[sdiy] cute hand held scopes on eBay

[Tom Wiltshire]

Hi Richie,

On 23 Feb 2012, at 21:27, Richie Burnett wrote:

> Hi Tom, and all.
> 
>> I quite like the fact that my own digital scope just puts linear sections between samples. It makes it obvious that I'm zoomed in to the limit of the data, and that I shouldn't go making assumptions about what's inbetween those few points. Dot mode does this too, but the display isn't so clear.
> 
> My point regarding the sinc-interpolation is that it is technically the correct way to display a continuous waveform in between the discrete sample points.  Assuming the signal in question has been properly band-limited before being sampled (by front-end analogue anti-aliasing filter), then sinc-interpolation actually reconstructs the original continuous-time signal from the discrete sample points.

I understand that it's technically correct. I just like the technically-incorrect way because it reminds me that what I'm seeing is sampled, not continuous. The sinc interp gets me back to a continuous signal, which isn't what I'm viewing. I'm looking at a load of samples of my input.

>> If I had a beautiful-looking sinc-interpolated image to look at, I might well start confusing it with reality, and I definitely wouldn't be able to tell where the actual datapoints were.
>> Tastes differ, I guess.
> 
> It's not confusing anything for reality, it is reality.  Done properly sinc interpolation will show you the original band-limited signal by drawing a continuous smooth line through the sample points.

That assumes that I didn't lose anything of interest when the signal went through that front-end anti-alising filter. I agree that what the sinc-interp shows is what the scope's AD sees, but what the scope's AD sees isn't necessarily the same as what I feed in, if I push it.

> The thing is that sinc-interpolation isn't actually that hard to do.  It is just post-processing of the sampled data before it is displayed on the screen.  That's why I think that it is a shame that the ARM DSO product is let down in this final step of displaying the captured data!

Hey, it's firmware. We should be able to have lines, dots, lines with dots in different colours, or full sinc interp on some menu somewhere! Everyone's happy!


Tom