[Lowfer] Re: 185.3 party tonight!

[Alberto di Bene]

Jay Rusgrove wrote:

> Results were pretty good - if you are careful with your measuring technique. I
> have no idea how the program determines what it displays as a signal level, but
> dragging the cursor over a seemingly constant white line yields varying signal
> levels of  several dB.

Argo dB display is computed using as reference either a value you can choose
with the mouse, or the ADC full scale value. In other words, 0 dB corresponds
to the value of monochromatic sinusoid that fully excercise the 16-bit ADC.
Difficult to achieve, in practice, due also to sound card non linearities.

Moving the cursor over a signal line gives the dB strength of that signal, at that time.
So it's normal to see variations. It means that the signal wasn't constant in time.

> By dragging the cursor over an entire section of a line
> or character you can easily find a maximum level. When you compare one segment
> with others that are stepped up or down by 10 dB using the maximum level method
> you can probably be within  2 dB (or better) of the actual step without a great
> deal of difficulty. As the signal gets weaker and approaches the background
> noise this figure gets worse. Errors of 3 dB or more are possible.

The dB reading of Spectran (which uses the same algorithm of Argo) has been
checked in a lab with a professional Rohde & Schwartz generator, and found
to be accurate within 0.1 dB - Actual readings in noisy conditions give of course
much worse results than this.

Hope this helps

73  Alberto  I2PHD