Model-8121C uses a logic circuit to generate "Pseudo  Random  Noise", it  is  the  newest   and the most advanced design to generate audio white noise. It has much flatter frequency response than using diode. Model-8121C has: (1)White noise source.(2)Weighting filters.(3)Built-in power amplifier.(4)" TRUE RMS " voltage meter circuit.  It can generate five different kinds of noise's spectrum, including White Noise, Pink Noise and three kinds of "Weighted Noise",  conforms  to  following  nation's  standards:  (1)  JIS C5531, IEEE 219, CNS 4785 (2)  EIA 426B (3)  DIN 45573 2. Since the Noise Generator (Model-8121C) is mainly  designed for loudspeaker testing, thus, it is equipped with a 50W or 100W power amplifier, and "TRUE RMS" voltage measuring circuit , to measure the "TRUE RMS" of output noise. Never use a general VTVM, AC millivoltmeter or a digital voltage meter to  measure  noise  voltage,  unless when these meters have specially noted that  its AC testing function is a TRUE RMS mode, otherwise, the testing result of  RMS  value is  only precise for a sinusoidal signal, because these  meters  are designed  for sinusoidal RMS voltage testing. If these meters are used to test  nonsinusoidal signals (including noise signals), the results of measured RMS values will be inaccurate, therefore, the 8121C is equipped with "TRUE RMS" testing circuit, to measure the noise output voltage. Since noise voltage itself is not highly stable, thus, the voltage meter's  indicator  needle  will  have  a  slight  deflection,  don't  mistake  this  as 8121C  was  faulted.  3. Model-8121C has also provide a selectable "Filtered Noise Sig. Output" terminal (9), which  has the following two usages: (1) It can connect externally with another power amplifier ; (2) It can connect externally with another weighting filter, usage of an external weighting filter is: When users or other national standards required " weighting filter network with special specification ";then, users can connect (9) to  the external needed filter, for necessary filtering, then, connect the output of an  external filter back to the built-in power amplifier input terminal (10) of Model-8121C, for power  amplifying, thereby  measuring the premeasure