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5.18 How can I use an oscilloscope to set the gains in my system? [AO]


This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

5.18 How can I use an oscilloscope to set the gains in my system? [AO]

This section assumes you are already familiar with your oscilloscope and
will not go into setting it up. If you haven't already, spend a few
minutes with your scope's manual.

You'll need a test disc with a variety of test tones. I use the official
IASCA test disc, but there are some of the "Bass Discs" that have test
tones as well. You do NOT want to use sweeps, only pure tones. There is
an AutoSound 2000 disc (#101?) which has a signal which is unclipped for
20 seconds, clipped for 5, and then unclipped for the last 5 seconds. I
have never used the AutoSound 2000 discs, but know them to have
excellent recommendations (as well as all the test tones you could ever
need). Viewing this track on your scope's display could be useful if
you've never seen clipping on an oscilloscope display before.

You start by finding the clipping level of your head unit. Many of the
better head units will not clip the pre-amp outputs, even at full
volume, but it's always better safe than sorry. Disconnect the RCA's
from your head unit. Pop in your test disc and skip to the track with a
1 KHz tone. If your CD player has a repeat function, set it to repeat
just this track. That way you won't have to skip back at the end of the
tone. Set your bass, treble, fader, and balance all to center. Turn the
volume all the way up. Probe your right and left front (and rear if you
have them) one at a time. Your scope should show a wave, either a sine
wave or a clipped sine wave. If you have a standard pure sine wave then
all is good, and you're ready to proceed. If you have a clipped wave
then you need to turn down the volume, one click at a time until you
see a perfect sine wave on your scope's display. Remember this point, as
this is the highest you can EVER turn up your head unit. After you set
the level for one of your outputs the rest should be the same, but check
them all just to be sure. The results will be the same if you leave the
RCA's plugged into the head unit and disconnect them at the other end
(from your amp/EQ/processor/whatever) but unless you have a remote
control you'll be running back and forth to change the volume.

If your head unit has subwoofer pre-amp outputs you'll need to test them
using a different tone. I usually test subwoofer outputs at a level
midway between the crossover points. For example, if your subwoofer
outputs are crossed over at 80Hz then you would want to use a 40Hz test
tone. Other than that the procedure for testing subwoofer pre-outputs is
the same as testing front or rear outputs.

Once you've found the clipping level of your head unit it's time to
proceed down the signal chain. If you are using an EQ, preamp, DSP, or
other processor (NOT including crossovers) test them next. Leave your EQ
set the way you usually use it. Hook up the processor and probe all the
outputs of your processors in the same method as you did your head
unit. You should probe each of the outputs using tones that match the
bands of your EQ. For example, if you have a 9 band EQ with bands at
50/100/200/400/800/1.5k/3k/6k/12kHz you would probe your EQ 9 times,
once with a 50 Hz test tone, once with a 100 Hz test tone, and so on. If
your EQ also includes a crossover you'll need to follow the crossover
procedure. If any of these processors are clipping you will probably
need to turn down your head unit's volume control or make any
adjustments on that unit that you can. For example, if you are testing
an EQ and you have any bands excessively boosted, try bringing down
that band first. That may be causing your clipping.

To test your crossover you need to probe each output using a test tone
that is midway between the high and low pass. For example, a channel
which is crossed over between 100 Hz and 20 KHz (like a front channel)
would be tested at 9950 Hz. Since you'll be hard pressed to find a 9975
Hz test tone on your CD use the 10Khz tone. For a rear channel crossed
over with a lowpass of 3500 Hz you would use a 1750 Hz tone. As you
again would have problems finding a 1750 Hz tone on a CD use a 2 KHz
tone. For a subwoofer channel lowpassed at 70 Hz you would use a 35 Hz
tone. This one you may find on your CD, if not use 30 Hz or 40 Hz.

Assuming your crossover has level settings you will want to turn the
level for whatever channel you're testing all the way up and probe the
output. Assuming the output is clipping, back the level down slowly
until you see a perfect wave on your scope.

Now it's time to check your amp's outputs. Hook up your amps and play
the same test tones you were using on the crossover. Disconnect the
speakers and then turn the gains all the way up. Probe the first
channel's output. Adjust the gain the same way you did your crossover.
Back the gain down slowly until your wave isn't clipping anymore.

Voila! You are done. You have just effectively eliminated clipping from
your system. If you turn your bass or treble up, or boost up a channel
on your EQ you may introduce some clipping. After major EQ work you may
want to redo this procedure, starting at the EQ.


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