Please 'Boom' Responsibly As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while. |
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Click on the picture to view a larger version of it. Use the back button to return to this page. An oscilloscope employs a cathode ray tube (CRT) similar to the picture tube in a television. The CRT 'shoots' an electron beam at a phosphorous coated screen. When the electrons hit the screen they excite the phosphorous atoms and cause them to give off light. The beam is constantly scanning across the screen. When there is no input signal, the beam just scans in a horizontal line. Feature and common settings There
are a number of switches on a scope which have to be set
correctly for the information on the display to be of any
use. I generally use the scope for trouble shooting car
audio problems. These are the most common settings for
me. Inputs Most scopes have 2 input 'channels'. They are labeled channel A and channel B. They can be used individually or together. Focus The focus control simply allows you to keep the beam in focus. (big surprise!) Vertical and horizontal position controls These controls simply allow you to center the beam on the display. Timebase: The timebase determines
the time it takes to scan 1 division (from side to side).
For audio, I generally use 0.2 milliseconds; for
switching power supplies, 2-5 microseconds (depending on
the frequency at which the power supply is oscillating). Trace intensity The trace intensity allows you to adjust the beam to a comfortable brightness level. When the timebase is set for very short times (very fast scanning speed), the display may appear dim. If the scanning is slow, the display may be uncomfortably bright. If a very bright display is used often, it will also reduce the life of the display by burning a line on the screen. Volts/division: Determines the
sensitivity of the input amplifiers. It allows you to
adjust for the best resolution. For car audio 10v/div is
the most common, lower settings (more sensitive input)
are used for preamp level troubleshooting Trigger source The scope must be 'triggered to display a stable waveform. There are several options for the trigger source. The most common trigger source is the signal on the input being used. If you are using the channel A input, the trigger source would be set to 'channel A'. This is the configuration which I use most. If you are using both inputs, you can select either channel as the trigger source. Trigger level: For the waveform to be 'locked' on the screen, the signal has to be of a sufficient level. If you want the scope to be triggered (locked onto the signal) when the voltage of the waveform reaches a certain point, you can set the 'trigger level' so that it will trigger properly. For car audio work, this control is usually set to its center '0' position. It will cause the scope to trigger on the weakest of signals. This control is more commonly used when working with video. Trigger mode The trigger mode allows the scope to lock onto different types of signals. My advice... use the trigger mode which gives you the best results for the waveform being monitored. AC/DC input coupling You should remember that we talked about high pass crossovers and the fact that a high pass crossover blocks low frequencies. You should know that a crossover is actually a filter. The input to the scope can be switched to go through a high pass filter or to bypass the filter. When switched to pass through the filter, the scope is A.C. coupled and the D.C. component of the signal is removed. When the scope is D.C. coupled, the signal is not passed through the filter. Vector input The vector input is used when you need to compare two signals. When the scope is in the vector mode, a voltage applied to one of the inputs will cause the beam to move in the vertical plane. Input to the other input will cause the beam to move in the horizontal plane. There is no scanning in the vector mode. VOLTAGE MEASUREMENT If the input of
the scope has a positive or negative voltage, the beam
will still scan across but will be deflected up or down
from the reference line in proportion to the voltage
applied. The volts/division selector allows you to keep
the beam from being deflected off of the top or bottom of
the display. The v/d selector also lets you compensate
for a small voltage so that you may view the signal with
better detail. The sine wave on the display is
approximately a 1000 hertz test tone. If the scope is set
for 10 volts/division (and it is), the voltage of the
sine wave is approximately 50 volts 'peak to peak' which
is equal to 25 volts 'peak'. The signal is swinging as
high as +2.5 divisions and as low as -2.5 divisions. A
total of 5 divisions at 10 volts/division. TIME MEASUREMENT The
time/division control tells the scope to scan at a
predetermined rate of speed. If it is set at .2
microseconds/division (as it is in the picture). The time
that it takes the beam to scan one horizontal division
will be .2 microseconds. When using the scope for viewing
audio waveforms, it is usually adjusted so that the
scanning beam looks like a straight steady line. For
viewing extremely low frequencies, it may be necessary to
adjust the timebase (volts/division) control to a point
that you may see the beam scanning across. If you set the
timebase control to 100 milliseconds/division, it will
take 1 second to scan across the whole display (10
divisions*.1 seconds/division). I haven't mentioned it
yet but it is easy to determine the frequency of a sine
wave if you know how long it takes to complete a full
cycle. The frequency is the reciprocal of the time it
takes to complete one cycle. If it takes 1/1000 of a
second to complete one full cycle, the frequency of the
signal is 1000 hertz. 1/500 of a second (or .002 seconds)
for 1 cycle would be 500 hertz. PERIOD OF A WAVEFORM If I didn't mention it earlier, the period of a waveform is the time it takes to complete 1 full cycle. To find frequency of a waveform if the period is known, simply divide 1 by the period (1/period). You can also
determine the frequency of a sine wave with a scope. The
diagram below is (supposed to be) the screen of a scope.
As you can see, there are there are 10 divisions from
left to right (and up and down). If you know the timebase
setting and the timebase, you can calculate the frequency
of the waveform. The text on the diagram indicates that
the timebase is set to .1 milliseconds per division. This
means that the 'beam' is scanning across at a rate which
will move the beam through 1 division in .0001 seconds.
You can see that the wave, from 1 peak to the next, spans
3 divisions, which means that its period is .0003
seconds. If we divide 1 by .0003 (1/.0003), we get 3333
cycles per second (3333 hertz). Please note that this is
NOT a very precise way to measure frequency. A frequency
counter would be MUCH more accurate. A.C. coupling In the
picture below, the scope is set for A.C. coupling. Let's
say that it is the speaker output of a high power head
unit. You should remember that the speaker output signal
is 'riding' on 6 volts of D.C. The D.C. is filtered out
of the signal so the signal is vertically centered on the
reference. D.C. coupling In this
picture, the scope is D.C. coupled. You can see that the
signal looks the same but is shifted up. You should
notice that the horizontal center of the sine wave is
located at 3 divisions above the reference. At 2
volts/division, the horizontal center of the sine wave is
at 6 volts (just as it would be on a real scope). The following demo shows how changes in the time base and voltage selector will change the way the waveform is displayed. The signal is a 1khz 1vpeak (2vp-p) sine wave. Depending on the speed of your processor, there may be a second or more delay between the time you push a button and the time the display is updated. Click HERE to make it fill this window.
This scope has a few added features. The signal is the same 1vpeak (2vp-p) 1khz signal except for the fact that it has 6 volts of DC offset. This is the type of signal that you'd find on the speaker output wires of a typical high power head unit. Since the volts/division includes DC, selecting a voltage lower than 2 volts (while DC coupling is selected) will result in the sine wave disappearing past the top of the screen. Click HERE to make it fill this window.
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If you find a problem
with this page or feel that some part of it needs
clarification, E-mail
me. This is a link to this site's home page. |
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Please note that the waveform would start at the left
side of the display and would extend to the other side of
the display. It would NOT begin and end as I have it
drawn. 
