Test Drive: Symetrix 628 Digital Voice Processor

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by Jerry Vigil

symetrix-logoHow are you processing your voice tracks?  Do you run your mic through a compressor/limiter and make EQ adjustments at the console?  Do you also run the mic through a multi-effects box to apply a noise gate?  This is a pretty common setup for a lot of producers, unless you’re fortunate enough to have a voice processor.  They eliminate patching from box to box, and they keep the signal path short and noise free by putting everything in one neat little package, the kind of package Symetrix is well known for.  You’ve probably seen their 528 in a studio somewhere (maybe your own), and you may recall a Test Drive [November 1993 RAP] on the 601, a monster digital voice processor that also included a bank of delay and modulation effects.  The new 628 Digital Voice Processor from Symetrix is a beautiful combination of the two, taking the 528 up to the digital level with some nice features, and bringing the 628 down to a less complex and more affordable unit.  The 628 packs a pre-amp, de-esser, expander/gate, compressor, and parametric equalizer in a single rack-space unit.  At the top of its list of features is the ability to store settings to one of 119 user preset locations, a major plus for any studio that records many voice talents. 

Rear panel connections include balanced XLR mic and line inputs as well as a balanced TRS 1/4-inch line level input.  You can only use one or the other, mic or line input.  The PHANTOM POWER On/Off switch accommodates both dynamic and condenser type microphones.  You’d expect a single output from this single channel processor, but the 628 offers two balanced line level XLR outputs labeled Analog Left and Analog Right.  There is no stereo processing taking place; the output at both connectors is the same.  In effect, you have a second output that can be used simultaneously with the first should you have a need to split the output.  One use for the second XLR output might be to set it to mic level (with an internal jumper).  That way the box is capable of being connected to either the line or mic input of the console.  Nice flexibility.  And if you need unbalanced line level outputs, there are two 1/4-inch outputs next to the XLR outs.

This is a digital processor, and you’ll also find digital outputs on the rear panel.  You get AES/EBU on an XLR connector and S/PDIF on RCA.  The DIGITAL OUTPUT SELECT button chooses between the two, and the SAMPLE RATE SELECT switch selects between 32kHz, 44.1kHz, and 48kHz sampling.  The MIDI/RC-1 IN and MIDI OUT/THRU jacks do not provide MIDI control of parameters.  These connectors are used to perform MIDI dumps of preset information from one 628 to another.  The MIDI/RC-1 IN also accepts the optional RC-1 Remote Controller, a very handy device that provides instant switching between the unit’s first 11 presets.  The RC-1 is basically a MIDI controller that sends only eleven program change commands and can be used to access the first eleven programs on any MIDI capable box.  A twelfth button sets the 628 to bypass mode.  The RC-1 gets its power from the 628 via the special cable provided, but a 9V DC input on the back of the RC-1 accepts an external power source for using the RC-1 with units other than the 628.  The MIDI CHANNEL SELECT switch (1-16), MIDI PROGRAM DUMP button, and the AC Power connector wrap up the rear panel tour.

The 528 user will feel right at home with the 628’s front panel which divides the various components of the processor into separate sections.  At the far left is the Mic Pre-Amp section.  The MIC/LINE button switches between mic and line level inputs.  Another button activates a 15dB pad to accommodate mics with high output.  The GAIN control adjusts the mic level input from +15dB to +75dB and the line level input from -12dB to +12dB.  A red CLIP LED lights when the pre-amp is overloaded.  The analog input is sent to a 20-bit A/D converter, and the digital signal is sent to 24-bit processing for the effects. 


Next in line is the de-esser.  Press the ACTIVE button to engage the de-esser.  A green LED lights to indicate it’s in line.  (The de-esser, expander/gate, compressor, and EQ sections all have independent ACTIVE buttons and green ACTIVE LEDs.)  There are adjustments on the de-esser for FREQUENCY and THRESHOLD.  These knobs, like all the knobs on the 628 (with the exception of the GAIN control), rotate infinitely and activate the 3-digit LCD Master display at the far right where parameter values are displayed.  In other words, there isn’t an arrow on the knob pointing to parameter values printed on the panel around the knob.  The result is, you cannot glance at a knob and see where it’s set.  To view a parameter’s setting, turn that knob left or right one click.  This displays the current value.  Further clicks in either direction will change the parameter setting.  The de-esser’s THRESHOLD control sets the level at which de-essing occurs (from -60 to 0dB) and the FREQUENCY control sets the frequency that will be affected (.8 to 12kHz in 100Hz steps).  A dedicated 6-segment LED meter indicates the amount of de-essing/gain reduction occurring.

Next is the Expander/Gate section, again with an ACTIVE button and dedicated six-segment LED gain reduction meter.  There are three controls on the expander/gate.  The THRESHOLD is adjustable from -60dB to -20dB in 1dB steps.  The RATIO is adjustable from 1:1 to 9.99:1 in gradually or logarithmically increasing steps, and the RELEASE time is adjustable from .25 to 5 seconds in 50ms steps.  The Ratio and Release controls were not on the earlier 528 and enable using this section as a fast acting noise gate as well as a downward expander.

