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voice processor

mantyi themba

Nov 30, 2014
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hi everyone please help me. I am doing a design project using a voice processor chip (ISD1420p) using the attached. the problem is that I am not getting a clear and loud sound at headphones, instead I get a noisy sound with a low sound that I recorded. I then used an audio amplifier to amplify the output, still no changes what can I do?
 

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Harald Kapp

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Do you have a clean power supply? What value do you use for C11? What do you use for volume control (VR2)?

It would generally be very helpful if you'd annotate the schematic with component values.

I then used an audio amplifier to amplify the output, still no changes
Amplifying a noisy signal will not improve the signal to noise ratio. You need to get a clean audio signal in the first place, that is at the output of the chip.
 

davenn

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You need to get a clean audio signal in the first place, that is at the output of the chip.

and if its noisy at the chip output, then you need to sort out what problem there may be with the microphone input side
 

KrisBlueNZ

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Nov 28, 2011
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Here's the suggested application circuit from the data sheet. The design from electronicsproject.org looks very similar. Coincidence? LOL

ISD1400 application circuit.png
The only differences are:
  • the trigger circuit: the electronicsproject.org design use a relay instead of a pushbutton to trigger playback; this could be done without the relay;
  • the speaker circuit: the electronicsproject.org "designer" didn't understand bridge-tied load amplifiers and has messed up the connection of the headphone socket and volume control;
  • the electret microphone circuit: I think the electronicsproject.org "designer" just made a mistake or a deliberate change when copying the circuit.
To fix the headphone output, make the following changes.
  1. Disconnect everything connected to the "SP+" and "SP-" pins (14 and 15).
  2. Don't use the "SP-" pin (pin 15) for anything.
  3. Add a new capacitor, 100 μF 16V electrolytic, with its positive terminal connected to "SP+" (pin 14).
  4. Connect the negative side of the new capacitor to the clockwise end of a 50Ω potentiometer. This will be the volume control.
  5. Connect the negative end of the volume control potentiometer to the 0V rail (VSSD, VSSA).
  6. Connect the tip and ring contacts of the headphone socket to the wiper of the volume control potentiometer.
  7. Connect the sleeve of the headphone socket to the 0V rail.
The electret microphone circuit should work as shown but if you want to change the electronicsproject.org design to match the data sheet's suggested design, disconnect the negative side of C12 from the 0V rail and connect it instead to the -RECLED pin (pin 25). And check that the component values match the diagram from the data sheet.

Electret microphones are polarised, and for this circuit to work, the negative side (usually also connected to the case of the microphone) needs to be the side that connects to C13 and R26.

If you still have problems after making those changes, post the details of the electret mic you used and I'll suggest some other tests you can make.
 
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