Interfacing Professional Microphones to Computer Sound Cards

Many users of computer sound cards purchase a professional microphone to improve upon the performance of the microphone included with the sound card. But because interconnection procedures in the computer world differ from those used in professional audio, it is not always easy to make a professional microphone work with a computer. To be successful in connecting a microphone to your computer, you must know some things about both your microphone and your sound card. The following three pieces of information are important to know about each product, and can be found by consulting the product literature or by contacting the manufacturer’s Technical Support department:

As is the case in most other aspects of computing, there are significant differences between the sound cards used with Apple and IBM-compatible computers. Additional information for users of Apple Macintosh computers is contained at the end of this document.

Contents:

Signal Level

Professional microphones put out a very weak signal - less than 1/1000th of a volt, or 1 millivolt. Audio inputs on sound cards, even though they may be labeled "Mic In" or be identified by a small microphone-shaped icon, often are not designed to accept such a low signal level. Most sound card inputs require a minimum signal level of at least 1/100th of a volt (10 millivolts); some older 8-bit cards need 1/10th of a volt (100 millivolts). This discrepancy means that if a typical professional microphone is connected to a sound card input, the user will have to shout into the microphone or hold it just an inch or so away (or both) in order to produce a strong enough signal for the sound card to "hear."

There are two possible solutions. One option is to increase the sensitivity of the sound card input, so that it can more easily detect the signal from the microphone. The software supplied with some sound cards allows the user to increase the sensitivity or "gain" of the input, either with a click-and-drag input level control or a set of check boxes that double, triple, or quadruple the sensitivity. (Note: Increasing the sensitivity of the input will always add some noise, so use only as much additional gain as necessary.)

If the input sensitivity cannot be increased, another option is to amplify the microphone signal before it goes into the sound card input. This can be done by running the microphone signal through a device called a mic preamplifier or mic-to-line amplifier. An example of preamps like this are the ARTcessories MicroMIX or the Rolls MP13. A microphone mixer can also be used if it has an output that will provide adequate signal level to the sound card input. (In this case, the mixer is being used only for its preamplification function and not its mixing capability.) Either way, you will have to know the typical output level of the microphone (found on the microphone’s specification sheet) and the sensitivity of the sound card input in order to know how much amplification is needed, and whether a particular mic preamp or mixer will do the job.

Impedance

Impedance is an electrical characteristic similar to resistance. It is important because the relationship between the impedance of a microphone and the impedance of the sound card to which it is connected can have a significant effect on how much of the microphone’s signal is actually transferred to the sound card. For acceptable results, the output impedance of the microphone must be less than the input impedance of the sound card. If the impedance of the microphone is the same or higher than the input impedance of the sound card, some or all of the microphone’s signal strength will be lost (an effect called ‘loading’.) The higher the microphone’s impedance is compared to the sound card’s, the more signal will be lost. Connecting a high impedance (also called ‘High Z’) microphone to a sound card with an input impedance of 600 ohms will result in so much signal loss that the talker’s voice will be inaudible. Professional microphones typically have an output impedance of less than 600 ohms and most sound cards have an input impedance of 600 to 2,000 ohms, so impedance is not usually a problem.

Connector and Wiring Scheme

The most visible problem encountered when connecting a professional microphone to a sound card is that different connectors are used. Because of their limited width, computer sound cards can only accommodate very small connectors. The 3.5 mm (1/8") "miniplug" used on most Walkman-type personal stereos is the most popular type. The standard 1/4" and XLR connectors used on professional microphones are far too big to fit into a single card slot.

Just as important as the type of connector used is the wiring scheme used. Notice in the photo above that the XLR connectors have three connection points (either pins or sockets). Professional microphones with XLR connectors use an industry-standard "balanced" wiring scheme, with two of the pins used to carry audio and the third as a ground connection. There is no standard for the wiring of the 3.5 mm miniplug connectors used on sound cards, so the actual wiring scheme varies depending on the manufacturer of the card.

The 3.5 mm miniplug is commonly available in two different configurations. Most sound cards use a three-segment version, often called a "stereo" connector since it can be used to carry two separate channels of audio in addition to providing a ground connection. When used as a microphone connector, the end portion of the connector (called the Tip usually carries the audio signal; the center portion (called the Ring) is sometimes used to carry low-voltage dc power required by the microphone supplied with the sound card; and the third section (called the Sleeve) is used as the ground connection. On the two-segment or "mono" version, the Tip of the connector carries audio and the Sleeve is used for ground. DC power cannot usually be supplied through a mono 3.5 mm miniplug.

Some sound cards have an additional stereo input labeled "line in. " This is designed to accommodate the stereo signal from a VCR, CD player, or tape deck, and is not suitable for use as a microphone input.

