Installation manual

Once the signal leaves the low pass crossover it is routed to the Sub Harmonic
Synthesizer. I bet that most have never heard of such a device but they are common in
pro audio and have been made for car audio however in much simpler forms. The basic
principle is to take a band of frequencies and synthesize them down to half their original
frequency (one octave down). We have taken this one step further. The signal is fed into
two separate filters, each tuned to a different frequency and these are individually
synthesized and then via two level controls and a master level, the synthesized signals
are mixed back with the original signal.
The two level controls at 32Hz and 45Hz may be set to suit the taste of the listener and
then the master level sets the amount of synthesized signal mixed into the main signal
path. The effects of this are truly amazing.
The compressor follows and this to the best of our knowledge has never been
incorporated in a car stereo product. Of course compressors are common fare in the pro
audio industry. Owing to space limitations we were not able to include variable ATTACK,
RELEASE and RATIO controls. Instead these parameters have been set for the band of
frequencies which RA is processing, namely those below 100Hz. A variable
THRESHOLD control is included with a front panel LED which flashes when compression
begins.
OK here is a short electronics lesson for those who want to know a little more about
compressors.
Dynamic Range is the ratio of the specified maximum signal level capability of a system
or component to its noise level; usually expressed in decibels. Human hearing has a very
high dynamic range. A human is capable of hearing anything from a quiet whisper to the
sound of the loudest jet aircraft. The difference can get to over 100 decibels (a factor of
10,000,000,000 in power!) If we want to reduce this dynamic range we use a downward
compressor (commonly called a compressor). The compressor will “kick in” at a specified
signal level determined by the THRESHOLD control. At this level the compressor will
prevent the output level from increasing linearly with increasing input level.
On the next page is a graph showing the relationship between the input signal and the
output signal. With the compressor out the ratio is 1:1 meaning that if the gain structure of
the device is say 5x, put in 0.1v get out 0.5v, put in 1v get out 5v and so on. (No you
cannot put in 10,000v and get out 50,000v)! If the gain structure was 1x, then put in 1v
and get out 1v. The graph shown shows a 1x (unity gain) structure.
With say a ratio of 4:1 means that a signal that would be 4dB above the threshold is
reduced to 1dB above by the compressor.
The THRESHOLD control on RA does not control the absolute level but only the signal
level which enters the side chain of the compressor.
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