The hardware course 1 YBET

19. The sound board

19.1. Sampling - 19.2. Audio recording and production - 19.3. Wave table and its midday - 19.4. DSP - 19.5. 3d sound - 19.6. Connector industry - 19.7. Impedance, codec - 19.7. Conclusion

19.1. Sampling.

The sound (music, word...) is propagated in the form of waves, it is an analogical phenomenon. To convert it numerically, a sound board uses an analog-to-digital converter (ADC). Into opposite direction, a digital-to-analog converter (DAC) transforms the numerical values of the sound board into analogical to produce sound. Analog-to-digital conversion, or sampling, can produce errors. The quality of the digitized signal is initially tributary of the sampling rate. A frequency of 44,1 Khz corresponds to the standard audio CD. Coded in 16 bits (precision of sampling), this makes it possible to produce 65.000 different values. The human ear of a person let us say age walls is limited near 22,1 Khz. Certain sound board have a frequency of 48 Khz to obtain a quality DAT. The number of bits used to code sampling is also determining. The boards its use, in general, a coding on 16 bits. But there are more precise boards coding on 24 bits or even on 32 bits with processes of interpolation. The quality of a sound board also depends on particular audio parameters: the rate of distortion and the report/ratio signal-to-noise. This last measures the relationship between the aural signal and the background noise. It must be higher than 90 decibels. The rate of distortion indicates the percentage of error which undergoes the sound signal. It must be lower than 0.005 %.

19.2. Recording and production of the sound

A sound sound board must be able to record and read sounds in same time. This function, called Duplex Full, became essential with the development of the multimedia applications and telephony on lnternet. The musicians will be interested especially in the function Full Duplex hardware present on the top-of-the-range audio boards. It makes it possible to mix several sound sources and to directly record them, without loss of information, on the hard disk (Direct-to-disk).

To reproduce a sound starting from numerical data, without one being able to perceive his artificial origin, is not easy. The old models of boards its used the synthesis FM or frequency modulation. This method combined several waves of different frequencies, but the result was poor. Today the majority of the audio boards use the synthesis by sound tables of samples or wave tables. With this process, the digitized sounds can be stored in a specific memory, then added and modified by requesting the processor little. The table of wave must be compatible with the general standard MIDDAY.

19.3. Wave table and midi

The standard MIDI (Musical DIGITAL Instrument Interfaces) makes it possible the electronic instruments to communicate between them. A wave table compatible with the standard General Midday manages 128 different instruments. A compatible GM board is thus able to reproduce the sound of any instrument. The quality of this reproduction depends on the number of samples present on the memory of the board. More the memory allocated with the wave table is important, better will be the restitution. The banks wavetables (wave table) can be stored on a read-only memory (Rom). But in this case the bank cannot be modified. On a read-write memory (RAM) embarked, it can be replaced starting from the computer. It can also be stored on the read-write memory of the computer but with the detriment of the general performances. Generally, all the notes of musics by instruments are not digitalized, only the note "it" is digitalized by octave, the other notes coming from extrapolation.

Approximately, the sound with the format was created in 3 different ways according to the evolution. The first audio boards the bottom-of-the-range one used simple programming and used the processor to the maximum. This principle is called synthesis FM. The processors of the type MMX use a decompression better than the synthesis FM, for a processor use of 30 % of the working time. The second method, already used by the boards sounds of mark of the type Creative Labs SB32 used a table, a kind of memory in ROM, the WAVETABLE. According to the digital signal entering, the circuit generates the aural signal of exit on 16 channels (output signals), the news go up to 128 simultaneous channels. Consider that these channels are each time of the instruments.

19.4. DSP

Currently, the principle rather uses a specialized processor of microprocessor type which uses a similar principle, but more complete: the DSP (DIGITAL Processor Signal). According to the digital input signal, the specialized processor goes required in a Rom memory the output signal. Until there little evolution, except that the circuit is also connected to a memory RAM. You can thus charge of new sounds in this memory RAM, or why not replace the sound in Rom by new in RAM. Notice that this theoretical diagram represents generally only one circuit, except in the extensible smart cards. On the top-of-the-range boards, the DSP can be even reprogrammed to carry out complex operations.