Dynamic Vs. Condenser Microphones

Different types of microphones use different methods of converting the acoustic energy created by a sound source (such as your voice) into electrical energy that can be amplified, processed, recorded, or transmitted. The two most popular types of microphones for professional use are the dynamic and the condenser (sometimes called an electret ). The primary difference -- as far as sound cards are concerned -- is that condenser microphones require a source of dc power to operate. Dynamic microphones do not require any external powering.

The type of power needed by the condenser microphone and the way that it is provided are important issues that may affect whether a particular professional microphone will work with a particular sound card, and how the cable connecting them together should be configured. One type of power, called bias voltage , provides power for a small transistor inside the microphone element or ‘head’. The other type is called phantom power , and is used to operate a small preamplifier which slightly amplifies the signal or provides frequency contouring. The preamplifier may be housed inside of the microphone handle or -- in the case of small lavalier or gooseneck microphones -- in an external tube or pack.

[NOTE: the preamplifier used by professional condenser microphones is not the same as the microphone-to-line amplifier mentioned earlier, which also goes by the name ‘preamplifier’.]

Some professional condenser microphones are designed to accommodate an internal battery, while others require phantom power from a microphone mixer or power supply. The microphones supplied with computer sound cards often operate on bias voltage supplied by the sound card through the Ring portion of the stereo miniplug connector. So far, sound cards cannot provide the phantom power used by many professional condenser microphones.

Connecting the Microphone to a Sound Card

To connect a professional microphone with a three pin XLR output connector to the 3.5 mm miniplug mic input of a sound card, a special cable must be purchased or made. For the microphone to work properly, the cable must have the proper type of connector for the sound card (two conductor ‘mono’ or three conductor ‘stereo’ miniplug) and be wired correctly. The correct wiring scheme depends on the type of microphone and the wiring of the sound card’s microphone input. Cable wiring for some common microphone types and sound card connectors are illustrated below.

Connecting Professional Dynamic Microphones

The wires that are connected to pins 1 and 3 of the XLR connector should both be connected to the Sleeve of the mono miniplug. The wire that is connected to pin 2 of the XLR should be connected to the Tip of the miniplug.

If the soundcard uses a stereo miniplug, the configuration is slightly different. The wires that are connected to pins 1 and 3 of the XLR connector should both be connected to the Sleeve of the stereo miniplug. The wire that is connected to pin 2 of the XLR should be connected to the Tip of the miniplug. No connection should be made to the Ring of the miniplug, because dynamic microphones do not require external dc power.

Sometimes it is impossible to tell if the connector on a sound card is of the mono or stereo variety. If a cable that is equipped with a mono connector is plugged into a sound card input that uses a stereo connector, the microphone should still work. This is because the Ring portion of the sound card jack will make contact with the Sleeve portion of the miniplug on the mic cable, which will connect any dc bias voltage to ground.

Connecting Professional Condenser Microphones

Connecting a professional condenser microphone to a sound card can be complicated, because there are so many variations between different brands of microphones in terms of bias voltage requirements. (Phantom power is a defined audio industry standard and is usually the same regardless of the brand, but no sound cards are able to provide it.) Here are the possible situations:

If the microphone can operate on an internal battery, no external source of power is needed and the mic can be connected to the sound card using the same wiring scheme as for a dynamic type.
If the microphone is a handheld or gooseneck style with an internal preamplifier that requires phantom power (because a battery cannot be accommodated), it cannot be connected directly to the sound card. These microphones must be connected to a dedicated phantom power supply or a microphone mixer that has this feature; the output of the power supply or mixer is then connected to the input of the sound card using the same method as for a dynamic mic. An example of a preamplifier like this is the ARTcessories MicroMIX or the Rolls MP13.

Other Microphone Issues

How long can the microphone cable be? Because computer sound card inputs use the unbalanced wiring scheme, microphone cables longer than 15 feet will usually pick up electromagnetic interference or cause the sound to become muffled. To preserve sound quality, use the shortest mic cable possible.
Is ‘polarity’ important? If pin 3 of the XLR connector is wired to the Tip of the miniplug instead of pin 2, the polarity of the signal will be inverted. The microphone will sound the same to the human ear, but voice recognition software will probably not recognize the sound waveform, resulting in a high error rate.

Using Sources with non-XLR Connectors

If the microphone or other audio source to be used is equipped with something other than a three-pin XLR connector, a little research must be done to find out which portion of the connector carries the audio and which is connected to ground. The audio signal should always be routed to the Tip of the miniplug connector on the sound card, and the ground should be connected to the Sleeve of this connector. No connection should be made to the Ring on stereo connectors. Cables for this application are available that terminate in a mono 1/4" phone plug on one end and a stereo 3.5mm phone plug on the soundcard end, with no connection to the Ring. A standard audio patch cable combined with an adapter like the Hosa GMP-113 can also suffice.

