How to use an Oscilloscope (CRO) to identify clipping

Here's a quick & dirty intro to the CRO (Cathode Ray Oscilloscope), how to set it up properly & how to identify clipping.

For this test I've used:
BWD-509B CRO
Digitor 707 Digital Multimeter (DMM)
Alpine CDE-7831 Headunit (with BBE, BASS control, 2x 4V Pre-outs)
dB Drag's dB Jam 6 CD for test tones

Before we start FAQ:


Q. Can I use this same method for testing amplifier clipping?
A. Yes, you use the same method. Just disconnect the speakers from their terminals and connect the CRO probe to them instead. You can also connect the DMM to find out the voltage output. Just make sure the headunit is running unclipped before you start

Q. Do I need to run test tones? Can’t I use music instead?
A. No. Music doesn’t give a clean, clear signal and makes it hard to identify the trace. You must use a tone source (CD or generator)

Q. Can I do this for my full range amps too?
A. Yes, just use a higher range test tone around the center of the frequency range your amp will be driving.

Introducing the CRO

Here's my CRO. It’s an Army Surplus BWD509B single trace CRO that’s not too flash and over 20 years old. But it does the job, handles more power than I can ever feed it, is reliable and cost me a carton of beer.

CRO's looks pretty daunting, so what does everything do?

Intensity Knob (Top Right) - This turns on the CRO and adjusts the brightness of the trace on the screen. Adjust it however you like.

Red Dot above Intensity - Indicates if the CRO's turned on. This is handy as the CRO can take a few seconds to warm up and start showing a trace.

Three Knobs Under Intensity - These are used to adjust the traces horizontal position on the screen, adjust focus, on screen start & end points & on screen trace width. Adjust these so the trace is clear, and the trace starts on the left of the screen and goes across to the right all the way.

Volts/CM Knob (Bottom Left) - This adjusts the sensitivity of the CRO in Volts, and adjusts the size of the waves on the screen.

AC/DC Switch (Under Volts/CM Knob) - This switches between AC & DC volts. You want to measure sine waves, so put this to AC.

Vertical Position Knob (Center Bottom) - This adjusts the vertical position of the trace on the screen. Adjust it so the trace is roughly centered.

Time/CM Knob (Bottom Right) - Adjusts the bandwidth of the display. A bandwidth out of range will results in a CRO trace that doesn't pulse and/or display a signal.

Trigger Level (Under Vertical Position) - This is used to adjust the trigger level if you are testing a conductor carrying multiple signals. For our purposes, Automatic Trigger is the best setting.

Three Sockets Bottom Right - These are used for calibration & advanced CRO techniques we don’t need, so you can safely ignore them.

Not pictured is the CRO probe. A CRO probe looks a bit like a pen, when you pull the top down a little hook pops out, that’s the CRO probe tip. Connect this to the outer ring of the RCA; the little alligator clip is for the reference, so connect it to the center pin of the RCA. As we are measuring a sine wave, the CRO is in AC mode and polarity isn't an issue. This is the same when CRO'ing an amp.


Here are three examples of incorrect CRO’ing techniques:


This is what happens when you try to CRO with music instead of a test tone. Notice how the wave is not uniform and very jagged? This makes it difficult to know what you’re measuring. The line through the middle indicates silent sections (image taken with a slow shutter)


This is a nice sine wave; however the Volts/CM sensitivity is set too high and can make it difficult to define the tops of the waves. The line indicates silence (slow shutter speed, silence is due to track change)


Another nice sine wave, however this time sensitivity is set too low and the peaks/crests of the waves are being chopped off by the screen.

 

 

CRO’ing the Headunit

To do this I've connected the CRO probe to one of the headunit RCA's (tip on ring, clip on pin) and adjusted the CRO to the correct ranges.
The DMM has been connected to the other one.
I am running a 60 Hz test tone out of the headunit. 60 Hz is the accepted standard for testing in sub-bass regions. This headunit is for a podium, so sub-bass range clipping/pre-out voltage is what I'm interested in.
The headunit setting have all been set to 0 (EG. volume), flat (EG. EQ settings) and off (EG.BBE)

All images taken with a slow shutter speed, so sorry about the blurriness.

Slowly winding up the volume, at 34/35 it shows a trace like this:

This is a nice clean trace showing a properly formed wave. Note the DMM voltage is showing 3.6V.

Upping the volume to 35/35 (full) shows a trace like this:

It is difficult to see clearly, but the crests of the waves are slightly mis-formed with an irregular ripple on top. This indicates that the headunit has started to clip. Note the DMM is now showing a full 4V. 4V @ Onset of Clipping is its rating, Alpine knows headunit.

Now with 34/35 volume, upping the BASS Control from 0 to +1 show a trace like this:

INSTANT SQUAREWAVE! Never use BASS Boosters or BASS Controls. This is why. This is a badly clipped signal which is square waving; running speakers with a signal like this will result in damage to them. Note the DMM reading now, 5.0V! This is due to the nature of square waves increasing the Duty Cycle of an AC signal.

For curiosities sake, I've reduced the volume to 20/35 with BASS Control +7 (maximum). This is the trace:

Even worse square wave! Again, this is why I say don’t use bass boosters or up bass controls. Again, this signal will cause damage to speakers. Note the DMM again, 5.8V this time. The larger square top means an even longer AC duty cycle; hence the voltage has again risen.

Finally, what happens if we have 34/35 volume, everything flat and turn BBE on:

Another square wave which will cause damage. Voltage is up to 5.9V now, hence the duty cycle has increased again due to even worse clipping.