Signal WAV is not compressed. This involves 172 KB 1 seconds in 44,1 Khz, 16 bits stereophony's, that is to say 31 MB for a 3 minutes song. The standard midday does not record the sounds itself, but the parameters which are used to manage these sounds. Its MIDDAY in some kind is thus compressed. Results, the 3 minutes song takes nothing any more but a few kilo on the hard disk. There exists some shareware which transforms one with the other. Moreover all files midday allow a modification of one or more notes without re-recording the piece in entirety.

The sounds boards are characterized by the number of ways in polyphony. With possibilities of addition of boards allowing a better treatment (Wave Blaster).

19.5. ITS 3D

The amateurs of plays attach a great importance to the "spatialization" of the sound. This sound effect makes it possible to position an object in space and to give the impression which it moves. The sound environment can be even modified according to the environment of the play. This realism will be really appreciated with four loudspeakers. Unfortunately each manufacturer uses his own standard of his 3d. Your board must be at least compatible with the standard Direct Sound 3d. This API created by Microsoft forms part of 1a family of DirectX. Creative Labs into drawn an extension, named Audio Environment, more sophisticated. It can manage numerical effects in real time. Other manufacturers use API, created by Aureal Audio, which offers to little meadows same the characteristics: standard A3D.

19.6. Connector industry

The boards its fit in a bus ISA (old models of audio boards) or currently in a bus PCI. The sound board can also be integrated directly on motherboard of computer. If bus ISA has a too low flow for the greedy applications like the its 3d, it can be enough for office automation applications.

The internal connectors generally gather an entry line (stereo chain), an audio input an entry joystick and an exit loudspeaker. On certain boards, a bridging on the board makes it possible to use this exit loudspeaker like exit line (weaker output voltage). Currently, the majority of the boards do not have any more entries line, replaced by a second exit loudspeaker (effects 3d). Let us announce that the exit joystick is often used as input/output midday via an adapter provided or not.

19.7. Impedance, codec...

19.7.1. Sound improvements.

Certain apparatuses, including the boards its the top-of-the-range one are compatible stereo Dolby. This system allows a better reproduction of the music.

19.7.2. Voltage and impedance.

Each output channel (and thus entry) is characterized by a maximum output voltage and an impedance (internal resistance of the power source). This impedance limits the output current according to the power supply. The inputs/outputs CD, cassettes and line identical into analogical and are standardized with 250 mV (even if an amplifier or a table of audio mixing reserves distinct channels). The apparatuses can be connected on any entry "line". On the other hand, a turntable for disc vinyl uses a tension definitely weaker and a strong impedance. The entry must be specific. The recording on a PC thus passes by a correct amplifier (entry on the amplifier by an entry platinizes, left on recording LINE) or by a table of mixing used by the DJ in wiring for sound.

19.7.3. Codec.

Codec is found as well in video acquisition in sound. The codec is the mechanism which makes it possible to transfer an audio analogical signal in digital signal (readable by a PC). This codec is special for each board and east includes in the driver. The codec can be hardware, software or mixed. In the last case, other functions are included in the mechanism like the transformation into dolby or even the suppression of the "noise" in the recordings of the discs vinyls.

Attention, when you record discs with format WAV, you will not be able inevitably to read again them with an other sound board, precisely with cause of these codec.

19.8. To conclude.

      The majority of the current motherboards integrate an audio board. It generally uses the codec hardware AC' 97 which is satisfied to manage audio flow (analogical/digital transfer), whereas the processor treats sound effects. For the majority of users, these sound boards are largely sufficient. These sounds boards even make it possible to leave the music on professional amplifications (mobile wiring for sound kind) average and strong power. On the other hand, for elaborate applications multi-media, the use of DSP directly decoded by the electronic circuit of the sound board is obligatory, including for the musicians who use the entries midday.

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