We continue in this set of
tutorials for the audio test software suite, Visual Analyser. So
far, we've had a general introduction and a
closer look at the Scope
section. In this article, we'll stick with the Scope section,
but bring in the second channel. We'll be picking up where we left
off, so if you haven't gone through the previous tutorials, I'd suggest
doing that first.
As before, make sure that we have:
The soundcard output
connected to speakers and soundcard input
Both channels of the Wave
generator set to 1000Hz, and both enabled
Switching to the Second Channel
Press the Settings button at top left of screen.
Just in from the top right of that new window you'll see a rather
nondescript little box marked Channel(s), and it's currently set to A.
Press the options arrow and select B.
Woah, that was a surprise, wasn't it. All the
green disappeared, and is replaced by red for right. The Ch B (R)
panel is now enabled, and the Ch A (L) is greyed out. We probably
don't need to explore this further, as it's otherwise the same as the
left, with just one exception, which we'll leave to the next section.
So let's go right on and select "A and B". As we might
expect, both red and green light up, and both control panels are
accessible. If you tick both channels' Values box, you'll see both
columns of measurements.
But here's something! Note that the Left channel,
in green, displays on the right of screen, while the right
channel, red, shows up on left of screen. Alfredo?
The one difference between the A and B channels that I
mentioned above is the tickbox in the B channel marked Trig Left.
To investigate that, set both of the Wave generator's frequencies to
1001 Hz. This causes both waves to drift to the left. Tick
the Trig box on the right channel and bring the Trig slider down until
the red trace locks. You might also want to fiddle the Delta Th%
value to optimise the triggering. The green A channel trace still
drifts to the left. (If it doesn't, it could be you still have it
set to trigger by itself, if so, untick its Trig box.)
We could obviously also use the A channel trigger system
to trigger it, but there's an easier way. Just tick the Trig left
box. The left signal snaps into lock, and the words "Triggered by
Right Channel" appears where its own trigger controls used to be.
So, handy, eh? Supposing you are tracing a signal
through a faulty preamp. You feed the output of the sound card
into the preamp, and feed that also to the Scope B channel. You
set the B channel triggering to provide a good stable image, and set it
to trigger the A channel too. Now, as you trace through the preamp
looking for where things are going wrong, the A channel will always be
nicely triggered, no matter the level or distorted nature of the signal.
Note though that this cross triggering only works if the
signals are of the same frequency, or at a multiple. If we set
Channel A to 1002Hz, it takes off again. But if we then set it to
2002, it falls back into lock.
That brings us to this observation, that in VA, we can
have two timebases working at once, one for each channel. This is
in contrast to older and low-price CROs where both channels shared one
timebase, and so you could only look at things that were harmonically
We're going to pass over the next two settings, B/A and
A/B, as they don't relate to the Scope section. So, we'll go on
This is a real throwback to the days of cathode ray
oscilloscopes where the two sets of beam director plates were called X
and Y. It enables simple measurements of phase angles, comparisons
of frequencies, curve tracers for semiconductors and tubes, and so on.
You can see some of these applications at:
(Note that before you go applying large voltages to the
input of your soundcard, as is shown in the curve tracer application
above, you'll need to protect it! We'll come to that.)
To explore some of these possibilities, select the XY
option from the channels selector.
Set both wave generators to 1000Hz, and you'll get a
line at 45 degrees.
Introduce a 90 degree phase difference between the
waves (box at centre of Wave window, then press Apply). Now we
have a circle.
Increase the phase difference to 180degree (antiphase),
and the line is now at 45 degrees the other way.
Try out 270 degrees and 360 degrees before returning
the phase difference to zero.
Nudge A up to 1000.1Hz and you'll see how the
pattern rotates to reflect the ever-changing phase difference.
If you care to count, it will rotate in 10 seconds, the reciprocal
of the 0.1Hz difference between them.
Now shift A up to 2000, which should give us a
bowtie, the two loops reflecting the 2:1 ratio of frequencies.
Bring A up to 2000.1 and it starts to rotate again.
Make A 3000.1 and it has three loops.
Bring B up to 2000 and you'll see a more complex
pattern. One edge has two loops touching it, and the other
three loops, reflecting the 3:2 ratio of frequencies.
You and the kids can have lots of fun playing with other
Use the Settings/Main window to choose A+B mode.
Another colour change, this time to yellow. Notice that the B
channel controls are greyed out, and the singe image on screen is now
much bigger. We're seeing Channels A and B being summed into one
signal. We might do this as a phase check in a stereo system.
If the signals are in phase, they will add. If out-of-phase, they
will cancel. Prove this by introducing a 180 degree phase shift in
the Wave Generator!
Remember to reset the Wave Generator
phase back to 0 or you won't get the right results in the next section.
Use the Settings/Main window to choose A-B mode.
The colour for this mode is a violent pink!
This is a really useful facility with a number of
As a differential measurement. Supposing we
want to know the signal voltage across a resistor, neither end of
which is earthed. We put a probe on each end and use A-B mode
to measure the voltage across it.
To measure the signal level on balanced lines, eg in
a studio or professional sound situation. Put a probe on each
leg of the balanced line and A-B mode will tell us the actual
voltage between the legs.
To set channel gain really accurately. Feed
the same signal into both channels and put a probe on each output.
If the gain is equal in each channel, you should see exactly
nothing. Increase the Zoom on Channel A to see precisely how
much nothing you have!
To check between the input and output of a device to
see what effect it's having at different frequencies or levels.
A good tip for differential tests. Firstly put
both probes on one point. If you don't see nothing, something is
messing up the gain equality from that point onwards. Get that
right before doing your differential readings.
Just the reverse of A-B mode. Still hot pink, just
the channels have been swapped over.
A cross B
I think this means A multiplied by B, i.e. A x B - it
certainly acts like that (Alfredo?). I'm not sure of any
application for this in the Scope mode, but it might be more useful in
the Voltmeter mode. And it's brown!
B cross A
Same as the above. Since the product of two
numbers will always be the same, A x B = B x A, I'm not sure why we need
this option. (Alfredo?) (?)
All good stuff, eh? We now have the ability to do
things like compare a waveform at input and output of a pre-amp, to
check phase changes, to measure differential and balance signals,
compare frequencies and much more.
Now, keep in mind that the various channel options we
looked at above also effect the other measurement tools, such as
spectrum analyser, voltmeter, etc. But we'll look at those effects
in their own context.
In our next exciting episode, we might look at
Calibration. Time to put this thing to some real work!