Adjustable Parametric Equalizer Hardware Description

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1 Adjustable Parametric Equalizer Hardware Description Adam Grunke April 27, 2004 ETEC 474 Professor Morton

2 Introduction The Adjustable Parametric Equalizer (APE) allows the professional audio engineer to select and adjust a frequency using a single rotary encoder knob. With a 31 band graphic equalizer (EQ), which is one of the most common method of equalization in sound reinforcement, the entire audio signal will be passes through 31 filters and is then summed together. Most often, the engineer only needs to use three or four of the 31 filters available. With the APE, the audio signal only passes through four filters which reduces power consumption. The filters four are switched capacitor filters (SCF) and can be used as a high notch filter (NF) or a narrow band pass filter (NBP). The present settings will always be saved so they won t be lost when the unit is powered down. The APE can be used for the following live and studio sound applications: equalization of the main stereo mix out of a sound console, two separate monitor mixes, equalization of two different instruments, as a crossover or the output of channel one can be fed into the input of channel two which allows for eight EQs on one instrument. The APE has both a balanced XLR and a balanced ¼ Phono jack for inputs per channel. This allows the APE to be used with a variety of high end systems quickly and without any adaptors. The low frequency roll off starts at 10 Hz and the upper frequency rolls off at 25 khz. The APE operates with one rotary encoder and three push buttons that allow the user to adjust the frequency and gain of the filters. The APE fits in a standard single rack unit (1RU) which is 1.75 x 19 x 8.5 (4.4cm x 48.3cm x 21.6cm BDM

3 The BDM is there for any future upgrades to the APE. User Interface PUSHBUTTONS There are three pushbuttons for the user. The SW1 is the mode select pushbutton, SW2 is the enter pushbutton, and SW4 is the backspace pushbutton. Each pushbutton is normally open and connected to PB0, PB1 and PB4. Each pushbutton is single-pole, single-throw (SPST) and is tied to 5 volts. When the button is selected, the pin on the 9S12 goes to ground. ROTARY ENCODER The user rotates the knob to select the center frequency and gain. The rotary encoder has two channels. Channel A goes into PB2 and channel B goes into PB3. When the encoder is rotated clockwise, the active low signal on channel A leads the active low signal of channel B by 90 degrees. The reverse is true for counter clockwise rotation. LCD The LCD displays the different center frequencies and their gains. The LCD also shows which channel is being used and what settings the user has stored or loaded. Audio Input and Audio Output INPUT Each channel of the APE has both balanced XLR and ¼ inch Phono inputs and outputs. Pin 2 of the input is connected to the inverting input of an LF351 op-amp. Pin 3 of the input is connected to the non-inverting input of the LF351. The signal on pin 2 is the inverse of pin 3. The output of the op-amp is the sum of the two signals after the originally inverted signal was flipped to match up with the signal on pin 3. Balanced inputs removes noise collected through cables since the noise of one of the signals gets inverted and added to the other noise signal.

4 OUTPUT The output of each channel is also balanced. So the signal mush be sent out twice: one inverted and one non-inverted. The resistor and capacitor values chosen around the outputs cut out frequencies below 10 Hz and above 25kHz. Filtering Maxim IC s MF10 switched capacitor filters are controlled by resistor values. The center frequency, Q, and gain can all be adjusted with resistor ratios. Each MF10 package has two filters and each filter uses six resistors to adjust its parameters. Since this is an adjustable parametric EQ, digital potentiometers are used instead of fixed resistors. Analog Device s AD5204 digital potentiometer has four digital potentiometers per package and each digital pot is addressable. The AD5204 receives an 11 bit data word. The first three data bits select which digital pot to adjust and the remaining 8 bits select the resistance. Each digital pot on the AD5204 has 256 steps which provides a wide enough range of resistances to produce the proper center frequency ratios. Each of the 12 AD5204 packages are connected to the SPI of the 9S12. Each package is individually selected by a dedicated IO. All of Port H and PP4 through PP7 are used to select which digital pot the 9S12 is writing to. The APE does both band pass and notch filtering. The notch filter requires an external op-amp. The output of that op-amp is sent to a click-less audio switch. There are four switches per package on the SSM2404. The output of the band pass filter and the output of the notch filter for ½ of the MF10 is sent to separate switches. The switches are always opposite. So if the band pass output is what the user wants, the band pass switch is closed and the notch switch is open. The output of both switches goes to the next filter in the channel or the output if it s the last filter.

5 Parts List Item Quantity Part Description Designator 1 1 Microcontroller - MC9S12DP256B U1 2 4 Programmable Filter - MF10 U Digital Pot - AD5204 U Low Voltage Reset - MAX6314 U Switch IC - SSM2404 U Op-amp - LF351 U Jack - XLR Male J Jack - XLR Female J Jack - 1/4 inch phono J Crystal - 16MHz X BDM Connector P Capacitor - 330pF C1,2,4,7, Capacitor - 1uF C3,5,6, Capacitor - 0.1uF C9,10,11,12,13, Capacitor - 22pF C14, Capacitor uF C Resistor - 100k,1% R Resistor - 10k, 1% R9-20,33,37, Resistor k, 1% R21-22,27, Resistor k, 1% R23-24,29, Resistor k, 1% R25-26,31, Resistor - 20k, 1% R34, Resistor k, 1% R35,36,39, Resistor - 18k, 1% R Resistor ohm, 1% R Resistor - 3k, 1% R Resistor - 22k, 1% R Resistor - 47k, 1% R LCD Module LCD Pushbutton Switch SW1,2, Rotary encoder, ECW1D-B24- BC0024 S3

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