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The OMLmic is a low distortion implementation of an electret microphone. The design uses a simple modification to a common electret microphone capsule to achieve good sound quality at a very low cost. This is particularly useful for high volume applications, such as close-mic'ing acoustic instruments and drums. A discussion of electret microphones is given [[http://www.openmusiclabs.com/learning/sensors/electret-microphones/|here]], and is a good background for some of the technical details to follow.
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==== Features yet implemented ====
 * All initialization routines for the codec, microcontroller, and memory - The codec is capable of multiple sample rates, and these can be easily changed to trade-off between audio quality and processing time. The codec also has an on-board headphone driver and microphone input which can be enabled, although the off-board hardware is much better. Both input and output filters can be modified, along with gain and internal signal path.
Electret microphones have very low pick-up noise, and a good SNR for high signal levels. They also have a wide frequency range which can be relatively flat, depending upon the capsule. This makes them very useful, especially considering their small size. But, their application in this regime (high signal levels) is limited due to their early distortion characteristics. Luckily, there are some easy ways to extend this range.
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 * FTDI cable header - This will allow for connection to computers, and future bootloading capability. All of the necessary components are attached to mimic the Arduino bootloader. There are 3 main sources of distortion in an electret microphone application. The first is non-linear flexure of the electret diaphragm. This is impossible to change for a particular element without attenuating the sound waves before they hit the diaphragm. Fortunately, this distortion is usually much smaller than the second contributor, which is distortion in the JFET amplifier within the capsule. Reducing this source of distortion is the main focus of the OMLmic. The last source of distortion is the final amplification stage, which should be negligible in a good mixer or pre-amp.
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 * Clip and level LEDs - The level LED comes on at approximately -35dB, and becomes brighter with more signal. The clip LED turns on, and stays on for 1 second, every time the signal goes above approximately -3dB. 0dB is when distortion begins. ==== JFET distortion reduction ====
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 * Variable gain input stage - The front end presents an input impedance of 520k to AC signals, and has a variable gain of 0db to +34dB. The OMLmic uses 2 methods for JFET distortion reduction. The first is a common technique of converting a 2 terminal electret module into a 3 terminal module so the internal amplifier can be run as a source-follower (rather than the standard common-source configuration). According to the datasheet for the JFET used inside of an electret microphone (2SK596 - linked below), the input signal level can only go to 10mV before the output distortion reaches 1%. Not only is this a very low sound level, equivalent to someone talking at normal volume at a distance of a few feet, 1% distortion is already a very large level, and far better is achievable.
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 * Socketed op-amps - Just in case you want to fiddle around with the sound. We suggest the TLC074 for good frequency response and output drive, but unless your listening on headphones at the output, you probably won't notice too much difference with the less expensive TLV274. Cutting a trace on the bottom PCB of the electret capsule allows the JFET source to be pinned out, and a seperate ground lead attached. But, due to the low bias current of the JFET (usually around 200uA), the added benefit is quite minimal. The reason for this, is that in a source-follower configuration, the distortion is due to gate to source voltage (Vgs) variations as the drain current (Id) varies. In an ideal situation, Vgs would remain constant, and distortion would be eliminated. But, since there is no access to the gate terminal within the capsule, it is difficult to exert any control over Id (which can vary by up to 50%), and the resulting Vgs.
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 * Optional jumpers - There are jumpers for installing a hard bypass switch between the input and output. There is also a jumper for installing a foot-switch in parallel with the rotary encoder push-button.

==== Features not yet implemented ====
The current MICrODEC implementation is fun to use, but there are a number of things that would make it even better.
 * Internal USB - There is currently an FTDI cable header onboard, but it would be far more convenient if the case did not need to be removed to take advantage of it.

 * Bootloader - Once there is USB, there can be bootloader! This would allow for downloading and trading of MICrODEC programs by those who aren't interested in the details of AVR programming.

 * MIDI - An optional MIDI board would allow the MICrODEC to be controlled by external pedals, synths, and computer software. Sample MIDI code has been written and tested, so merely a break-out board is required. Currently, there is only one USART for both MIDI and USB, so a solution to allow for both is needed.

 * Arduino compatibility - Although this would seriously reduce the full power of the processor, it would also allow for a wider user base.

