EXTENSION CHORDS

THE COMPRESSION FACTOR

EXTENSION CHORDS

Allen Hester

In the recording studio, it is not all done with just instruments and voices. Much of what we hear coming through our home stereo speakers and car radios has been processed through any number of echo units, compressors, limiters, aural exciters, parametric equalizers, and so forth—signal processors. These are the electric outboard accessories which can transform a bland, weak-sounding vocalist or group into something better; maybe not much better, but better.

Signal processors are not intended as magic potions that can make hit sounds out of just anything; some would argue that if the feeling is not there in the performance of the musicians, then no amount of 'fixing in the mix' can save a lifeless recording. But signal processors do have their place in the studio, a very important place as part of the aural picture that musicians, engineers and producers try to weave around a melody. In general, most good producers would agree that the thing to do is not rely too heavily on any effect, but to use them sparingly, much as a good chef would use fragrant spices in preparing a meal. (Phil Spector being the exception.)

Compression is one such signal alteration, a process long in use in the recording studio, which has recently found its way into small effects for use with guitars, basses and so forth. One of the first units was the Dan Armstrong 'Orange Squeezer,' a tiny box that interfaced between the input jack on the guitar and the guitar cord. This particular product is no longer being made, but many studio players (Jeff Baxter comes immediately to mind) still use the trusty Orange Squeezer.

More recently, compressors have been incorporated into amplifiers. Norlin's Lab Series amps have on-board compression, which can be switched in or out, and which has a variable threshold and an LED indicator light on the front panel so that the guitarist knows .when his signal is being compressed. Also, several manufacturers of outboard effects pedals have a compressor on the market, such as MXR, Boss and Ibanez.

However, when it comes to studio and home recording units, most of which are rack mountable and intended strictly for use in a fixed installation such as a studio, the brand name that immediately comes to mind is dbx (the small letter? are theirs, not mine!), dbx makes several units, varying in price and function from a sirpple unit such as the Model 161 all the way up to the 'Cadillac' in the line, the Model 165, which features a totally new circuit called 'Over Easy' compression.

'All other compressor/limiters utilize a threshold, which is the point of gain change. Below this threshold, the signal passes through unchanged. When the signal exceeds the threshold level, it is compressed at the pre-selected compression ratio. Below the threshold, the signal is unchanged. Above the threshold, it is immediately and drastically changed.

However, the dbx 'Over Easy' concept allows the signal to pass through the threshold point and gradually adds the desired amount of compression over the range of several decibles. The result is compression that is practically inaudible, which is the goal of any compressor/limiter.

Most conventional compressors use some form of peak detection, with transient response characteristics which have a disturbing effect on program material. The true RMS level detection of the dbx 165 closely simulates the impulse response of the human ear, so that even at high compression ratios, the processed signal is more natural sounding.

Traditional gain reduction is accomplished by sensing signal level at the output and applying a correct signal, via a feedback loop, to the output. At progressively higher compression ratios, the feedback loop gain increases, distortion is increased, and eventually instability or oscillation occurs. To avoid this problem, many conventional units limit the maximum amount of feedback, thus restricting the maximum compression ratio to some lower value, such as 10:1 or 20:1.

However, the dbx compressor can achieve even infinite compression with complete stability. At infinite compression, a signal will not exceed the present threshold, regardless of input level

changes. In conventional compressors, attack and release times are fixed. Even if they are adjustable, they cannot vary with changes in the program. The dbx design makes it possible for the attack and release times to follow the signal envelope, meaning that the attack and release rates of the dbx units follow the rate of level change in the program material. No manual attack/release adjustments are required, so the operation is simplified, and the naturalness of any given sound is preserved. However, on the Model 165, attack and release rates can be manually adjusted if the engineer wants that capability. The front panel has a master/slave switch so that the unit switched to 'master' will control all the functions of the 'slave.'

The Model 165 was designed with a balanced high impedance differential input stage and an \unbalanced output, terminating in a Jones barrier strip. It is a single-channel compressor/limiter that is strappable for ihe true stereo operation.

The 165 is the only dbx compressor that offers the option of manual or automatic attack and release rates. A special input is provided to the level detector to allow for the creation of special effects.

Down on the other end of the price spectrum, the Model 160 is a professional studio product with a balanced, high impedance differential input stage and an unbalanced output, a Jones barrier strip, automatic ground loop compensation, plus built-in thump and pop protection * to prevent turn-on and turn-off transients from appearing at the output.

The 161 is an even lower priced unit for the home recording studio. It has an unbalanced input and output, terminated in phono jacks; it lacks the turn off/on protection, but is identical to the model 160 in all other operational aspects.

Whatever your budget, if you are into recording you will need a compressor, and dbx has one to suit your needs. Write to them at 71 Chapel St., Newton MA 02195.