(this ain't gonna be as easy as you might think)
General Signal Information:
If a CD player produces 1 volt with a 0dBfs signal, the output will never be more than 1 volt (disregarding tone controls/equalization which will be covered later). Many CDs (especially rap) will be recorded with significant portions at 0dBfs. This will look like clipping if the waveform is viewed with a 'scope (or in a program like '') because there will be 2 or more consecutive points of sampled data at its maximum positive or negative level/value. Most pop music is not recorded with clipping (never 2 or more consecutive points of sampled data at its maximum positive or negative output value) and will generally have at least 1dB of headroom when encoded. Pop music will generally have only about 15dB of dynamic range in the middle of a song (lead in/out excluded). This means that the signal level will be between 0dBfs and -15dBfs for most of the song. If you listen to high quality recordings like those from Sheffield Labs (or other audiophile labels), the dynamic range will be much greater. This is because, many times, they don't use any processors like compressors (a compressor is essentially an automatic volume control that can reduce the dynamic range of a signal or can keep the signal from going above a given level). This means that some of the softer sounds are just that... soft and the loud sounds are... loud. This means that the triangle may produce an output level of maybe -40dBfs and the drums -1dBfs. If the deck produces 1 volt with a 0dBfs track at full volume, the drum will produce a 0.891 volt signal and the triangle will produce a 0.01 volt signal. The image below shows the relative signal levels.
You've probably noticed that TV commercials seem to be much louder than normal programming. It's not that the maximum level of the commercial is any louder. It's that the commercial is recorded with very little dynamic range. Almost all of the recorded audio will be within a few dBs of the maximum level. The signal is compressed to have a very small dynamic range.
General CD Player Information:
In this diagram, you can see the 1 volt signal produced by the 80hz 0dBfs track. If the bass is boosted 6dB (1/2 on many decks), the output would be 2 volts. If the bass is boosted 12dB (maximum boost on some decks), the output voltage will be 4 volts. If this were a 2 volt deck (that produced 1 volt output with a 0dB track with no boost) it would be clipped for anything above 6dB of boost. A 4 volt out deck could produce the signal without clipping.
As a side note, if you had a 2 volt (max) deck that produced 1 volt (at full volume) with the 0dBfs track (no boost) and you wanted to be able to use the full range of the bass and treble controls, you could fade away from the RCA output you were using (the front in this case). In other words... If you were using the front RCA outputs and you wanted to prevent them from clipping, you'd fade to the rear. This would reduce the gain of the signal going to the front preamp section of the head unit (internally) and would therefore reduce the overall level of the front outputs. If you faded the front signal level down 50%, you would only get a 0.5 volt out signal with the 0dBfs track (no boost) but the 80hz track could be reproduced at full (12dB) bass boost without clipping the output of the deck. Using the fader is the only way to prevent clipping on some (maybe most) decks while using the tone controls at full boost.
General Processor Information:
Peak voltage is the voltage from the reference (0 volts) to the top of the waveform (when viewed on an oscilloscope). If you don't fully understand this, go back to the page of the site. You need to understand this before moving on. This diagram is from that page.
Gains on Crossovers:
In the following demo, you can see that there are 2 sine waves. The one on the left shows the output of the head unit. The one on the right is the output of the signal processor (EQ, Crossover...). You can adjust the volume of the head unit (0%-100%) and the output gain of the processor (click on the slider to lock and unlock it). The signal processor's gain control has a range of ±12dB. The center position is 0dB gain (or no gain and no change in the signal level). Notice how the signal output of the processor changes in relationship to the head unit as you set the output gain. You should also notice how little gain it takes to cause the processor's output to reach the point of clipping for processors without switching power supplies. You can set the head unit's max output voltage by entering it into the text field above the volume control.
Gains on Equalizers:
Gains on Amplifiers:
This doesn't mean that one type of power supply regulation is better than another. Highly regulated power supplies, unregulated power supplies and loosely regulated power supplies have their advantages and disadvantages. There's no 'best' when it comes to power supply design.
Let's assume that the deck (at full volume) produces a 1 volt signal with the test tone with all of the tone controls set flat. If we boosted the bass by 6dB, the output voltage would be 2 volts. If we boosted the bass by 12dB, the output would be 4 volts. This should still be unclipped for this head unit. If we drove this signal into the cheaper processor (with only a 10 volt regulated supply - and a 2.5 volt maximum output), the output signal from the processor would be clipped (unless it had some sort of 'input' gain control). The head unit would not be able to be used at full volume with this processor. The gains on the amplifier would have to be set to about 2.5 volts or less (assuming that the gain's markings indicated the amount of signal needed to drive the amp to clipping). If the gains were set to anything higher than 2.5 volts, the processor would clip before the amp reached full power (even though the signal from the head unit is still 'clean'). If the bass boost were set back to +6dB, the volume control could be used throughout its range. To use the bass boost at any setting higher than +6dB, you'd need to go to a processor with a switching power supply. It's important to know what your equipment is capable of so that you can know how to set all of the gains.
