A Fourth-Order Speech Filter (Based on Texas Instruments Application Note)
Note) Fig. 1: Generating stable Vcc/2 from Vcc The frequency range of 300 to 3400 Hz is often where human speech can be found in an audio spectrum. It is essential, particularly in teleph…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
A Fourth-Order Speech Filter (Based on Texas Instruments Application Note) is a electronics project. Note) Fig. 1: Generating stable Vcc/2 from Vcc The frequency range of 300 to 3400 Hz is often where human speech can be found in an audio spectrum. It is essential, particularly in teleph…
- Source pages
- 235-238
- Named parts
- 4
- Build goal
- Working, tested prototype
Tools you need
- Digital multimeter
- Wire stripper and side cutters
- Soldering iron with a fine tip
- Current-limited bench supply
Use eye protection, good lighting, and a clean insulated surface throughout the build.
Safety precautions
- Disconnect every power source before changing a connection.
- Check component polarity, pinout, and supply voltage twice.
- Use a current limit for the first power-up.
- Keep liquids, loose metal, and uninsulated wires away from the bench.
Gather, identify, and understand every part.
Use the standardized inventory, then open What's this? to learn each part's role, advantages, limitations, handling, and specifications.
What's this?Image, role, pros, cons, handling & specifications
Operational amplifier
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for Operational amplifier; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
LM324
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for LM324; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
SEMICONDUCTOR LEARNING VIEWLM358
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for LM358; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
NEARLY
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for NEARLY; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
Connect one verified path at a time.
Explore the named components and standardized signal flow, then use Source Check to verify exact physical pins, values, and topology before applying power.
Trace this circuit.
A Fourth-Order Speech Filter (Based on Texas Instruments Application Note): interactive parts, standardized terminals, responsive anchored wires, student explanations, and the original circuit reference in one shared system.
Trace before wiring
Follow power, ground, inputs, processing, and outputs in that order. Never guess a pin from package shape alone.
Connect with power off
Make short, labelled connections and share a common ground only where the schematic requires it.
Inspect every joint
Check continuity, polarity, adjacent shorts, and loose connections before the first power-up.
Open all source diagrams and build views 3 visuals
Confirm the hardware-only control path.
This project does not include firmware in the source. The circuit itself provides the required behaviour.
How to connect
- Match every controller label to the circuit view and source pin map.
- Join grounds before signal wires when separate low-voltage supplies are used.
- Keep motors, relays, pumps, and other loads on a suitable driver and external supply.
Common mistakes
Reversed VCC/GND, board-label versus GPIO-number confusion, missing common ground, and charge-only USB cables.
Troubleshoot
Disconnect loads, continuity-test one path at a time, then test with a current limit.
Software preparation
No IDE, board package, library, or firmware upload is required for this project.
If you add a programmable controller as an extension, document its pin map separately.
How to upload code
The original design is implemented entirely in hardware, so proceed after verifying the circuit and supply.
Assemble, deploy, test, and troubleshoot.
Use the complete source notes in build order, then pass the final checks before calling the project finished.
Build in functional stages
- Power and regulation
- Controller or processing stage
- Inputs and sensors
- Outputs and loads
- Enclosure and strain relief
Power up safely
- Inspect unpowered continuity first
- Apply the lowest safe current limit
- Measure supply rails before signals
- Add one load at a time
- Record expected and actual results
Work from simple to complex
- Confirm power, ground, polarity, and orientation
- Compare each pin with the source
- Test inputs separately from outputs
- Replace only one variable at a time
- Power off before every correction
Follow the documented instructions.
These notes come from this project's source and remain in their original order.
Project overview
Project build noteNote) Fig. 1: Generating stable Vcc/2 from Vcc
The frequency range of 300 to 3400 Hz is often where human speech can be found in an audio spectrum. It is essential, particularly in telephone lines, to restrict the frequency response to fall within this range as it is required. Another project that utilizes a filter like this is "Digital Speech Security System," which was featured in EFY Electronics Projects Vol. 19. On the other hand, it relies on dedicated filter ICs, which, in addition to being expensive, are difficult to come by. Not just for the project that was specified, but also for a variety of other speech circuits, this circuit will prove to be highly beneficial. A fourth-order filter that is able to accept an audio input signal with an amplitude of up to 2.5V can be built using two integrated circuits (ICs). One of these ICs would be a quad op-amp such as LM324, and the other could be a single op-amp CA3130/CA3140 or dual op-amp LM358 operating off a single supply of 5V as Vcc. This filter would be able to Using a unity-gain voltage-follower circuit like the one illustrated in Figure 1, which is biased at half the source voltage developed across R2, one may generate a stable voltage of Vcc/2, which is equal to 2.5V. Bypassing ripples in the supply voltage is the responsibility of the capacitor C1, while the output of the operational amplifier is the responsibility of the capacitor C2. The voltage Vcc/2 is connected to the places that are corresponding to it in Figure 2.
Fig. 2: Fourth-order bandpass speech filter for 300 Hz to 3400 Hz The filter circuits that are found at the input of op-amps make use of conventional capacitor values and resistor values with a 5% tolerance. Even while the differences in the components do have a very little impact on how the circuit works, those differences will be nearly impossible to detect. The response curve of the filter is shown in Figure 3, and it can be seen that the rejection of 50/60Hz by the fourth-order filter is larger than 40 dB, but the rejection by the second-order filter is approximately 15 dB. The filter has been developed to have a roll-off of 0.5 dB at 300 Hz and 3 kHz respectively.
Fig. 3: Frequency response of 2nd and 4th order bandpass speech filter The response of the filter has been tested and found to be good.
You built A Fourth-Order Speech Filter (Based on Texas Instruments Application Note).
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
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