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manual abstract
100 FERNWOOD AVE. • ROCHESTER, N.Y. 14621 (716)544-5191 • TOLL FREE (EXCEPT N.Y. STATE) (800)828-6308 TABLE OF CONTENTS Page 1. INTRODUCTION / UNPACKING 2 2. FRONT AND BACK PANEL LAYOUT 3 3. INPUT, OUTPUT, AND POWER CONNECTIONS 3 ,.4. EXPLANATION OF PARAMETRIC NOTCH FILTERS 4 5. THE SC-68'S CONTROLS FILTERS 6 SETUP SYSTEM 7 6. USING THE SC-68 TO REMOVE FEEDBACK 9 7. DEFINITION OF TERMS 11 8. TROUBLE SHOOTING TIPS 14 9. SPECIFICATIONS AND BLOCK DIAGRAM 15 20. GRAPH OF KEYBOARD / MUSIC STAFF FREQUENCY NUMBERS 16 1 control clockwise is equivalent to moving the notch up the keyboard, and counterclockwise moves it down. The Bandwidth control (Octaves) determines the number of notes, or range of frequencies, which will be included in the notch. This Is shown in the illustration as a bracket encompassing roughly 2 1/2 notes on the keyboard, since the Octaves control is set at .3, a fairly broad notch. Turning the control clockwise would make the notch progressively narrower, so that at its narrowest setting of .03 Octaves, less than one note of the scale would be effected. As noted earl ier, each of the 3 filters is identical except for the range of frequencies it is designed to cover. Any filter may be adjusted as desired without regard to the other filters, with one exception: instead of setting two sharp filters to nearly the same center frequencies, it is preferable to use just one filter at the desired frequency and use a slightly broader bandwidth setting. Some people make the mistaken assumption that if one filter set to 3 kHz can produce a 50 dB notch, then 2 filters set to 3 kHz should produce a notch of 100 dB. In fact, it doesn't work that way, but even if it did that would be missing the point. Each filter by itself is capable of producing a notch accurate enough and deep enough to satisfy any real-world requirement. Setting two filters to the same frequency simply isn't necessary. SETUP SYSTEM CONTROLS Because a parametric notch filter is particularly useful as an anti-feedback tool in sound reinforcement applications, a special setup system has been incorporated into the SC-68 to make the sound man's job even easier. Essentially, the system consists of a limiter which holds feedback at a listenable low level, switchable listening modes to compare filtered and flat audio, and a meter which indicates, in dB, the improvement in system headroom which has been achieved. As a whole, the SC-68 system is somewhat comparable to having a separate limiter-compressor patched into a parametric equalizer, except that in the SC-68 both the limiter and EQ parameters are optimized for the specific job of feedback control. The operating mode of the SC-68 is selected by means of push-button switches next to the meter. Only one mode is used at a time. In the SETUP mode, the Setup system is activated. In the OPERATE mode, the entire setup system is disabled and the SC-68 functions as a straightforward notch filter. A brief description of the operating controls is given in Figure 7. (On the next page) PEAK LED The SC-68 peak indicator will flash on whenever signal levels in excess of +14 dBV are present. With an overall system headroom of +20 dBV, this'light will warn you when you are within 6 dB of clipping. The Deak detection circuit monitors the circuit at 10 individual points, including the input, output, and all 8 filters. Since the SC-6.8 functions at unity gain or at a loss, a /lashing peak light will indicate that high-level signals are occuring somewhere in the audio chain prior to the SC-68. 7 FILTER CONTROLS The SC-68 uses 8 filter secions, with the only difference between filters being the range of frequencies spanned by the center frequency (Hz) control. Once you understand one filter section, you will understand them all. Figure6 Typical filter section, SC-68 Referring to the above illustration of a typical filter from the SC-68, the IN/OUT switch allows quick A/8 comparisons of filtered and flat audio signals for each filter section, making it easy to test the effect of any filter without disturbing the other filters. Immediately below the switch, the Amplitude (dB) control determines the depth of a notch at the center frequency. Its effect is shown on the accompanying graph, in which a notch is centered at about 440 Hz, and produces a cut of 6 dB at the center freqency, corresponding to the -6 setting of the Amplitude control. Turning the control clockwise will decrease the depth of the notch, eventually obviating the effect of the filter when it is set to “0". Turning the control counter-clockwise will make the notch deeper, producing a more pronounced cut at the center frequency. The Center Frequency (Hz) control, selects the frequency at which the notch will be deepest. Frequencies above and below the center frequency will be proportionately less affected than the center,frequency. Frequencies which are outside the range of the filter will be unaffected. In this illustration...