Schoeps DSP-4P, August 2002The Schoeps DSP-4P ($3,850) is described by the company as a digital microphone processor for the PolarFlex system. What does the PolarFlex system do, you might ask? 5/14/2004 8:00 AM Eastern
The Schoeps DSP-4P ($3,850) is described by the company as a digital microphone processor for the PolarFlex system. What does the PolarFlex system do, you might ask? The PolarFlex system presents an interesting proposition: Two pairs of vertically coincident omni/figure-8 or back-to-back cardioid capsules may be used in combination to create two virtual microphones. Also, the user may, by combining the patterns, create a pair of virtual microphones capable of any pattern between, and including, the omni and figure-8 patterns.
In addition, the frequency response of the combined pairs can be collectively altered past the confines normally associated with any particular mic pattern by creating highly adjustable crossover points, which provide a multiplicity of single-band, dual-band or 3-band equalization circuits.
While Schoeps does not mandate that its microphones be used for the PolarFlex system, the company does suggest the use of small-diameter condenser microphones. Schoeps suggests the CCM 2S omni, CCM 8 figure-8, CCM 4V or CMXY 4V cardioids from the Compact Series, or CMC 62S omni, CMC 68 figure-8 or CMC 64V cardioid from the Standard Series. Schoeps was kind enough to provide pairs of CCM 8 and CCM 2S mics and mounting rigs along with the DSP-4P. (The total cost of this rig would be $8,765, including all mics, cables and the DSP-4P.)
The Schoeps DSP-4P microphone processor is housed in a compact chassis (8.6x3.5x9.6 inches) that weighs six pounds. The 24-bit digital I/Os are optimized for high-quality digital audio and use Motorola 56009 DSP. The AD/DA converters are from AKM. On the back panel, there are two rows of XLR connectors. The top row, all 24-bit digital, consists of two AES/EBU inputs (four channels of digital audio), two AES/EBU outputs (four unprocessed digital audio channels) and one AES/EBU processed stereo output. Internally, all audio signal processing is handled by fourth-order Infinite Impulse Response (IIR) filters used with 56-bit DSP accumulators.
In addition to being a digital audio input, the first XLR is also used as the master digital input to synchronize the DSP-4P's internal A/D converters to an external clock when the analog inputs are used. Multiple DSP-4P units can be operated together in master/slave configurations. The second XLR also functions as the slave input, switching the DSP-4P to Digital Input mode when a digital signal is detected.
The bottom row of analog XLRs consists of two 5-pin female XLR connectors that can be used either as mic inputs for the two pairs of mics or as four analog line inputs. The 48VDC phantom power delivered to these input XLRs cannot be defeated. To prevent the phantom power from damaging the output circuitry of any attached devices, Schoeps suggests using isolation transformers when interfacing line-level equipment. Analog outputs 1 and 2 are 5-pin XLRs that carry the four channels of unprocessed audio. Analog output 3 carries a 5-pin processed stereo output.
Also on the back panel are toggle switches to select 44.1/48kHz operation and pads to accommodate +20dB mic (for 10mV/PA condenser mics) or +10dB line inputs. Mics that are more sensitive than 10 mV/Pa may also use the line input. An IEC standard power receptacle is used, and the power supply can be operated at 110 to 120 VAC or 220 to 240 VAC. Power consumption is 15 VA.
The front panel has five multifunctioning knobs; a two-position switch determines their usage. The PATTERN/CR.-FREQ. position allows the three upper knobs to be adjusted to create three different polar patterns: one for lows, one for mids and one for highs. Patterns are selectable in 11 steps, from omni to figure-8. Each of the three patterns can also be turned off, which is the equivalent of introducing an 18dB/octave cut across the particular frequencies established by the crossover frequencies.
The fourth and fifth knobs are used to move the two crossover frequencies, thereby creating 1, 2 or 3-band operation. The crossover frequencies are adjustable in 1/3-octave steps from 100 to 1k Hz and 1 to 10 kHz.
