Creator Ci40 Microphone and Stereo Inputs

The Creator Ci40 platform is designed to allow developers to easily create connected projects based on MIPS hardware. For more projects, documentation and open-source code for the Creator Ci40 check out www.creatordev.io

In this project we investigate several modes in which the Creator Ci40 board can be used in audio related projects. The project looks at:

1) Using headsets & microphone

2) Feeding stereo audio from other sources

3) Stereo microphone or audio source input

4) Creator Ci40 audio input configuration options

• Headset microphone connection

• Microphone Plug-In-Power

• Audio input gain control

5) Appendix

• Audio input electrical parameter summary

Step 1: 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.

Step 2: 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 in the figure. Use of screened cable is recommended to minimise noise.

Step 3: 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 R9.

• R94............................Function

• 0R..............................12 Mic input connected to audio LEFT input (Default)

• Not Fitted................CN12 Mic input disconnected

Step 4: ​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

• 2.2k..................____..................RIGHT Audio Input – PiP Enabled (Default)

• Not Fitted.........____.................RIGHT Audio Input – PiP Disabled

• _______...........2.2k..................LEFT Audio Input – PiP Enabled (Default)

• _______...........No Fit...............LEFT Audio Input – PiP Disabled

Step 5: 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

• OFF..................................................Pre-Amplifier gain 0dB (Default)

• 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 is shown in the figure

Step 6: Audio Input Electrical Parameter Summary

• Parameter..........................Value........................Condition

• Audio ADC...........................................................AKM AK5720VT 24bit

• Max input signal level........2.0 Vpp...............Pre-Amplifier OFF (0dB)

• (0dBFS digital level)..........0.35 Vpp..............Pre-Amplifier ON (+15dB)

• Input Resistance..................>2kΩ....................PiP Enabled

• .................................................>15kΩ...................PiP Disabled

• PiP voltage............................3.3V......................PiP Enabled

• .................................................None....................PiP Disabled

• Input coupling....................................................Both ch. AC coupled to ADC

Special thanks to hardware engineer Pete Money from the Creator team for his efforts in putting this article together.