236 Chapter 7
There are also occasions when a deliberate reduction in the channel separation is
advantageous, as, for example, in lessening “ rumble ” effects due to the vertical motion of
a poorly engineered record turntable or in lessening the hiss component of a stereo FM
broadcast. While this is also provided by the circuit of Figure 7.70 , a much less elaborate
arrangement, as shown in Figure 7.71 , will suffi ce for this purpose.
A further, and interesting, approach is that offered by Blumlein, who found that an
increase or reduction in the channel separation of a stereo signal was given by adjusting
the relative magnitudes of the ‘ L R ’ and ‘ L R ’ signals in a stereo matrix, before
these were added or subtracted to give the ‘ 2L ’ and ‘ 2R ’ components.
An electronic circuit for this purpose is shown in Figure 7.72.
7.18.5 Filters
While various kinds of fi lter circuits play a very large part in the studio equipment
employed to generate the program material, both as radio broadcasts and as recordings
on disc or tape, the only types of fi lters normally offered to the user are those designed
to attenuate very low frequencies, below, say, 50 Hz and generally described as “ rumble ”
fi lters, or those operating in the region above a few kHz, and generally described as
“ scratch ” or “ whistle ” fi lters.
Three such fi lter circuits are shown in Figure 7.73. Of these, the fi rst two are fi xed
frequency active fi lter confi gurations employing a bootstrap type circuit for use,
respectively, in high-pass (rumble) and low-pass (hiss) applications, and the third is an
2K22K2100 K
(Log)‘R’ output‘L’ input‘R’ input‘L’ outputFigure 7.71 : Simple stereo channel blend control.