Studio Technologies Gen II - Manual (Page 11)

Generation II User Guide Issue 3, July 1991
Studio Technologies, Inc. Page 13
Band-Reject Filter
The band-reject filter is made up of four
sections of op amp. The 3dB points of the
filter are 400Hz and 2.2kHz, with an 11dB
dip at 1.1kHz. This filter is set to attenuate
signals in the voice band, while leaving low
and high frequency audio signals unaf-
fected. This filter is the reason why the
Generation II in the music & voice mode
adds simulation primarily in the non-voice
region of the audio spectrum. The signal
leaves the band-reject filter and proceeds
to the system status and front panel mode
switches.
Status Switch
Spread out over several of the schematic
pages is the system status switch. It is a
4-pole, 3-position slide switch. What it does
is really quite simple. In the fixed—music &
voice position: the output of the band-reject
filter is sent to the input of the compressor,
the bucket brigade delay’s clock oscillator is
set to the high frequency position, and the
wiper of the stereo intensity trim pot is sent
to the summing/gain and difference/gain op
amps. In the fixed—music position: the
output of the line input stage is sent to the
input of the compressor, the oscillator is set
to the low frequency position, and the wiper
of the stereo intensity trim pot is sent to the
summing/gain and difference/gain op amps.
In the front panel position: the mode switch
and stereo intensity potentiometer on the
front panel are active.
Front Panel Mode Switch
If the status switch is set to the front panel
position, the front panel mode switch is
active. The switch selects if the band-reject
filter is in or out of the signal sent to the
compressor, and controls the BBD’s clock
oscillator frequency. This allows the users
to select if the Generation II operates in the
music or the music & voice mode.
Audio Delay
A large part of the Generation II’s circuitry
involves time delaying the audio input
signal. The “heart” of the time delay is an
integrated circuit that implements a bucket
brigade delay (BBD) line. A BBD is an
analog technique that samples a signal and
stores an analog representation of its level.
This analog signal is then passed, “bucket
to bucket” to achieve a delay. The more
buckets, and the more time the signal is
stored in each bucket, the longer the time
delay. The length of time the signal sits in a
bucket is inversely related to the frequency
of its clock signal. The higher the clock
frequency, the shorter the time delay. An
anti-aliasing filter is required to prevent the
BBD from receiving input signals greater
than one half the lowest clock frequency. A
low-pass filter on the output of the BBD
prevents clock signal from being present in
the audio output. A compandor scheme is
used to improve the BBD’s signal to noise
ratio. The input signal is compressed prior
to being sent to the BBD, and then ex-
panded afterwards. Charge pumps help the
compressor and expander circuits respond
to signal transients.
Compressor
Audio enters the delay circuitry via an
inverting buffer op amp associated with the
compressor portion of an integrated circuit
compandor. A simple resistor/capacitor pre-
emphasis network precedes the inverting