In the following demo, there's a display similar to what you'd have on a spectrum analyzer. Each LED indicates the relative level of the signal at each individual frequency. When all LEDs are at the same level, all frequencies in the signal will be reproduced at the same output level (assuming that they were recorded at the same level). At points on the display where the LEDs are at a higher or lower level (referenced to the 'flat' part of the spectrum), the signal output level at those frequencies will be higher or lower than they would be if the bass control were bass set to 'flat'. Let's assume that the vertical dimensions of the display correspond to the range of the output of the head unit. If the LEDs go beyond the top of the display, the head unit is clipping (if there's a low frequency signal in the recorded material). As you can see, the output will remain 'clean' at any position on the volume control as long as the bass is set flat (or for negative gain). As the bass is increased, the volume control can't be used at its full max position without driving the output into clipping. The controls work just as they did in the previous demo.
As a refresher... The following image shows what a 'clean' signal looks like and what a 'clipped' signal looks like. The top is clipped (you can see the flattened top and bottom of the wave). The bottom waveform has clean peaks (no clipping). This image is a short segment of a complex audio signal. For most of this discussion, we'll use more simple sine waves.
The volume on the 'head unit' below is set (it can't be changed in this demo) to 57 and the bass to +10dB, you can see that all of the LEDs for the frequencies below 100hz have gone off of the top of the display. If you played a 0dBfs sine sweep signal, as long as the swept frequency was below ~100hz, it would be clipped. As the swept frequency went above 100hz, the signal level would be unclipped. In the following demo, move your mouse from left to right under the row of LEDs. You can see how the level of the sine wave changes. This is how the output level would change during the sine sweep. Of course, the width of the sine wave would also change during a real sweep. If you were listening to a well recorded piece of music (with a flat frequency response and the highest peaks at/near 0dBfs) and there was some low bass content, the low bass output from the head unit would be clipped.
A sine sweep is a sine wave signal that generally starts at a low frequency and increases in frequency. Some sweeps are from 20hz to 20khz. Others will cover a much smaller part of the audio spectrum. For most sweeps, the signal level is constant throughout the sweep.
As you can see in the following demo, there are 2 sliders. The top is volume for the head unit. The bottom simulates the gain control on signal processors/amplifiers. As you can see, If you clip the signal coming out of the head unit, the output of anything downstream will also be distorted (no matter the gain position).
Gain Setting Overview:
A system with a flat response is one that will reproduce every part of the spectrum at precisely the same level as every other part of the audio spectrum. This means that if you have a disc that has every instrument recorded at the perfect level (whatever that may be) it will be reproduced that way when played through your system. This is especially critical when listening to instruments like the piano. If the response isn't flat, some notes will be really loud and others may be barely audible (although the system would have to be pretty bad for the difference in levels to be that significant).
Note on the Spectrum Analyzer:
As a side note, having a 2 channel scope (most have at least 2 channels), you can have one channel of the scope on the RCA signal and the other on the speaker output. The ground for the scope should probably be connected to the chassis of the vehicle. Don't connect the scope probe's grounds to anything. The display on the scope might be a little noisy but you'll still be able to see the clipping. If you were to connect the ground for one scope probe to the RCA shield ground and the other scope probe ground to a speaker signal wire (like the right negative on many amplifiers), you'd probably damage the head unit's shield ground (and possibly the amplifier). Also... The voltage settings for each of the oscilloscope's channels will have to be different to be able to get good enough resolution to clearly see the clipping.
Since the highs amp will be covering most of the audio spectrum, we'll use it as the reference. Turn bass amplifier off. You can remove the remote wire from the amplifier's remote terminal. Be sure to insulate the wire so that it doesn't ground out and damage the head unit. Removing the RCAs will also do the trick. Set the volume of the highs to play at a low to moderate level and use the equalizer and a pink noise track to get a flat response on the spectrum analyzer. Only concern yourself with the frequencies above the crossover point (commonly that's about 100hz). The head unit's tone controls should be set to flat and loudness contour should be switched off.
Now, using the scope (connected to the RCA outputs) and the pink noise disc, let's set the head unit so that it is just at the point of clipping. Leave the bass and treble flat. Make a note of the volume setting and turn the volume down. Reconnect your highs amp and connect the scope to the speaker output of your highs amp. Turn the head unit back on and the volume to the maximum where it was not clipping. Play the pink noise and increase the gain on the highs amp just to the point where the amp starts to clip. Since pink noise is somewhat tough on tweeters, you may not want to take any more time than necessary.
Remove the RCAs from your highs amp (don't touch the gains). Set the bass on the head unit to about -1/2 of its range (-5 for a control that will go ±10). Adjust the low frequency amp to have a flat response (just as you did for the highs amp - using pink noise and the spectrum analyzer). This may be difficult to do because of a lack of adjustability on the EQ and the transfer function of the vehicle.
Turn the volume down and reconnect the highs amp. Turn the gain all of the way down on the amp. Using the spectrum analyzer and pink noise, set the gain on the low frequency amp so that the level above and below the crossover point (~100hz) are about the same. The following demo shows how the volume, bass gain and highs gain will affect the level on the spectrum analyzer. Of course, the display won't have a separate set of LEDs for each amp. I made them overlap so that you could see the curves and where they'd intersect.
Analyzer Mic Location:
If You Disagree:
Don't Forget the Fader: