        Operation of the Symetrix "Voice Processor"

     The Symetrix model 528 "Voice  Processor"  is  used  to
adjust  the  signal  level and, in some cases, the timbre of
microphone signals to be digitized and recorded onto  a  DAT
tape  and/or  into an SGI soundfile.  It is also possible to
patch a monophonic analog signal from some auxiliary device,
such  as a synthesizer, into the Symetrix, then process this
line level input instead of a  mic  signal.   This  tutorial
helpfile  summarizes  the  basic  studio  operation  of  the
Symetrix.  For more complete hardcopy  information  on  this
unit, consult section 3 of the _E_C_M_C _U_s_e_r_s' _G_u_i_d_e.

_1.  _I_n_p_u_t _a_n_d _o_u_t_p_u_t

 =>  _I_n_p_u_t:

     The  Symetrix  is  a   _m_o_n_o_p_h_o_n_i_c   preamplifier   that
includes,  on its rear panel, an input jack for a microphone
signal and another jack for a line  level  auxiliary  input.
The Sennheiser condenser microphone is normaled (permanently
wired) into the rear panel mic input of  the  Symetrix.   If
you wish to use a monophonic analog signal from some auxili-
ary device instead of the mic, patch the output of this dev-
ice  into  the  Symetrix  input  jack on the patch bay. This
input jack is the third jack from the left on the lower  row
of the patch bay.

     Note: Microphone and line inputs  cannot  be  processed
simultaneously  (mixed)  by  the  Symetrix.  Patching a line
level signal into the unit breaks the normaled  mic  connec-
tion.

 => _O_u_t_p_u_t:

     The Symetrix output on the patch bay is  normaled  into
the  _C_S-_1_1_5  _m_o_n_i_t_o_r  _s_e_l_e_c_t_o_r  . In turn, the output of the
_C_S-_1_1_5 is normaled to the inputs on the Otari DAT  deck.  If
the  (monophonic) _S_y_m_e_t_r_i_c signal is selected at the _C_S-_1_1_5,
this signal is normaled as  a  "split-mono"  signal  to  the
Otari  deck  (that  is,  the  monophonic  Symetrix signal is
routed to both the left and  right  channel  inputs  of  the
Otari DAT deck).

_2.  _A_d_j_u_s_t_i_n_g _t_h_e _s_i_g_n_a_l _l_e_v_e_l _a_t _t_h_e _S_y_m_e_t_r_i_x

     o+ There are two gain pots on the Symetrix  that  affect
signal  level:  (1)  a  _M_I_C  _G_A_I_N pot at the far left of the
unit, and (2) an _O_U_T_P_U_T _G_A_I_N (_d_B) pot on the right  side  of
the unit, next to a row of _O_U_T_P_U_T _L_E_V_E_L (_V_U) LED meters.
     o+ Set the _O_U_T_P_U_T _G_A_I_N pot initially to 12 o'clock (0 dB
     gain)
     o+ Now adjust the _M_I_C _G_A_I_N preamplifier pot to bring the
     mic  signal  up to a good line level. (The MIC GAIN pot
     has no effect on auxiliary line  input  signals,  which
     should already be at line level.)
     o+ Make coarse gain adjustments with  the  Symetrix  MIC
     GAIN  pot,  finer adjustments with the OUTPUT GAIN pot.
     However, never overdrive either  of  these  amplifiers.
     Neither pot should ever be wide open.


_3.  _U_s_i_n_g _t_h_e _D_e-_e_s_s_e_r, _c_o_m_p_r_e_s_s_o_r/_l_i_m_i_t_e_r/_e_x_p_a_n_d_e_r  _a_n_d  _E_Q
_c_i_r_c_u_i_t_s _o_n _t_h_e _S_y_m_e_t_r_i_x

     In general, it is best to keep recording paths as  sim-
ple  as possible, never routing a signal through any circuit
that is not needed. However, using the  _c_o_m_p_r_e_s_s_i_o_n  circuit
on  the Symetrix can help to avoid clipping of signal peaks;
the _d_e-_e_s_s_e_r circuit can attenuate  objectionably  prominent
mouth  or  breathing  noises  from vocal signals; and the _E_Q
(equalization) circuits can be used to brighten a dull  sig-
nal, or to take the edge off an overly shrill signal source.

_4.  _D_e-_e_s_s_e_r _c_i_r_c_u_i_t

     The _d_e-_e_s_s_e_r circuit,  used  primarily  when  recording
vocal  sources,  senses  and limits high frequencies, in the
range of 3 kHz through 8  kHz,  whenever  these  frequencies
exceed  a  pre-defined  threshold.   Frequencies within this
range that are most often associated with vocal sibilent and
fricative  consonants.   _D_e-_e_s_s_i_n_g can be especially helpful
when one employs very close micing of vocal signals.

     To use the _d_e-_e_s_s_e_r circuit:

     o+ Push the button at the far right of the  Symetrix  _I_N
so   that   _D_E-_E_S_S   is   selected   rather   than  _C_O_M_P/_E_X_P
(compressor/expander).  You  cannot  use  the  _d_e-_e_s_s_e_r  and
_c_o_m_p_r_e_s_s_o_r/_e_x_p_a_n_d_e_r simultaneously.

     o+ At the _D_E-_E_S_S portion of the Symetrix panel,  to  the
right  of  the _M_I_C _G_A_I_N, push the bypass button _I_N to defeat
the bypass.

     o+ Adjust the top blue _F_R_E_Q_U_E_N_C_Y pot  (variable  between
800  Hz  and  8 kHz) and the bottom red _D_E-_E_S_S _R_A_N_G_E pots to
clean up the signal. The red _D_E-_E_S_S _R_A_N_G_E pot varies from  0
dB  attenuation  (no  effect)  when the pot is at 5 o'clock,
fully clockwise) to - 20  dB  attenuation  (maximum  effect)
when the pot is at 7 o'clock, fully counter-clockwise).

_5.  _C_o_m_p_r_e_s_s_o_r, _l_i_m_i_t_e_r _a_n_d _e_x_p_a_n_d_e_r

     To use the _c_o_m_p_r_e_s_s_o_r/_l_i_m_i_t_e_r/_e_x_p_a_n_d_e_r  module  on  the
Symetrix:

     o+ Push the button at the far right of the Symetrix  _O_U_T
so that _C_O_M_P/_E_X_P is selected rather than _D_E-_E_S_S

     o+ At the _c_o_m_p_r_e_s_s_o_r/_e_x_p_a_n_d_e_r portion  of  the  Symetrix
panel  (labeled  _C_O_M_P _T_H_R_E_S_H_O_L_D at the top), push the bypass
switch _I_N.  This defeats the bypass to this circuit.

     o+ Adjust the three rotary pots for desired  _c_o_m_p_r_e_s_s_i_o_n
of  high intensity signals and/or for further attenuation of
low intensity signals.

     o+ The _E_X_P_A_N_D _T_H_R_E_S_H_O_L_D (black) pot controls the  inten-
     sity threshold level for low level signals that will be
     further attenuated, variable  between  0  dBm  (pot  at
     5:00, fully clockwise, maximum effect) and -60 dBm (pot
     at 7:00, fully counterclockwise, no effect).  This  can
     be useful to reduce background noise while recording.
     o+ The _C_O_M_P _T_H_R_E_S_H_O_L_D (green) pot controls the intensity
     threshold for high level signals to be attenuated. This
     pot can be varied between +20 dBm (green pot  at  5:00,
     fully  clockwise,  no  effect) to -50 dBm (pot at 7:00,
     fully counterclockwise, maximum effect).
     o+ The _C_O_M_P_R_E_S_S _R_A_T_I_O (red) pot determines the  compres-
     sion  ratio  (the  higher  the  ratio,  the greater the
     compression). The ratio is variable from 20:1 (greatest
     compression,  pot  at  5:00,  fully  clockwise)  to  no
     compression (pot at 7:00, fully counterclockwise).

     Typical settings of these three pots for _l_i_m_i_t_i_n_g  peak
signal levels would be:

     _C_O_M_P _T_h_r_e_s_h_o_l_d (green pot): somewhere between 3 dB  and
     6 dB attenuation (about 12:00)
     _R_A_T_I_O (red pot): 20 : 1 (5:00, fully clockwise, maximum
     effect, to avoid clipping of sudden signal peaks)
     _E_X_P _T_h_r_e_s_h_o_l_d (black pot):  7:00,  fully  counterclock-
     wise, no effect

     To use only the _e_x_p_a_n_d_e_r (no compression)  as  a  noise
gate, the following settings might be useful:

     _C_O_M_P _T_h_r_e_s_h_o_l_d (green pot): +20 dBm  (fully  clockwise,
     5:00, no effect)
     _R_A_T_I_O  (red  pot):  7:00,  fully  counterclockwise,  no
     effect
     _E_X_P _T_h_r_e_s_h_o_l_d (black pot): fully clockwise, 5:00,  max-
     imum  noise  gating.   These settings will reduce back-
     ground noise, but also make signal decays  more  abrupt
     (often  used  in  pop music recordings to give a signal
     more "punch").   If  decays  in  the  sounds  seem  too
     abrupt,  lower  this setting, gradually turning the pot
     counterclockwise.

     Compression and  expansion  can  also  be  combined  to
reduce  the  intensity  of  both  very hot signals (to avoid
clipping) and of very low level  signals  (to  reduce  back-
ground noise or to exaggerate decays):

     _C_O_M_P _T_h_r_e_s_h_o_l_d (green pot): somewhere between 3 dB  and
     6 dB attenuation (about 12:00)
     _R_A_T_I_O (red pot): 20 : 1 (5:00, fully clockwise, maximum
     effect, to avoid clipping of sudden signal peaks)
     _E_X_P _T_h_r_e_s_h_o_l_d (black pot): fully clockwise, 5:00,  max-
     imum noise gating.

For more details on compression and noise  gating,  see  the
Symetrix manual.

     Whenever the compressor is active, red LEDs will  light
up  on  the  Symetrix  _O_u_t_p_u_t  _L_e_v_e_l display, indicating the
amount of compression.

_6.  _P_a_r_a_m_e_t_r_i_c _E_q_u_a_l_i_z_e_r_s

     An _e_q_u_a_l_i_z_e_r (_E_Q) is an amplifier that operates only on
a  selected  band  (range)  of  frequencies within a signal,
increasing or decreasing the  intensity  of  this  frequency
band.   With  a _p_a_r_a_m_e_t_r_i_c _e_q_u_a_l_i_z_e_r, the operating "parame-
ters" (center frequency, bandwidth and  amount  of  gain  or
attenuation)  are  user-variable  rather  than fixed pre-set
values.

     Notch filtering (EQ attenuation) is  normally  used  to
reduce objectionable frequencies (such as 60 Hz power supply
hum) and narrow bandwidths are customary. When boosting fre-
quency  bands,  wider  bandwidths  (e.g. .7 octave) are more
common.

     The Symetrix provides three-stage equalization:

     (1) a low frequency  band  (center  frequency  variable
     between 16 Hz and 512 Hz)
     (2) a mid range band (center frequency variable between
     196 Hz and 6.3 kHz)
     (3) a high frequency band  (center  frequency  variable
     between 686 Hz and 22 Hz)

The controls for each of these three bands include  pots  to
determine  _b_a_n_d_w_i_d_t_h  (variable between .05 and 3.3 octaves)
and amount of _g_a_i_n or _a_t_t_e_n_u_a_t_i_o_n (variable between + 15  dB
gain and -30 dB attenuation).

     Note: Be very careful when  using  any  EQ  circuit  to
boost  a  frequency band. It is possible to distort frequen-
cies within the selected band, even though the overall  sig-
nal  level  appears well within acceptable limits.  Use your
ears!

