Nilfisk Alto BK 2.0 Benutzerhandbuch

5

The human ear can detect the noise generating by falling leaves as well as the roar generated by the taking off space 
shuttle. Unfortunately no analog, nor digital device can reproduce such wide spectrum. Please look at Chart. 1 and you 
will see the difference if dynamic capacity of various devices when compare to the human ear. More problems occur 
when handling high level signals and low level signals. When you reach the high level limit you may incur in distortion 
because of the dynamic range of the instrument therefore a certain "reserve" must be maintained to avoid distortion. 
This reserve is known as "headroom" and it is usually set at 10-20 dB. Would not be easier just to reduce the operating 
level? Yes it would but you would put low level music signals at the same level of the basic noise floor so the overall 
quality of the signal would be highly deteriorated. Please look at Chart. 2 and note the Usable dynamic range (including 
headroom) versus high level distortion generated by peaks, and Noise floor level.

Chart.1: The dynamic range capabilities of various devices

Chart.2: The interactive relationship between the operating level and the headroom

So the operating level must be as high as possible but not high enough to generate distortion.
Thriky uh?

There is a very easy way to obtain this. With the volume knob in hand you increase the volume during 
low passages and decrease the volume during loud passages. Unfortunately even the Great Houdini 
would not be fast enough to monitor carefully the speed of the musical signal and it would be impossible 
to detect signal peaks and consequently levelling them out. Manual control is therefore out of question. 
The answer is AGC (automatic gain control); a device that will monitor the signal in real time and that 
will adjust the gain for the best S/N ratio without producing distortion. So, the name of this device is 
"Compressor/limiter".

Try to measure the dynamic range of musical instruments. You will find out that your ear will handle such 
range. Your audio equipment will not and distortion and overloading will be generated.

To avoid these, compressor/Limiters will be used. Both Compressors and Limiters more or less do the same 
job but Limiters brutally limit the audio signal above a set threshold, while Compressors handle signal in a 
much more civilised way and over a wide range of levels. If an audio signal exceed the threshold set by the 
User the limiter will kill any audio signal above that threshold. Period!  Also Compressors perform their function 
when the audio signal exceed a certain threshold but the signal is not killed brutally. The audio signal will be 
reduced in gain in proportion with the amount above the set threshold.