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Using Creator Ci40 Microphone & Stereo Audio Inputs

The Creator Ci40 platform, (125-3306) available from RS now, is designed to allow developers to easily create connected projects. For more projects, documentation and open-source code for the Creator Ci40 check out www.creatordev.io

In this article we investigate several modes in which the Creator Ci40 board can be used in audio related projects. The article covers

  • Using headsets & microphone
  • Feeding stereo audio from other sources
  • Stereo microphone or audio source input
  • Creator Ci40 audio input configuration options
    1. Headset microphone connection
    2. Microphone Plug-In-Power
    3. Audio input gain control
  • Appendix
    1. Audio input electrical parameter summary

 

Using headsets with a (Mono) microphone

By default the Creator Ci40 is supplied configured for mono microphone input on the 3.5mm headset socket CN12.

The CN12 microphone connection has Plug-In-Power (PiP) (also known as Phantom Power) for powering a standard electret condenser microphone.  

Additionally the Creator Ci40 also has an audio pre-amplifier for providing extra gain when using microphones. The pre-amplifier is enabled using switch SW3.1 (refer to the Board Configuration Options later in this article)

The Creator Ci40 headset socket has 4 contacts and is configured for use with CTIA standard headsets. If you have a headset that is OMTP standard you need to use an OMTP-CTIA adaptor which are readily available.

 

Feeding stereo audio and/or microphone signals from other sources

If you want to feed stereo audio into the Creator Ci40 from another source, such as a mobile phone, tablet, or stereo microphone you can use the on-board stereo audio ADC.

The Creator Ci40 has a stereo audio ADC. The inputs to the ADC are accessible via pads on the underside of the PCB. Stereo audio signals can be fed into these pads.

Note: Some re-configuration of the Ci40 board may be needed depending on your application.

Connections:

Pad Reference

Signal

TP40

Audio LEFT Input

TP41

Audio RIGHT Input

TP42

Audio Ground

 

A typical connection to TP40, TP41 and TP42 is shown below.

Use of screened cable is recommended to minimise noise.

 

Creator Ci40 audio input configuration options

The microphone input on the headset connector CN12 is connected to the audio ADC LEFT Input. If a standard stereo headphone jack, with 3 contacts, is inserted into CN12 then the microphone input will be shorted to audio GND so the LEFT input channel will not function.

To allow simultaneous use of stereo headphones and stereo audio input the microphone connection from CN12 should be disconnected using R94

 

R94

Function

Default

0R

CN12 Mic input connected to audio LEFT input

Not Fitted

CN12 Mic input disconnected

 

 

Microphone Plug-In-Power

Both LEFT and RIGHT audio inputs have Plug-In-Power (PiP). PiP provides a low current dc supply, in this case at 3.3 Volts, to power microphones and this voltage shares the same conductor as the audio.

Most audio sources will not be affected by the PiP voltage. However if you experience issues with your audio source then the PiP can be disabled using R62 and R93

R62

R93

Function

Default

2.2k

-

RIGHT Audio Input – PiP Enabled

Not Fitted

-

RIGHT Audio Input – PiP Disabled

 

-

2.2k

LEFT Audio Input – PiP Enabled

-

Not Fitted

LEFT Audio Input – PiP Disabled

 

 

 

 

Audio Input Gain Control

The audio ADC has a pre-amplifier which can provide an additional +15dB gain for use when connecting to microphones or other low level audio sources.

The pre-amplifier can be enabled using SW3.1

SW3.1

Signal

Default

OFF

Pre-Amplifier gain 0dB

ON

Pre-Amplifier gain +15dB

 

 SW3.1 should be set ON when using headsets or microphones. This is not the default state

 SW3.1 location:

Audio Input Electrical Parameter Summary

Parameter

Value

Condition

Audio ADC

 

AKM AK5720VT  24bit    

            Max input signal level

(0dBFS digital level)

2.0 Vpp

0.35 Vpp

Pre-Amplifier OFF (0dB)

Pre-Amplifier ON (+15dB)

Input Resistance

>2kΩ

>15kΩ

PiP Enabled

PiP Disabled

PiP voltage

3.3V

None

PiP Enabled

PiP Disabled

Input coupling

 

Both channels are AC coupled into the ADC

 

In a future article we'll discuss how to turn your Creator Ci40 into a streaming Wi-Fi speaker.

Happy Creating 

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Paul Evans joined Imagination Technologies in January 2012 and is now a Director of Business Development. Prior to joining Imagination, Paul worked for Altera Corporation for 11 years in marketing. He holds a BEng in digital electronic engineering from the University of Kent at Canterbury in England.
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