Test Drive: The Symetrix 601 Digital Voice Processor

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by Jerry Vigil

Look around your studio. If you're one of the lucky ones that has a fairly "new" studio, it's loaded with digital gear. You're pulling music beds from CDs. You're producing on a digital workstation. You're mastering digitally to DAT. As analog quickly becomes the exception and digital the norm, one thing remains the same. You still need microphones to record the human voice, and what the microphone converts the sound to is an analog signal. If you're going to be "digital," that microphone needs to get plugged into a box like the Symetrix 601 Digital Voice Processor.

The single rack-space unit flaunts a distinctive front panel consisting of 31 grey buttons, each of which has a small red LED at its center. If you like lots of lights, you'll like the way the 601 lights up when the power is turned on! Why so many buttons? The trend in the past few years has been to make digital processors with only a few controls on the front panel. The result is the need to constantly scroll through various parameters and menus in a display screen until you get to the one you want. The 601 brings a lot of these parameters back to the front panel for quick and easy access. This comes in very handy because the 601 delivers a lot of processing power in one package. You get fully parametric EQ, shelving EQ, notch filtering, dynamic noise filtering or noise reduction, de-essing, delay, stereo synthesis, gating, expansion, compression, and AGC (automatic gain control).

The 601 accepts either a mike or line level analog signal (or both), converts it to 18-bit digital audio, and performs its 24-bit digital processing at a rate of over 50 million instructions per minute (MIPS). The mike and line level analog inputs are both balanced XLR types. The 601 also accepts digital inputs (44.1kHz or 48kHz) in either the AES/EBU or S/PDIF format via XLR jack for AES/EBU and RCA jack for S/PDIF. There are two balanced XLR analog outputs (left and right channel) and two stereo digital outputs, XLR for the AES/EBU format and RCA for the S/PDIF. Rear panel buttons select S/PDIF or AES/EBU. The 601 is a true stereo processor utilizing two digital signal processors or DSPs. The digital inputs are stereo. The analog inputs are mono, but once the analog signal is converted to digital, it is split, and identical signals are sent to each of the two DSPs. Though the 601 has two DSPs, they are not individually accessible. Whatever processing is being applied to one channel is being applied to the other, and adjustments made to a parameter effect both DSPs identically. The delay line is the only exception to this. Phantom power is provided and turned on and off on the rear panel. MIDI input and output jacks facilitate MIDI program change as well as MIDI parameter editing.

The Symetrix 601 provides 128 factory presets and 128 user memory locations. For the radio producer who is recording voice tracks from several different people on a regular basis, the ability to save and recall a variety of settings instantly is a major plus. The first thing on the Test Drive to-do list was to put a voice track on a continuous loop and scroll through all 128 of the presets. Surprisingly, there were a large number of useable "effects" that weren't expected. Most of these utilized the unit's delay section. Though the delay section is simple, it still provides several effects you'd normally go to an "effects box" to get. More on the delay section later. Many digital processors with factory presets provide descriptive "names" for the presets usually shown in a large multi-line LCD display. The 601 doesn't display preset names on its small, 4-character LED display. However, names and descriptions for the 128 factory presets are included in the owners manual.

At the far left of the front panel are two input level controls, one for the mike level input and the other for the line level input. A unique feature of the 601 is its ability to use both analog inputs simultaneously. This, in effect, makes the 601 a mixer of sorts. Red LEDs next to the level controls indicate whether the unit is set to line input, mike input, line/mike inputs (mixer mode), or digital input. A ten-LED bargraph above the level controls displays input headroom and includes a Clip LED to indicate overloading of the analog inputs.