At the center of the front panel are the Compressor controls.  This section also has its own 6-segment LED gain reduction meter.  The THRESHOLD is adjustable from -60 to 0dB in 1dB steps, the RATIO is adjustable from 1:1 to 14.8:1 in gradually increasing steps, and the RELEASE time is adjustable from .25 to 5 seconds in 50ms steps.

The Parametric EQ section offers three bands with controls for FREQUENCY, BANDWIDTH, and CUT/BOOST.  A button toggles between the three bands, and the selected band is indicated with a red LED for LOW, MID, and HIGH.  The low band frequency is adjustable from 20 to 500Hz in gradually increasing 1Hz to 10Hz steps.  The bandwidth is adjustable from .3 to 4 octaves in increasing steps from 1/100 octave to 1/10th octave, and cut/boost is -15 to +15 in 1dB steps.  Bandwidth and cut/boost parameters are the same for all three bands.  The middle band frequency is adjustable from 160Hz to 6300Hz in 10Hz steps below 1K and 100Hz above 1K.  The high band frequency is adjustable from 680Hz to 20kHz in 10Hz steps below 1K and 100Hz steps above 1K.  (The high band tops out at 16kHz when 32kHz sampling is set on the rear panel.)  Because the bands are overlapping, it is possible to boost/cut up to 30dB on a very wide range of frequencies!

The Master section at the far right offers an OUTPUT LEVEL knob (+/-15dB) and an 8-segment headroom meter with a clip LED.  The large 3-digit LED display is easy to read from a distance.  The PRESET control scrolls through the 127 presets.  The 128th (or preset #0) is a bypass mode which can be copied and saved to other locations to “reset” them to zero processing.  To load a preset, dial it up and press the LOAD button.  If adjustments are made to the preset, the LED flashes to indicate it has been edited.  To store it, turn the PRESET control to the desired location then press the SAVE button.  The display will read “Sto,” and a second press of the SAVE button actually stores the preset.  The unit is shipped with eight “ready to go” presets at locations 120-127.  These are read only and provide good starting points for various applications.

We tested the 628 on both a Neumann U87 and a Shure SM5B.  Because of the 628’s digital nature, setting up levels was an important first step, and the result was a great sound from a very quiet box.  The pre-amp is transformerless, has no capacitors, and incorporates filters to reduce RF interference.  The compressor performed wonderfully, keeping the signal clean even at large amounts of gain reduction.  The fact that there are only three parameters for both the compressor and the expander/gate make them both easy and fast to set up.  The de-esser was equally easy to use.  With a digital readout of parameter values, it is simple to zero in on offending frequencies to de-ess by going to the EQ first, setting the boost to about 10dB, then scrolling the frequencies until the target frequency jumps out at you.  Then, reset the EQ, go to the de-esser, dial in that frequency, and adjust the threshold to set the amount of de-essing. 


Though playing with the EQ requires regular clicks of the knobs to see what current settings are, the overlapping bands, wide range of bandwidths, and great sound make up for the small inconvenience.  Overall, the 628 is extremely easy to use and can be used right out of the box without a glance at the manual, which by the way, is very well written and goes the extra mile to explain, in terms users can understand, the basics of audio processing.  A half-hour with this manual, and you’ll be ready to take on any dynamics processor with more confidence.

The 628 handles all the basic mic processing needs very well, but, as mentioned earlier, perhaps its handiest feature is the ability to store settings to 119 user presets.  In the radio production environment, this is a wonderful tool.  Each talent on your staff (or talent pool) can have his/her own preset which eliminates both the time setting up the processing each time the talent arrives, and any inconsistencies that would likely creep in from session to session.  The RC-1 Remote Control speeds things up even more by putting the eleven most used presets just one button press away.  And for the producer/voice talent who has several EQ settings for different effects on his/her own voice, the 628’s preset feature is a great time saver. 

Obviously, the 628 with its RC-1 remote are ideal for the on-air studio.  Eleven presets available on a small desktop remote control are enough to handle most on-air staffs, and the main unit can be safely stashed in a rack nicely hidden in the studio furniture somewhere.  If more security is desired, a simple but clandestine front panel button/knob combination disables the front panel controls.  For either studio, you can’t go wrong with the 628, and once you discover the value of a box like this, you’ll wonder how you managed without one. 

List price on the 628 is $1,199, and the RC-1 Remote Control lists for $199.  Reported specs on the 628 include a frequency response of 20Hz-20kHz and THD+Noise at .05%.  The A/D and D/A converters are 20-bit Delta-Sigma.  By the way, the 628 can also function as an analog to digital converter should you merely want to take a mic through a great pre-amp and output a digital signal to a digital console or the digital input of a recorder.

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