[LAU] Soundcard Oscilloscope

[Will Godfrey]

On Sat, 24 Mar 2018 10:32:13 +0100
Fons Adriaensen <fons at linuxaudio.org> wrote:

>On Fri, Mar 23, 2018 at 10:06:25PM +0000, Will Godfrey wrote:
>
>> Bypassing the global filter gives a smaller, high frequency ring at all
>> corners of the square - very unusual.  
>
>Not at all. A 'perfect' square wave has infinite bandwidth, and
>therefore can't exist in a sampled format as everything above
>half the sample rate has to be removed.
>
>What you see on the edges of a bandlimited square wave is just
>the effect of the antialising filter. Calling this 'ringing' is
>sort of a misnomer - there is no resonance as you would have
>with a high Q filter. Also it doesn't correspond to any single
>frequency even it looks a bit like it does. 
>
>Imagine the signal that is the difference between a 'perfect'
>square wave and the same after a sharp 'brick wall' lowpass
>filter. That difference is just the sum of all the harmonics
>that have been removed by the filter. In other words, of all
>the harmonics that are NOT in the bandlimited signal. 
>
>If you want to avoid this (for aesthetic reasons, there is no
>point otherwise), a more gradual lowpass filter (which also
>attenuates harmonics within the audible range) will do that,
>producing 'rounded' corners instead.
>
>Ciao,
>

Following on from this I decided to dig out my seriously ancient home-built
Signal generator, and do some tests.

Running sine waves into the KA6 working at 48kHz sample rate, simple scope was
showing a flat response to 20Khz then a quite rapid drop, as I expected.

Switching to 96kHz I had expeted just a more relaxed roll of, but in fact it
was flat up to 40kHz, and then again a rapid drop.

Now that's the top linit for the KA6, but I'm wondering if an interface that
samples at 192kHz would give me an 80kHz bandwidth - if so that would be more
than good enough for straightforward Audio testing.

My only disappointment with simple scope is there appears to be no way of
saving the current settings or window size. I've got a spare Rasperry Pi3 and
an embarrassing number of older monitors, so it would quite like to put these
together with a USB sound card for a steam-punk scope :)

As an aside, switching the generator to a square wave did produce what I'd
recognise as 'normal' ringing. I wondered if the generator had developed a fault
at first, but taking it to work this morning, I was able to confirm with a
decent scope that its waveform is clean and quite sharp edged, so that must be
due to the input circuitry of the KA6.

I was also pleasantly surprised to find that all the generator's frequency
ranges are still well within 1% and (on sine wave) measured amplitude is within
+-0.5dB except the 10 to 99.9Hz range, where it is about 4dB down at 10Hz.

Obligatory linky :)
http://www.musically.me.uk/images/Sig_Gen.JPG


-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.