The 31 buttons on the front panel are divided into six sections. They are: Parametric EQ, Dynamics Processing, Delay, Output, System, and Presets. The parametric EQ of the 601 is quite versatile. This is a 3-band equalizer. Bands 1 and 3 can be switched to shelving EQ. All three bands can cover the entire frequency range of the 601's EQ which happens to be from 31Hz to 21.11kHz. Each band can be adjusted from +18dB to -50dB. The bandwidth of each EQ band can be set from .05 octaves to 3 octaves. Any of the three bands can be turned on or off independently of the others by pressing the BP button for that band. When active, the red LED in the center of that band's BP button illuminates. When bypassed, the LED is out. When flashing, that band can be edited with the three EQ editing buttons -- Frequency, Level, and Width - and the large data wheel. Pressing Frequency lets you use the wheel to set the center frequency of the selected band. Pressing Level enables adjustment of the amount of boost or cut. Pressing the Width button lets you modify the bandwidth of the selected band.

The next section of the front panel is devoted to the Dynamics Processor of the 601. This section includes the de-esser, dynamic noise reduction, downward expander, compressor and AGC. There is a button for each of these and four other buttons used to select adjustable parameters of each of the five dynamics processors. These four buttons are Attack, Release, Frequency/Ratio, and Threshold. Pressing the NR button toggles the noise reduction between active and inactive or "out" as the display reads. When active, the Frequency and Threshold buttons become active and are used to adjust the "resting" frequency of the NR. The Threshold button accesses two threshold settings used to set the NR for the appropriate input. The 601's dynamic noise reduction circuitry uses a variable frequency low-pass filter to perform single-ended noise reduction.

Pressing the DS button activates the 601's de-esser. (The de-esser and noise reduction blocks cannot be used simultaneously.) When the de-esser is active, the Threshold becomes the only active parameter button. The Threshold setting sets the level where the de-essing actually takes place.

Pressing Expander activates the downward expander, and all parameter buttons become active -- Attack, Release, Ratio and Threshold. Attack time is adjustable between 0.1ms and 10 seconds. The release time can be set between 100ms and 10 seconds. The expansion ratio is adjustable from 1:0 (out) to 1:8.

While the expander reduces its gain for signal below the threshold, the compressor reduces its gain when audio is above the set threshold. As with the expander, the compressor provides adjustments of attack time, release time, ratio and threshold. The ratio is adjustable from 1:1 (out) to 10:1.

The AGC block cannot be used while the compressor is active and vice versa. Pressing the AGC button activates the AGC and all four parameter buttons. The ratio is adjustable from 1:1 to 4:1.

Thresholds on the NR and de-essing blocks are adjustable from 35dB to 0dB, and on the expander, compressor, and AGC from -100dB to 0dB. A note in the manual explains that "All of the dynamics blocks use a threshold parameter. Unlike analog processors that you may be familiar with, each of the threshold settings in the 601 reference to digital clipping (full-scale) rather than to some nominal signal level (like 0dBu). This means that you may not be able to directly translate threshold settings that you are familiar with from the analog world to the digital world." This is of no concern to most, but if you're one who sets up dynamics processing by the numbers rather than your ears, this is worth noting.

The next section of the 601's front panel provides six buttons devoted to the Delay block. This is a dual delay with feedback and delay-time modulation. Though quite simple compared to more elaborate delay algorithms found in digital effects boxes, the 601 provides quite a few nifty effects with this simple delay block. Modulation of the delay time enables such effects as chorusing and flanging and a very short delay simulates small room reverb. The Mix button sets the mix between the delayed signal and the direct signal. The Delay button is used to set the amount of delay. Pressing the button toggles between delay 1, delay 2, and dual delay modes. In each mode, the current delay time is shown in the display. Maximum delay is 330ms. The Feedback button sets the amount of feedback as well as the phase of the feedback. The Filter button activates the lowpass filter with a range from 600 to 18kHz. The Rate button sets the frequency or rate of the modulation generators, and the Depth buttons sets the modulation depth. These parameters provide several effects more commonly associated with digital effects processors. In fact, some of the effects were as good as those found on boxes designed more towards special effects. You get ping-pong delays, echoes with changing pitches, PA system effects, and even automatic panning achieved by modulating the output levels with the oscillator in the delay section.

