Now the fun part! Each channel pair of audio signals enters by way of the SymbiLink jacks, goes through a buffering op-amp, and on to the daughter boards where a Cirrus Logic codec awaits the analog signal. The codecs contain ADCs, or analog-to-digital converters, which do just what it sounds like. Our familiar analog squiggly lines are turned into ones and zeros, and lots of them, since this chip is capable of 24-bit/96kHz processing. Twenty-four bits is good, as opposed to 8 or 16 bits--the more bits, the greater the resolution and dynamic range. Ninety-six kilohertz refers to the sampling rate and is better than 44.1 or 48kHz, because the effective highest frequency handled will be slightly less than half of this rate, in this case 47.5kHz, which is more than enough for this situation. I actually don't know for sure that Zapco is using the highest bits and sampling rate, I'm just stating the capabilities of the chip.
After the signal is turned into a Matrix-like string of numbers, all the processing is done by mathematics. There's no signal-to-noise ratio, no phase shifts in crossovers and equalizers, because it's all numbers. The numbers are sent to a Freescale Semiconductor 56300 series 24-bit DSP chip where all the processing is accomplished. Compared to just a few years ago, this little guy is practically a supercomputer. Six channels of phase, time delay, crossover and parametric EQ are all that is being used now, but I suspect Zapco plans to tap some of the surround sound capabilities as well. After the math is finished (be glad you don't have to do it!), the numbers are sent back to the codecs to be converted by--you guessed it--digital-to-analog converters (DACs) back into the analog world. The analog signal is sent to the amplifier stage as usual.
Zapco sent the dash-mount controller, the DRC-SL, as well as the software to use a PC to control the DSP functions. I found the DRC-SL to be somewhat limited, mostly due to its half-DIN size, which allows text but no graphics in the user interface. I do like the fact that navigation and selection are accomplished using a single 5-way joystick. You move the button up, down, left and right, and push in to make your selections. The graphic user interface on the PC was easier to use, allowing you to see every activated function on the screen at one time, as well as seeing a frequency response that reflects your EQ and crossover selections.
Here's a quick run-down of the functions available:
- Input level (sensitivity) adjustable from 500 millivolts to 10 volts, this adjustment tunes the input signal to match the input level of the ADCs, which is about 500 millivolts.
- Output level from the DSP to the amplifier stage to maximize the dynamic range. There are input and output clip lights to guide you in setting these two levels.
- Channel selection, as pairs (1+2, 3+4, 5+6) or as individual channels. You select which channels to make adjustments to.
- EQ, crossover and Q-Bass On/Off selection.
- Ten bands of fully adjustable parametric equalization for each channel. The EQ can be the usual Peak format, in which you boost (or cut) the center frequency, or you can switch to Shelf format, which adjusts all frequencies above the "shelf" frequency.
- Lowpass, highpass and bandpass crossovers, with selectable slopes of 6, 12, 18 and 24dB in either Butterworth or Linkwitz-Riley alignments.
- Q-BASS, which is simply another band of parametric EQ with 18dB of boost, and frequencies more suited to subwoofers.
- All frequencies are adjustable in whole digits from 10Hz to 20kHz for the EQs and crossovers.
- Time delay from 0 to 22 seconds in 0.1-second increments, with a display that correlates time to distance.
Pretty cool, huh? You can set individual addresses on each amplifier and control up to 16 amplifiers from the same PC or DRC-SL in-dash controller. Another very cool feature is real-time adjustments. The amplifier responds as you move the numbers around, rather than making an adjustment and then "sending" or "updating" the processor. Yep, very cool. The DRC-SL does have access to combining the inputs using the "SOURCE" selection. You can send input channels to any or all of the amp channels with this selection, but I found no matching control in the PC software.