6.3. Audiosystems − sound card, microphone, acoustic system

Any modern PC includes a card − audioadapter or audiomap. Application of audioadapter (Sound Blaster) gave to the computer not only the stereophonics of sounding but also possibility of record on external carriers of audio signals. Disk carriers of PC quite not do for the record of ordinary (analog) audio signals, as is expected for a record only of digital signals, that almost not distort at their transfer on flow lines.

Basic components of modern audiomap. In modern audiomaps it is possible to distinguish four independent blocks − digital record/recreation, synthesizer, МРU and mixer.

Block of digital record/recreation, which is also called a map digital channel, or highway. It carries out conversions: analogue-digit and digit-analogue in the mode of programmatic transmission or by the channel of direct access DМА. It consists of unit which executes analogue-digital conversions directly − ADC/DAC (coder/decoder, codec), and control unit. ADC/DAC is or integrated in the complement of one of card microcircuits, or separate microcircuit is used (АD1848, СS4231, СТ1703 and other). From quality of ADC/DAC depends quality of recreation sound. In addition, it depends on input and output amplifiers.

The block of synthesizer is built on the base of microcircuits FМ-synthesis ОРL2 (YМ3812) or ОРLЗ (YМ262), on the base of microcircuits WТ synthesis (GF1, Wave Front, ЕМU8000 and so on) or that and other together. It operates or under control of driver (FМ, majority of WТ) − programmatic realization МIDI, or under control of own processor − hardware representation. Almost all FМ - synthesizers are compatible between themselves, and WT-synthesizer − not compatible. Majority of WT- synthesizers have built-in ROM with standard set of tools General MIDI (128 melodious and 37 shock instruments), and also RAM for loading of the additional digitised sounds which are used during implementation of music.

The block of МРU carries out the reception/transfer of data by external МIDI-interface, shown out on disconnector МIDI /Joystick .

The block of mixer carries out adjusting of levels, commutation and integration of the analogue signals used on a map. The previous, intermediate and output amplifiers of audio signals enter in the complement of mixer.

ADC periodically determines the level of audio signal and converts it into a digital code which is written down on external carrier as a digital signal.

The digital samples of the real audio signal are kept in computer memory, for example as WAV-fails. Red off disk a digital signal is given on a digit-analoque converter (DAC) which converts digital signals into analogue. After filtration of them it is possible to amplifier and give on acoustic columns for a recreation. Significant parameters of audioadapter is frequency of quantum of audio signals and bit of quantization (table. 6.1).

Table 6.1. Frequency of quantization

Frequency range, Hz	Type of signal	Frequency of quantization, kHz
400÷3500	Barely legible language	5,5
250÷5500	AV quality of language	11,025
40÷10 000	Quality of sounding of MW-set	22,040
20÷20 000	Sound of high quality	44,100

Frequencies of quantization show how many times on a second signals are sampled for conversion into digital code. Usually they present from 4 ÷ 5 to 45 ÷ 48 kHz.

The bit of quantization characterizes the number of quantization steps and changes the degree of number 2. So, 8-bit audioadapters have 2⁸ = 256 degrees, that obviously not enough for the high-quality code of audio signals. Therefore presently apply 16-bit audioadapters which have 2¹⁶ = 65 536 degrees of quantization − as at audio compact disk.

The parameters of synthesizer determine possibilities of map in the synthesis of sound and musics. During a receipt on the synthesizer of some guided information according to it a corresponding output signal is formed. Modern audioadapters synthesize musical sounds by two methods: friquency modulations FМ and by means of wave synthesis WT (Wave Table, choosing sounds from the table of sounds). Other method provides more natural sounding.

FМ (Freguency Modulation) − synthesis by means of a few generators of signal (usually sinewave) with an intermodulation. Every generator is provided by a chart of frequency and amplitude of signal controlling and forms the "operator" − base unit of synthesis. Mostly in sound cards apply a 2-operational (ОРL2) synthesis, sometimes − 4-operational (ОРLЗ). The chart of operators joint (algorithm) and parameters of every operator (there is frequency, amplitude, law of their time-history) determine a sounding timbre. FМ-range more comfortable for creation of new timbres, although possibilities of FМ -synthesizer of sound cards are limited enough as a result of the simplicity.

WT (Wave Table) − recreation of the written in the digital state soundings − samples. Instruments with small duration of sounding usually write down fully, and for other it can be writen only beginning/end of sound and small "medium" part which is then plaied back in the cicle during set time. For the change of sound pitch a digitization is plaied with different speed, and, that sounding character did not change here, instruments consist of a few fragments for the different ranges of notes. In the complex synthesizers they use a parallel playing of a few samples on one note and aftertreatment of sound − modulation, filtration, different effects and others like that.

