To understand what clipping is, you first need to understand what is being clipped. The sine wave or sinusoid is a function that occurs often in mathematics, physics, signal processing, audition, electrical engineering, and many other fields. It looks like this:
The height of the wave is called the amplitude and the distance between waves is called the wavelength. The wavelength also determines the frequency of the wave (the shorter the wave the faster they come) and is measured in cycles (waves) per second. In SI units, the unit of frequency is hertz (Hz), named after the German physicist Heinrich Hertz. This wave occurs in just about all aspects of bass.
The vibration of a string under tension is a sine wave. The string also generates harmonics (sympathetic vibrations) of the fundamental frequency of the wave. This can actually be seen under strobe lights. Shortening the length of the string or increasing the tension increases the frequency of the sine wave. This sine wave is transferred to the body of acoustic instruments via the bridge and is amplified (increase the amplitude) by the body.
In electric instruments, by a number of means (light, vibration, magnetic) the vibration of the string is converted into an electrical signal by the pickup(s). This electrical signal still has the same frequency of fundamental and harmonics, but the amplitude is converted in voltage. The larger tha amplitude of the vibration of the string, the greater the voltage from the pick-ups. Amplifying the signal increases the voltage, and volts x amperes (amps) = watts. All electrical circuits are limited by the voltage available. If the amplitude of the wave exceeds the voltage available in the circuit, the bits of the wave above the voltage available will be clipped off. Which looks like this:
Clipping can occur in any electrical component of your signal chain and will be heard as distortion. Putting hot pups in front of an active circuit could cause clipping. Hot output from pedals can cause clipping in other pedals. Some pedals are designed to clip (fuzz, distortion etc). Most of these devices do not have flashing lights though to tell us when they are clipping, although Ashdown pedals do have their trademark analogue meter. But lots of amps do - some even have more than one!
The clipping light is a voltage meter or more likely a VU meter (volume unit) as these are a slower measurement and will therefore be more noticable. If you look at a PA amp you will usually see LED's indicating the signal level - green for ok and red for clipping (yes it happens everywhere). What the single clip LED is a VU meter with just the first red LED present. The position of the clip indicator is important to understanding how it works. Most pre-amps have active tone controls that boost as well as cut the chosen frequencies, so if you turn all the tone controls up you are more likely to clip. Some amps have owners manuals that make this clear. From the Genz Benz SHuttle 6.0:
“O/L” LED – This “Over-load” LED shows the operating status of the preamp stage. When this LED flashes, overdriving of the input gain stage is occurring. Adjusting the Preamp Gain and Master Volume controls will affect the over-load condition. Again, over-driving the tube pre-amp may be a desirable tone for your playing style.
SETTING YOUR INPUT SIGNAL – Use the following guidelines to set your amplifier input stage for the type of bass used and your personal playing style.
1) Set your instruments controls (volume and tone) where you normally prefer to run them.
2) With the Channel Gain setting at “0” and your amplifier EQ positions centered at 12 o’clock, begin turning the gain control up to a level in which the O/L LED is just illuminating with your hardest notes played. It is okay for the RED LED to flash somewhat BUT it should not be on constantly unless an overdriven tube tone is desired. Now set your master volume settings for the desired output level (loudness) that you wish to produce.
3) Now, turn your attention to the EQ section and make the adjustments for the desired tone. Because this is an active EQ network any large settings of cut or boost will affect the gain level of the preamp and the O/L lights at the input stage.
4) After setting your EQ it may be necessary to go back and adjust your input gain level down accordingly. It should be noted that the Global Signal Shape circuits will not affect the input gain BUT will affect the Master output level and clip indicator if used with extreme settings.
The EBS HD350 does not have a clip LED but a peak LED:
GAIN - Control to adjust the instruments' signal strength to the right operating level in the EBS HD350. For optimum basic signal level, turn up the GAIN knob until the PEAK led starts flashing from the strongest signal from the instrument. Note: A correctly set GAIN is vital for the signal processing to work properly
The schematic in the owners manual shows the peak LED is before the tone controls. It does not seem unreasonable to assume then that the peak light on the EBS is set at a level where turning all the tone controls up wont make cause clipping, i.e. the peak light is actually -15db.
Clipping can also happen with the power amplifier. Many amplifiers have a limiter on the output stage. A limiter is a compressor with the parameters set to values that cause limiting rather than compression, a high compression ratio and a threshold set to maximum output voltage of the power amp. See the article on compression. Where a limiter is fitted there will often be an LED that illuminates when the threshold has been exceeded.
From the Genz Benz again: OUTPUT LIMITER – The SHUTTLE 6.0® contains an internal power amplifier “Soft Clip” Limiter. This Limiter allows simulation of the output stage saturation as the amplifier nears it maximum power. The limiter is a compound multistage analog circuit, is quite graceful in nature and is particularly musical sounding, even when driven hard. The output “CLIP/LIMIT” LED shows limiting action for the first 6dB above the maximum power threshold and then indicates power amplifier clipping as the power amplifier gradually transitions into gentle clipping.
The red “CLIP/LIMIT” indicates that the power amp has reached maximum power and/or the limiter threshold has been crossed. Under high output conditions this LED will light with the strongest pulses of the signal. Driving hard beyond this point will cause the amplifier to gradually begin to clip.
Although Genz Benz claim the limiter is musical and graceful, it wont be as musical and graceful as a fully specified compressor/limiter. The limiters in less capable amps are less likely to be musical and graceful.
The last component in the signal chain also produces sine waves. Instead of being limited by the voltage as are electrical components, there is a mechanical limit to the movement of drivers in speaker cabs. Therefore if a signal is put into a driver that exceeds the limits on cone sxcursion (how much it moves in and out), this is not termed clipping but the effect is similar and the resulting distortion sounds very similar.
It is important to listen to your sound for distortion and can be very difficult to distinguish between using a distortion pedal, pre amp clippng, power amp clipping and speakers hitting the excursion limit. If you cant tell the difference, turn down.
For more information; go to google and search for clipping.
