MICROPHONE AND SOUND CARD

Microphones are, predictably, used to collect data in the form of sound waves. They convert these compression waves into electrical energy. In digital systems, this analog electrical energy is converted, using an analog to digital converter (ADC) into a series of digital sound samples. In this section we examine the operation of microphones and then consider the operations performed by a typical

sound card to process the resulting analog electrical energy into a sequence of digital sound samples.

There are a variety of different microphone designs, the most popular being dynamic microphones and condenser microphones. All these designs contain a diaphragm which vibrates in response to incoming sound waves. If you hold your hand close to your mouth whilst talking you can feel the effect of the sound waves; the skin on your hand vibrates in response to the sound waves in exactly the same way as the diaphragm in a microphone vibrates.

A dynamic microphone has its diaphragm attached to a coil of wire; as the diaphragm vibrates so too does the coil of wire (see Fig 3.19). The coil of wire surrounds, or is surrounded by, a stationary magnet. As the coil moves in and out the interaction of the coil with the magnetic field causes current to flow through the coil of wire.

This electrical current varies according to the movement of the wire coil, hence it represents the changes in the original sound wave.

Condenser microphones alter the distance between two plates (see Fig 3.20). The diaphragm is the front plate; it vibrates in response to the incoming soundwaves, whereas the backplate remains stationary. Therefore the distance between the diaphragm and the stationary backplate varies;

when the two plates are close together electrical current flows more freely and as they move further apart the current decreases, hence the level of current flowing represents the changes in the original sound waves. Condenser microphones require a source of power to operate; this can be provided from an external source via the microphone’s lead or by using a permanent magnetically charged diaphragm.

Detail of a dynamic microphone.

Sound

Detail of a condenser microphone.

Fig 3.18

A dynamic microphone element.

This one has the magnet mounted within the wire coil.

Wire coil Magnet

GROUP TASK Investigation

Make a list of all the microphones you see each day. Can you determine whether these microphones are dynamic, condenser or some other design?

Consider the following:

The varying electrical current produced by a microphone is essentially the same as the raw analog signal output from all types of audio devices. Therefore it is possible to connect any of these different audio sources to one of the analog input ports on a computer’s sound card;

just be careful to connect to a port designed for the level of signal produced by the device.

There are usually a number of input ports on most sound cards suited to different levels of analog input signal.

Let us now consider the processes taking place once the analog signal from the microphone reaches the computer’s sound card. The signal is fed through an analog to digital converter (ADC), which predictably converts the signal to a sequences of binary ones and zeros. The output from the ADC is then fed into the digital signal processor (DSP), whose task is to clean up any abnormalities in the samples. The final sound samples are then placed on the computer’s data bus. The data bus feeds the samples to the main CPU, where they are generally sent to a storage device.

The major components involved in processing the audio data are the analog to digital converter (ADC) and the digital signal processor (DSP). Let us consider each of these components in more detail.

Analog to digital converters (ADCs) repeatedly sample the magnitude of the incoming electrical current and convert these samples to binary digital numbers; for audio data the size of the incoming current directly mirrors the shape of the original sound wave, hence the digital samples also represent the original wave. The ADCs used in many other devices, including scanners and digital cameras, are essentially the same as those found on sound cards; the CCDs in image collection devices produce varying levels of electrical current that represent the intensity of light detected at each photosite. The electrical signal is much the same as that produced by audio collection devices.

GROUP TASK Investigation

Examine the ports, and accompanying documentation, for either your school or home computer’s sound card. Describe the difference between each of the input ports and list suitable audio sources that could be connected to each port.

Fig 3.21

Creative’s Audigy sound card.

GROUP TASK Activity

Brainstorm a list of hardware devices that would likely include an analog to digital converter. Indicate the media type collected by each device and how different levels of electrical current could be used to represent the identified media type.

Most analog to digital converters contain a digital to analog converter (DAC). On the surface this seems somewhat strange, however the digital to analog conversion process is significantly simpler than the corresponding analog to digital conversion process.

The components and data connections within a typical ADC are shown in Fig 3.22;

this ADC performs its conversion using the following steps:

• At precise intervals the incoming analog signal is fed into a capacitor; a capacitor is a device that is able to hold a particular electrical current for a set period of time, this allows the ADC to examine the same current repeatedly over time.

• An integrated circuit, called a successive approximation register (SAR), repeatedly produces digital numbers in descending order. For 8-bit samples it would start at 255 (11111111 in binary) and progressively count down to 0.

• The DAC receives the digital numbers from the SAR and repeatedly produces the corresponding analog signal. The analog signals will therefore be produced with decreasing levels of electrical current.

• The electrical current output from the DAC is compared to the electrical current held in the capacitor using a device called a comparator. The comparator signals the SAR as soon as it detects that the current from the DAC is less than the current in the capacitor.

• The SAR responds to the signal from the comparator by storing its current binary number. This number is one of the digital sound samples and hence is output to the DSP. The SAR then resets its counter and the whole process is repeated.

So what happens to these sound samples once they reach the DSP? The DSP’s task, in regard to collected audio data, is to filter and compress the sound samples in an attempt to better represent the original sound waves in a more efficient form. The DSP is itself a powerful processing chip; most have numerous settings that can be altered using software. Most DSPs perform wave shaping, a process that smooths the transitions between sound samples. Music has different characteristics to speech, so the DSP is able to filter music samples to improve the musical qualities of the recording whilst removing noise. The DSP uses the sound samples surrounding a particular sample to estimate its likely value, if these estimates do not agree then the sample can be adjusted accordingly. Once the sound samples have been filtered the DSP compresses the samples to reduce their size. Some less expensive sound cards do not contain a dedicated DSP, these cards use the computer’s main processor to perform the functions of the DSP.

Capacitor

DAC Comparator

SAR

Analog Digital

Fig 3.22 Components and data connections for a typical ADC.

GROUP TASK Discussion

The processing performed by a sound card when collecting audio data involves most of the seven syllabus information processes. Describe the operation of a sound card in terms of these information processes.