Of Signal Levels and Insertion Loss
The signal levels of both outputs are approximately 8dB lower in signal level than the input.
This means a 5V peak input signal will be lowered to around 2V peak. This reduction allows
the output audio signal more space to rapidly increase in volume, which it may do under
certain conditions, without any significant risk of clipping distortion.
When the module's feedback control is at its minimum value the delayed output's signal level
pretty much follows that of the input. There are no surprises. However, increasing amounts of
feedback, both negative and positive, can produce massive changes in output signal level. The
actual peak output level is hard to predict as it can change with the harmonic structure of the
input signal, the fundamental frequency of the input signal, the delay time selected and the
modulation signals used. This rapid change in volume at certain frequencies is a distinctive
part of the flanging sound.
Any 5U modular system is powered from +/-15V so the maximum signal output of any
module cannot be more than +/-15V. In practice it actually works out somewhat less than this
and outputs of over +/-13V are unlikely. If the output wants to be above this value, but the
module cannot produce it, then the output clips. This means the module does what it is
supposed to do up to +/-13V and then any desired output greater than that is clipped at +/-
13V and goes no further. This abrupt flattening of the top and bottom of waveforms creates
distortion and is generally not wanted.
The Flanger module is quite capable, when feedback is applied, to produce signal levels far
greater than what goes in. If the output matched the input perfectly, ie. 5V peak ended up as
5V peak with no feedback, then when feedback was applied the output could easily try to
exceed the +/-13V. By lowering the nominal output level to approximately 2V peak the output
can rise by over six times before clipping sets in.
The two signal activity LEDs both monitor the signal level going into the BBD devices. The
module is calibrated so that the red LED will only light when the BBD audio driver circuitry
starts to overdrive. Keeping a signal under this level will help keep the signal as clean as
possible. It is, however, still possible, under certain conditions, for the output stages of the
module to distort. This should be obvious to the user as output stage clipping can normally be
heard. Clipping does not damage the module and some users may find the sound appealing.
When the delay time is very short the signal running through the BBD devices is not passed
through as efficiently as it is when the delay times are longer. Engineers say that the insertion
loss through the BBD increases at higher clock frequencies. The upshot of this is that you may
notice a small volume drop as delay times are shortened. While this has no real detrimental
effect, especially for chorus and pitch shifting effects, it does reduce the signal level available
for feedback. At very high levels of feedback, that is, close to or at oscillation, and with very
short delay times you will notice that the flanging effect is not as pronounced. In practice this
means that the position of the feedback control is important to getting the right sound at the
desired delay time.
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