Melody Modulation Circuit

Presented here is a circuit that alters a tune/melody to a different pitch, level without making the tune lose its basic features. The signal characteristics can be varied at different instants of time. The melody generated by melody generator IC can be delayed and its pitch can be varied at different time instants. This can produce different musical effects from the same melody.

The word ‘modulation’ is used to mean alteration of the melody by creating delay and other effects.

COMPONENTS

Component	Description (IC name)	Quantity
Phase Locked Loop (for VCO)	CD 4046	2
Multiplexer	CD 4051	1
Timer	555	1
Transistor (npn)	SL 100 BC547	1 1
Melody generator	UM 66(or any melody IC)	1
Resistors	2.2K ohm 10K ohm 40K ohm 200K ohm	2 2 1 2
Variable Resistor	10K ohm(or any)	2
Capacitors	470 pF 0.01u F 4.7uF	1 1 2
Diode	IN4148	1
Speaker	4 ohm	1

CIRCUIT DIAGRAM

WORKING

The circuit modifies the melody generated by the melody generator IC UM66. Voltage Controlled oscillators are used to generate oscillations when a constant voltage is applied. Phase locked loop ICs (CD 4046) (IC1 and IC2 in the circuit) are used for VCOs. PLL chips have VCO and Phase comparator. The voltage that is applied to VCOs can be varied by potentiometers. In the circuit two potentiometers 5K ohm and 10K ohm were used to vary the input voltage to VCO. These voltages are given to pins 9 of both IC1 and IC2. The VCOs outputs at pins 4 of CD4046 are time-multiplexed by a multiplexer IC CD 4051 (IC4 in circuit diagram). The selection signal for multiplexer is generated by 555 timer (IC 3) that is configured as an astable multivibrator. The duration of each high and low pulse from 555 timer is for 4.7msec. The duty cycle is made sure to be 50% by introducing a small-signal diode IN4148 between pins 7 and 6 of IC3. The multiplexer (IC4) gives the multiplexed oscillatory output at pin 3. This output is given to base terminal of the npn transistor through a 40K ohm resistor. Different values of resistors were used and tested, but a proper tone was obtained for 40K ohm. The collector terminal is given as input to melody generator UM66 (other melody generator ICs can be used). The output of melody generator is given to a 4 ohm speaker (SP) with the other terminal of speaker connected to 5 volts supply.

As the positions of both the potentiometers are varied, the same melody with many different variations in its tone, level, pitch, delay effects can be heard. Similarly other melodies can be varied by just placing that particular melody IC at the position of UM66 in the above circuit.

In case the melody generator IC gets over heated then use an npn transistor (BC547) to drive the speaker. Connect the output of melody IC to base of transistor through a resistor, ground the emitter, and connect speaker across collector and +5 Volts.

Configuration and Components Description

CONFIGURATION AND COMPONENTS DESCRIPTION

1. Phase Locked Loop (for Voltage Controlled Oscillator)

The Phase Locked Loop IC’s are connected with 200K ohms (R2 and R4) at pins 12. These resistors are ised for providing offsets. A high values of R2 or R4 means that there is only a small offset. To avoid complete silence when input is 0V, resistor R2 offsets the frequency range.

The output frequency from VCO (pin 4) when voltage at pin 9 is half the supply voltage is given by

f=1/ (R1*C1) = 1/ (2.2K*4.7u) = 96.71 Hz.

As the voltage at VCO’s input is changed by potentiometer, the frequency also changes.

2. 555 Timer

This is configured as an astable multivibrator.

The on-time is = R5*C3 = 10K ohm * 470 pF = 4.7 m sec.

Off-time is also the same 4.7 m sec (R6*C3).

The diode is used to block the path during the off-time while capacitor is discharging. The main idea is to have the same on and off times (50% duty cycle).C4 0.01uF is only to remove any noise if present in the circuit. It is not necessary to be connected.

The output of 555 timer is used as selection line for multiplexer, that multiplexes both the VCO outputs.

3. Multiplexer

CD 4051 is configured as 2:1 multiplexer, with both the VCOs’ outputs as inputs at pins 13 and 14 of MUX and a selection line that selects each of these outputs from PLL with the same duration. More effects can be added to the audio (melody), by including few more VCOs and multiplexing them and giving them as output to the speaker.

Many versions of UM66T are available which generate tone of different songs. For example, UM66T01 generates tone for songs ‘Jingle bells’, ‘Santa Claus is coming to town’ and ‘We wish you a merry X’mas’. Any melody IC can be inserted in the position of UM66 in the above circuit diagram to alter the melodies.