got invented. Since the signal >>> has infinite bandwidth, you should only have one station. >> >> on top of that, it would fold-over 0 Hz and interfere with itself. >> doesn't sound too practical to me. what was Armstrong thinking? >> > > SO THAT'S WHERE ALL THE GLOBAL WARMING IS COMING FROM!!!! > > I just knew it. > >>> Conveniently, it seems to fall off fast enough that we have more, >>> and not spaced all that far apart. (But 20 times the AM spacing.) >> >> hence a quite-a-bit better sound than from AM. maybe 20 times better. >> >> > > I've had a couple of AM tuners from the 1960s and > it'd shock you how pleasant AM could be. Bad things happen > to audio in radio stations and outside the > transmission towers/systems.
In every movie where the radio is softly playing a country song in the kitchen while mom wipes her hands on her apron and looks out the window to see that good looking stranger (who looks a lot like Clint Eastwood) coming up the dirt road... the radio is AM. FM just isn't romantic. -- Rick
On Wed, 17 Sep 2014 22:43:30 -0500, Les Cargill
<lcargill99@comcast.com> wrote:

<snip>
>I've had a couple of AM tuners from the 1960s and >it'd shock you how pleasant AM could be. Bad things happen >to audio in radio stations and outside the >transmission towers/systems. >
Back in the early 70s I was a young engineer at GM's Cadillac Motor Car division in Detroit. (Back when Caddies actually were made in their own factory, with their own separate engineering facility, etc.) I was astounded to discover that the fancy (for the era) Cadillac sound system had a horrible frequency response with little beyond about 7 kHz... and it started rolling off well below that. I asked one of the senior engineers, and was told that this had been arrived at by listener preference: When given controls to adjust the response (calibrated tone controls, I suppose), listeners overwhelmingly chose to cut out the high frequencies. The FM sound was thus pretty much the same as AM. The engineer's conjecture was that there was "something about the highs reflecting off all that glass." Wasn't until several years later that I realized it was more likely due to the wretched distortion from the output amp... nominally Class A, but certainly nothing like the purists drool over these days. That as before anyone used bridged outputs, so they only had a 12V total swing. (Used a huge Germanium "door knob" transistor, and drove it hard.) Ahh, the Good Old Days! Best regards, Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!