The next section of the front panel is the Output section which has two buttons. The Bypass button does as you'd expect. However, only the DSP is disabled; the signal still passes through the converters. The Level/Pan button is used to set the output level and left/right panning of the 601.

The next section is the System section which also has two buttons. The Global button has several functions, each accessed with successive presses of the button. It is used to set the digital input gain as well as the clock source for the internal converters and DSP. Inputs are selected with this button -- Mike, Line, Digital, or Mike/Line mix. Memory write protection is also set or disabled here. The MIDI button accesses the unit's extensive MIDI capabilities including MIDI program change and MIDI dump and load of programs in memory. This is also where the "realtime editor" is accessed enabling MIDI control of parameters not available from the front panel buttons.

The final section of the front panel controls is the Presets section which has four buttons. The Save button saves the program in the edit buffer to one of the user memory locations. The Compare button is used for A/B comparisons between the program in the edit buffer and one already stored in the 601's program memory. The Load button loads the program selected using the data wheel. The Leave Edit button takes you out of the parameter editing mode without changing any of the settings in the edit buffer.

The 4-character LED display next to the data wheel displays program numbers and parameter values as well as some prompts such as "donE" after a program has been loaded or saved, and "Prt" which alerts the user that the current memory location is write protected. A 10-LED bargraph above the display serves a dual function as an output level and clipping indicator, or as a gain reduction indicator.

The 601 is an interesting box, to say the least. The list price is $1,995. For some, that may seem like a lot for a mike processor, but on the other hand, this mike processor does some things the others don't. For example, how many 3-band digital parametric EQs have you come across that provide adjustment of the center frequency on each band from 31Hz to 21.1kHz? Since the 601 provides 18dB of boost on each band, you can therefore get as much as 56dB of boost at one frequency band, with a width as narrow as .05 octaves! This makes for some EQ effects not available from most any equalizer you've probably come across.

The dynamics section of the 601 sounds as clean as you'd expect a fully digital dynamics processor to sound. Most certainly, there will be users of the 601 or any similar device who will insist that their analog dynamics processor "feels" better to them. When it comes to how your own voice sounds going through any device, it's difficult for anyone to say one box is better than another. It's a very subjective matter, and the results will vary greatly, especially with a device like the 601 which provides literally an infinite number of possible set-ups for a given mike or individual.

The delay section of the 601 is an advantage because the majority of special effects most used on the voice in radio production are available in the 601's easy to use delay section. Having these commonly used effects in the same digital path as your dynamics processing and EQ can free up your digital effects box for other tasks. The 601 won't replace your reverb unit, but it will accomplish most of your delay, flanging, and chorusing needs.

Symetrix gets a giant kudo for bringing so many parameters to the front panel of the 601. It costs money to do that which jacks up the price of the device, but it's so much easier to see what the machine is doing to your voice, what effects are active and which ones are not. And editing the programs is extremely simple and fast.

Another pleasant surprise is the 601's operator's manual. It actually reads as though it were written for users of a more creative species rather than for the technical types. Several pages at the front are spent discussing, in plain English, the basics of equalization and its application to the human voice. It explains shelving EQ versus peaking EQ in terms your Sales Manager might grasp! You get a full and understandable explanation of noise reduction, downward expansion, compression and AGC. And if you want to venture into techie land, there's even discussion about matching levels versus matching impedances, XLR polarity convention, phantom power and more. And if that's not enough, there's a glossary in the back that defines in really plain English all the terms used in the manual that might be unfamiliar to some. While the manual isn't really necessary to get a lot out of the 601, one time through it lets you know how truly slick the Symetrix 601 really is.

Other reported specs of the 601 include a frequency response at 12Hz to 20kHz, THD at <.01% @ 1kHz, dynamic range >104dB, and a sample rate at 48kHz (44.1kHz with external clock). The conversion method is 18-bit linear with 64x oversampling.

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