 * C code - The current assembler needs to be ported to C for those who are into such things.
The OMLmic's second method overcomes this problem by using an op-amp to monitor Id outside of the microphone capsule, and then provide a feedback signal to keep Id and Vgs essentially constant. The schematic is linked below, and shows a low distortion op-amp (TLC071) measuring the current at the drain, and compensating the source voltage to keep the drain current constant. This reduces JFET distortion to .005%, and can provide moderate gain in the process.
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 * Adding another knob: [[AddingAKnob|Adding A Knob]]

 * Talking [[MicrodecMidi|MIDI]]

 * Talking to the [[MicrodecCereal|serial port]]
The dynamic range of the OMLmic can be extended by changing a few resistors. Depending upon your needs, this can be done with minimal change to the SNR or THD.
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 * [[attachment:MICroDEC_board.png|board image]] [[attachment:microdecv1.brd|board eagle file]]
 * [[attachment:MICroDEC_schem.png|schematic image]] [[attachment:microdecv1.sch|schematic eagle file]]
 * [[attachment:microdecv1.lbr|library eagle file]]
 * [[attachment:microdecv1.zip|pcb gerber files]]
 * [[attachment:microdec_bom.ods|parts listing]]
 * [[attachment:microdec1.zip|current firmware]]
 * [[attachment:microde
c_frontpanel_cutout.pdf|front panel pdf]] [[attachment:microdec_frontpanel_cutout.dxf|front panel dxf]]
 * [[attachment:microdec_rearpanel_cutout.pdf|rear panel pdf]] [[attachment:microdec_rearpanel_cutout.dxf|rear panel dxf]]
 * [[attachment:microdec_text_front.png|front graphics png]]
 * [[attachment:microdec_text_back.png|rear graphics png]]
 * [[attachment:OMLmic_board.png|board image]] [[attachment:OMLmic.brd|board eagle file]]
 * [[attachment:OMLmic_schem.png|schematic image]] [[attachment:OMLmic.sch|schematic eagle file]]
 * [[attachment:OMLmic.lbr|library eagle file]]
 * [[attachment:OMLmic.zip|pcb gerber files]]
 * [[attachment:OMLmic_bom.ods|parts listing]]
 * [[attachment:OMLmic_frontpanel.pdf|front panel pdf]] [[attachment:OMLmic_frontpanel.dxf|front panel dxf]]

OMLmic

About OMLmic

The OMLmic is a low distortion implementation of an electret microphone. The design uses a simple modification to a common electret microphone capsule to achieve good sound quality at a very low cost. This is particularly useful for high volume applications, such as close-mic'ing acoustic instruments and drums. A discussion of electret microphones is given here, and is a good background for some of the technical details to follow.

Electret microphones have very low pick-up noise, and a good SNR for high signal levels. They also have a wide frequency range which can be relatively flat, depending upon the capsule. This makes them very useful, especially considering their small size. But, their application in this regime (high signal levels) is limited due to their early distortion characteristics. Luckily, there are some easy ways to extend this range.

There are 3 main sources of distortion in an electret microphone application. The first is non-linear flexure of the electret diaphragm. This is impossible to change for a particular element without attenuating the sound waves before they hit the diaphragm. Fortunately, this distortion is usually much smaller than the second contributor, which is distortion in the JFET amplifier within the capsule. Reducing this source of distortion is the main focus of the OMLmic. The last source of distortion is the final amplification stage, which should be negligible in a good mixer or pre-amp.

JFET distortion reduction

The OMLmic uses 2 methods for JFET distortion reduction. The first is a common technique of converting a 2 terminal electret module into a 3 terminal module so the internal amplifier can be run as a source-follower (rather than the standard common-source configuration). According to the datasheet for the JFET used inside of an electret microphone (2SK596 - linked below), the input signal level can only go to 10mV before the output distortion reaches 1%. Not only is this a very low sound level, equivalent to someone talking at normal volume at a distance of a few feet, 1% distortion is already a very large level, and far better is achievable.

Cutting a trace on the bottom PCB of the electret capsule allows the JFET source to be pinned out, and a seperate ground lead attached. But, due to the low bias current of the JFET (usually around 200uA), the added benefit is quite minimal. The reason for this, is that in a source-follower configuration, the distortion is due to gate to source voltage (Vgs) variations as the drain current (Id) varies. In an ideal situation, Vgs would remain constant, and distortion would be eliminated. But, since there is no access to the gate terminal within the capsule, it is difficult to exert any control over Id (which can vary by up to 50%), and the resulting Vgs.

The OMLmic's second method overcomes this problem by using an op-amp to monitor Id outside of the microphone capsule, and then provide a feedback signal to keep Id and Vgs essentially constant. The schematic is linked below, and shows a low distortion op-amp (TLC071) measuring the current at the drain, and compensating the source voltage to keep the drain current constant. This reduces JFET distortion to .005%, and can provide moderate gain in the process.

Modifications, Additions, Enhancements

The dynamic range of the OMLmic can be extended by changing a few resistors. Depending upon your needs, this can be done with minimal change to the SNR or THD.

Documentation

Files: Version 1.0

Files: Datasheets

OMLmic (last edited 2011-04-04 06:48:17 by guest)