Digital Record & Replay Circuit Analysis Pt.1

Use Oscium's iMSO-204 digital oscilloscope to reverse engineer circuits like the digital voice recorder pictured above. Use the information you learn to improve the design or make something specific to your needs.

Learn How A Digital Voice Recorder Works

Pictured above is Radio Shack's digital record and replay module. Little circuits like this one allow tinkerers and hobbyists an opportunity to reverse engineer something that interests them and use that knowledge to build something new. The module, part number 276-1323, uses a single IC to record and playback up to 20 seconds of voice or other audio and costs about $12. With Oscium's iMSO digital oscilloscope accessories for iOS devices, your measurements at nodes of interest will teach you about how the circuit works.

Pin Assignment And Descriptions

Manufacturer data sheets contain a wealth of information that is incredibly useful for hackers, hobbyists, tinkerers, students and engineers. A simple pin assignment diagram, like the one below, reveals the conceptual fundament of the device described. Here is the manufacturer's data sheet for an IC that performs the record and replay function used in the Radio Shack module. It's unfortunate that the hardened epoxy blob on Radio Shack's module masks the component used, but the IC described in this data sheet is current and in production. PDF icon DATASHEET_apr9301v2-v2.3.pdf

Inside A Record/Replay IC

Another gem manufacturers include in their product data sheets are block diagrams of an IC's inner workings. These diagrams can be especially useful when troubleshooting a circuit that is not working or not working as expected. The figure below depicts the internal function blocks of a digital record/replay IC. Look at the Pre Amp in the IC. You can see that the amplifier receives an input waveform from the external microphone and an input reference level. The microphone output is gained up by the pre amp and then passed out of the IC for the next stage. There is an AGC input to the pre amp that is set by external components and has feedback from the next amplifier (driver amp) stage.

Application Circuit

The figure below shows the Record/Replay IC as it is used in the circuit for creating and replaying audio. Diagrams like this figure are useful in learning about a circuit's operation because the inputs and outputs are clearly depicted. The interaction and interdependency between connected devices can likewise be discerned by studying this kind of diagram. A curious tinkerer gains insight into a circuit's function from studying diagrams like the below.

Say you wanted to adjust the sensitivity of the microphone? From studying the diagram of the circuit you can see how the microphone is woven into the resistor/capacitor network connected to the IC. The chip manufacturer that created the diagram has indicated that series resistor between Vcc and the 22uF capacitor is selected and adjusted for specific microphones. The two 4.7K resistors in series with the microphone probably limit the current and set its operating voltage, but what happens if they are adjusted? Experiment and find out.

See The Module In Action

This 30 second video clip shows Oscium’s iMSO-204 measuring the voltage across the speaker leads when the module replays a bit of whistling.

What Comes Next

In Part 2 of this series we look at the voltage regulator used in this module.
  • Part 1: Record & Replay Circuit Analysis
  • Part 2: LDO Regulator Circuit Analysis
  • Part 3: Microphone Circuit Analysis
  • Part 4: Speaker Circuit Analysis