Basic properties of sound card. Frequency Response shows as far as well an audio system recreates a sound in all frequency range. An ideal device must identically pass all frequencies from 20 to 20 000 Hz. Although in practice on frequencies over 18 000 and below 100 there can be a decline of description on the value − 2 dB through the presence of low/high-frequencies filter, but consider that deviation below, than 3 dB impermissible.

A ratio signal/noise (S/N Ratio) is ratio of values (dB) of the undistorted maximal signal of card to the level of noises which arise up in the own electric charts of card. As a man perceives noise on different frequencies variously, a standart net was develped A−scaling. The greater ratio, the considerably more high-quality audio system. Lowering of this parameter to 75 dB is inadmissible.

Quantization noises. Residual noises, typical for digital devices which arise up as a result of nonideal conversion of signal from an analog into a digital form. This noise can be measured only at a signal presence and give as a level relative to maximally possible output signal in decibels. The less this level, the higher quality of sound.

Total nonlinear distortions (total harmonic distortion + noise) display influence of distortions, which are brought in by the apparatus of sound and noises amplification which are generated by a card. Measure in percents of the undistorted initial signal level. Device with the level of obstacles over 0,1 % consider off-grade.

Demultiplexing is a prime number which shows to which degree the left and right channels remain mutually independent. In ideal case, demultiplexing must be full (absolute stereo effect), but in practice there is penetration of signals from one channel into other. On high-quality stereo-device demultiplexing must be not less than 50dB.

The dynamic range is a difference in decibels between maximal and minimum signals, which a card can let pass. Usually a dunamic range is measured on frequency 1 kHz. In the ideal digital audiosystem a dynamic range must be about 98 dB.

Іntermodulation distortion. Express in the percents a ratio of amplitudes of distortions and test signal. The lower distortions level of parasite harmonics which are generated by amplifier the better. High-quality audio devices have intermodulation distortions no more than 0,1 %.

Potential strengthening. It is maximal amplification factor which is provided by the previous strengthener of sound card. It is desirable to have the high potential strengthening at low input voltage. The low voltage is considered value 0,2 V, that answers the typical output signal of domestic tape recorder.

Except these parameters for description of sound card point: bit, maximal frequency of discretisation, amount of channels (mono or stereo), methods of sound synthesis, compatibility, type of connecting, support polyphony and sound-effects and others like that.

The bit of map − it is the bit of digital sound transfer 8, 16 or 32 bits. 8-bit cards have quality of sound, near to phone; 16-bit already suitable for determination of "Ні-Fі" and theoretically can provide studio quality of sounding, although in practice it will be realized very rarely.

Maximal frequency of discretisation determines maximal frequency of written and recreated signal, which equals to the half of discretisation frequency approximately. So, for the record/recreation of language it is sufficiently 6 ÷ 8 kHz, musics of medium-grade − 20 ÷ 25, high-quality sounding − 44 kHz and anymore.

Under compatibility they understand mostly compatibility with the models Sound Вlaster − usually SBPro and SB16; SВРrо − 8-bit card, that allows record/recreation of one channel with frequency of discretisation 44,1 kHz or two channels with frequency 22,05 kHz; SВ16 − 16-bit card which assumes the record/recreation with frequency 44,1 kHz, and has automatic control of level from a microphone and programmatic adjusting of timbre.

Modern standards of 3D-sound. Most sound cards presently supports 3D-sound. At least, so producers declare. A sound card without maintenance of 3D-sound is presently doomed to the failure, even if other its characteristics lot better, than for competitors. As well as in time with 3D videos-accelerators, in the world of 3D-sound there are a few standards of its receipt. Direct Sound 3D ( DS3D). A firm Місrosoft gives DS3D as one of components Direct X, and also, as Direct 3D, that is responsible for creation and release of 3D graphic. Unlike of Direct 3D, a Direct Sound 3D supports the so-called "expansions" as ЕАХ. The in itself standard DS3D − it is the set of commands, by means of which the developers of programmatic foods determine, where there are sources of sound in space. The co-ordinates of sound sources in the process of implementation of these commands are passed on DS3D-compatible sound card which places them in space and processes then. Quality of 3D-sound which hatches depends on that, which is used sound card, due to what a sound is recreated through 4 (2) columns or through headsets.

In the complement of modern sound cards digital signalling DSP (Digital Signal Processor) or extended signalling processor АSР (Advanced Signal Processor) enters. For them use ideal algorithms for a digital compression and decompression of audio signals, expansion of stereo sound base and providing of three-dimensional sound (quadro-phonic).