In the GAIN/dB MODE position, EQ is achieved by cutting each of the three parts of the frequency spectrum a maximum of 5.5 dB in 0.5dB steps. Although EQ is entirely cut-based, one band can be raised by lowering the others and boosting the Overall Gain control. Both analog and digital input signals can be raised in 12 3dB steps to a maximum of 33 dB with this control. The fifth knob is used to configure the DSP-4P for Mode A (omni/figure-8 microphones) or Mode B (back-to-back cardioids).
A three LED display indicates -24dB, -12dB and -3dB levels, and suggests that the -3dB light be barely lit to prevent distortion. I tested it. They are right; even moderate -3dB illumination results in some nasty-sounding audio. Once you have achieved a particular setup, you can save it by using the Down/Up/Store buttons. The 10 presets only include directional characteristics and crossover frequencies. Level settings for each band and overall gain are not saved.
The DSP-4's four 24-bit AD/DA converters operate at 44.1 and 48 kHz, and are externally clockable for sample rates from 25 to 50 kHz. The idea here is that both virtual mic pairs can be recorded to four analog or digital destinations, or both, to be retrieved later for virtual processing. Of course, the DSP-4P can also be used simply as four mic/line inputs with 24-bit A/D converters.
During the recording process, you point the pairs of microphones so that the front end of each omni/figure-8 or back-to-back cardioid is aimed at the intended sound source. You might put the sound source somewhere between the two arrays, or you might position the arrays at a center point and record the audio as a surround environment. While a discrete 4-channel recording is occurring, you can use the DSP-4P's controls through its processed outputs to experiment.
The quality of the space itself becomes very important when using this technique. If your studio or intended recording site has any nasty little secrets, then the DSP-4P will make them apparent. My own experiments with the system were made with several acoustic guitar sessions and one with didgeridoo. I put the players between the pairs of mics and recorded the instruments in a typical stereo space. Changing the parameters changed the size of the space and how the instruments sat in it. In one case, a refrigerator on another floor created low frequencies (almost inaudible to the ear) that were very apparent through the Schoeps microphones. Adjustments to both the patterns and EQ reduced the LF to a tolerable level.
I kept wishing that a classical string or jazz quartet would call me for a session at a nice concert hall while I had the system available. Unfortunately, that didn't happen. Even though you can get four discrete channels of audio, I think it bears mentioning that any recording with multiple musicians would best be done with players who understand the importance of mixing by dynamic “give and take.” If everyone turns it up to 11 and wails, that's exactly what you'll get.
The documentation is fairly easy to understand, and I found the DSP-4P straightforward to operate. I did have a few moments of confusion while trying to remember whether I needed to be in PATTERN CR. FREQ. or GAIN/dB MODE positions, but they faded over time as I became more familiar with the device.
I see the appeal here for engineers and producers who may be leaning toward more minimal miking techniques as a way of eliminating multi-microphone phasing issues, and who have run into restrictions from frequency response or mic patterns. Similar to Mid/Side recording, this technique allows you to capture the performance for future manipulations. Sure, you'd take up four tracks where you might have only used two, but if you had the tracks, you'd probably have put up a couple of extra “just 'cause” mics, anyway.
I have not talked to anyone in the U.S. yet who has begun thinking about microphones and soundfields in this manner. Mid/Side recording is used in the U.S. to record location ambience for feature films, because at the time of recording, no one knows whether the audio will be married to a close-up, mid- or wide shot. Mid/Side is also valuable because it allows those decisions to be made during the post-production process.
In France more so than in the U.S., Foley is recorded as Mid/Side. Immediately, the Schoeps PolarFlex system offers more variations than the standard two-microphone Mid/Side method. I would not be surprised to find the DSP-4P in the sound department of the French film industry or in classical concert recordings before it receives acceptance in the U.S.
Where pattern and EQ have become both independent and important issues, the Schoeps DSP-4P is positioned to be a solution rather than a “just because we can” concept box. The DSP-4P demands that you use your ears; because of this, not everyone will appreciate its need or use, particularly those who employ a set-and-forget approach to recording. In a worst-case scenario, you still get four high-quality preamps and 24-bit AD/DA converters at about $950 a channel. By themselves, even without the flexibility offered by the PolarFlex controls, that's still a good deal.