Raising a Signal Out of a Mix

dbx 166XL COMPRESSOR / GATE	PROFESSIONAL PRODUCTS
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uration. Split the drum signal, sending one channel directly to the 166XLÕs INPUT and the other channel to an equalizer (e.g., dbxÕs 20 Series or 30 Series Graphic Equalizer). Then connect the equalizerÕs output to the 166XLÕs SIDECHAIN INSERT. The equalizer can be adjusted for boost with a peak of about 5kHz, causing the cymbal to be compressed on a very loud crash, stopping tape saturation at high frequencies, where there is less headroom. However, gentle tapping of a drumstick or brushing of the cymbal will not be affected. Assuming the tom-tom is a lower frequency instrument and can be better tolerated by the tape, it has less need for compression. Equalization in the Sidechain circuit means that the com- pressor is not triggered as readily by a loud tom-tom beat as by an equally loud cymbal crash. Refer to the next page for more Sidechain applications.

For drum kit submixes (e.g., mixing multiple drum tracks to two tracks while using both channels of a 166XL for com- pression), consider backing off the RATIO on each channel (down to 2:1) to avoid an excess of cymbal Òsplattering.Ó In larger multitracking systems, compress the kick and snare separately. A further possibility is to heavily compress a stereo submix of toms and leave the remaining percussives unaffected.

Since reducing dynamic range increases the average signal level by a small amount, a single track can be raised out of a mix by boosting its level slightly and applying compression. Start with a 2:1 RATIO and a relatively low THRESHOLD setting (-20dBu). Adjust both controls as necessary.

Compressors have also been used to bring vocals to the forefront of a mix in volume-restricted studios (e.g. home stu- dios). Start by adding a foam windscreen to the mic (if it doesnÕt have one). Set the RATIO to 10:1 and the THRESHOLD to -10dBu. With your mouth approximately 2 inches from the mic, sing the vocal part, but with less volume than normal. Use phrasing to give the part some intensity. An equalizer (e.g., a dbx 20 Series Equalizer or dbx 30 Series Graphic Equalizer) or a vocal effects device (e.g., reverb, delay, distortion) can be added to further define the performance.

It is also possible to separate certain vocals or instruments from a mono program already mixed: refer to frequency- weighted compression on page 9.

Note: When compressing a stereo program with a 166XL, the factors affecting a compression curve and the actual RATIO and THRESHOLD settings, are like those previously covered with reference to single channels of program material. However, it will generally be found that large amounts of compression are more audible in a mixed stereo program than they might be on the separate tracks that were mixed to create the program.

Preventing Analog Tape Saturation

With programs of widely varying levels, compression can prevent recording levels (e.g., cymbal tracks in a final mix or drum kit submix) from saturating tape tracks (see frequency-weighted compression, on page 9).

Preventing Digital Overload

Digital recorders and samplers produce audible distortion when they exceed their headroom (i.e., the range above their maximum operating level). The 166XL effectively ensures that audio input does not overload a digital recorderÕs A/D (analog-to-digital) converters. The 166XL can perform this function quietly enough for all digital media. To use the 166XL so that no changes in gain occur unless an emergency arises (wildly excessive levels), set Hard Knee mode On, the RATIO to °:1, and the THRESHOLD to the highest permissible level.

Note: PeakStop limiting can also be used to prevent raucous-sounding digital overload.

Speaker Protection (Auditoriums, Churches, Mobile DJs and Sound Systems)

Compressors are frequently used to prevent excessive program levels from distorting power amps and/or damaging dri- vers in a sound-reinforcement system (whether youÕre doing auditorium, church, or club sound engineering, or are a mobile DJ, or like to push the limits of your homeÕs audio entertainment center). Set the 166XL for limiting (Hard Knee mode On, with a RATIO of 10:1 or greater) and adjust the THRESHOLD to provide 15dB or more of compression (just a few dB below the input clip). For low-level signals, the 166XL wonÕt change gain, but if large signals come along, the gain will be reduced to prevent clipping and save sensitive system components from excessive heat buildup or other type of damage.

In circumstances where the 166XL is expected to cause no change in gain unless an emergency arises (wildly excessive levels), some operators set Hard Knee mode On, the RATIO to °:1, and the THRESHOLD to the highest permissible level. As with preventing digital overload, the 166XLÕs PeakStop limiter can be used instead of or in combination with the 166XLÕs compression.

As a general rule, compressors should be as close to the amplifiers as possible in the signal chain. If the 166XL is placed before the EQ (equalizer), for example, a potentially damaging boost in the EQ wonÕt be seen by the 166XL and the speakers may be damaged. (see Multi-way speaker systems, page 10). For maximum sound pressure levels, large sound reinforcement systems frequently use a separate compressor on each output of the electronic crossover(s). For a stereo sound-reinforcement system, one 166XL can be used for each stereo band (low, low-mid, mid, etc.).

Raising Average Level in PA Systems

Limiting (i.e., compression at high ratios like °:1) also benefits intelligibility by allowing low-level input signals to be reproduced through the system at higher volume. In a musical performance, this provides additional intimacy so that a