1/4"-to-1/4" cable shown with 1/4"-to-1/8" adapter

Microphones equipped with 1/4" plugs usually have audio on the Tip and use the Sleeve as the ground. These microphones often have a high impedance (about 10,000 ohms), which means that only a fraction of their output signal will be transferred to a low impedance (600 to 2,000 ohms) sound card input.

Special Information For Macintosh Users

In the case of Apple Macintosh computers, several things are different. Most Apple computers have one audio input and one audio output. These are identified on the back of the computer by a small graphic representation of a microphone (for the audio input) or a speaker (for the audio output.)

Drawing of the back of a Macintosh computer where sound output and input are identified

The sound input port is stereo and requires an ‘auxiliary level’ signal in the 100 millivolt range. This means that no standard professional microphone can be connected to this input without the use of an active preamplifier to boost the signal level. The sound output port is also stereo and also uses a miniplug connector, with the Sleeve as the ground connection. A set of headphones can be connected to this output and be driven at comfortable levels.

The newer Macintosh models, like Quadra’s and PowerPC’s, are equipped with a unique four-conductor 3.5 mm minijack that allows the computer to detect if the device that is plugged in is stereo or mono. The input settings in the Sound Control Panel will then be automatically configured for a stereo or mono device. This is a custom-made non-standard jack that is longer than a standard 3.5 mm connector. The Sleeve is used for ground, the first Ring is used for the left channel audio, the second Ring for right channel audio, and the Tip carries 5 volts of bias to the microphone. The bias voltage from the sound input port is used to power a special preamplifier mounted inside the standard Apple microphone that comes with the computer. The preamplifier boosts the microphone signal up into the 100 millivolt range.

Plugs actual size

Inside the Apple microphone

Connecting Microphones to the Macintosh

To interface a standard professional microphone with the Macintosh sound input port, a preamplifier must be used to boost the output level of the mic (typically less than 1 millivolt) to the level required by the sound card (about 100 millivolts). A standard 3.5 mm miniplug will fit into the sound input port in such a way that the bias voltage in the jack does not contact any of the conductors of the miniplug, and will not be fed to the microphone. A mono, unbalanced, auxiliary level signal that is carried by a one conductor shielded cable can be connected to the Macintosh sound input port by routing the audio to the tip of a mono miniplug and connecting the shield to the sleeve. The computer detects the presence of a mono miniplug and will set the input for mono operation.

A mono, balanced, auxiliary level signal that is carried by a two conductor shielded cable can be connected to the Macintosh sound input port by connecting pin 2 of the microphone’s XLR connector to the Tip of a mono miniplug, and pins 3 and 1 to the Sleeve of the miniplug.

A stereo, unbalanced, auxiliary level signal that is carried by a two conductor shielded cable can also be connected to the Macintosh sound input port. In this case, simply connect the shield to the Sleeve of a stereo miniplug, the Left channel audio conductor to the Tip, and the Right channel audio conductor to the Ring.

Tips on Voice Recognition

For accurate voice recognition, the software must receive clear, intelligible sound from the microphone. For this to happen, the microphone must be placed in an area where it receives relatively noise-free sound from the talker. The following guidelines will help you to get the best performance from your microphone and your voice recognition software.

Place the microphone close to the talker. As the background noise level increases, the ratio of signal to noise decreases and the performance of the voice recognition software degrades. The noisier the room is, the closer the microphone must be placed to the talker to provide sufficient signal-to-noise ratio for good voice recognition. In most situations, a talker-to-mic distance of less than one foot is optimum. In noisy environments, the mic should be within 6 inches of the talker’s mouth for good results; a headworn, lavalier/tie-clip, or gooseneck-type microphone is usually the best choice.
Use a directional microphone. Unidirectional microphones (referred to as noise-canceling by some manufacturers), which are less sensitive to sounds coming from the rear and sides, can help isolate your voice from ambient noise. Unidirectional microphones also help when the primary noise source is directly behind the microphone (such as the computer’s fan or hard drive). A unidirectional microphone aimed at the computer operator may still pick up noise from sources located behind the operator.
Use a windscreen or pop filter. Windscreens prevent air currents from the mouth from striking the microphone abruptly, which can cause a popping or thumping noise which cannot be interpreted by the voice recognition software. Condenser microphones are usually more sensitive to popping than dynamic types.

Summary

There are many variables that must be considered when interfacing audio equipment to a computer sound card. Keep in mind that your sound card might have a different input configuration than described here. If the technical information supplied with the sound card is unclear, call the manufacturer. In any case, the information presented in this document should help you to find a solution that works for your situation.

Written by Luis Guerra
Edited by Christopher Lyons and Gino Sigismondi

Apple and Macintosh are trademarks of Apple Computer, Inc.

Reprinted from Shure.

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