SOUND CARDS


A 16 Bit Sound Card.

Multimedia begins with a sound card. The first sound cards were 8 bit ISA versions, quickly followed by 16 bit ones. Creative Labs launched the sound blaster 16, which quickly became a standard that is still referred to today. Many other companies now produce sound cards but most of them are SB16 compatible, i.e. the I/O address (220H) the interrupt (IRQ 5) and DMA access are standard to all SB16 compatible cards.

Basically, a sound card consists of an FM synthesiser for playing back MIDI files, a D to A converter for playing back wave files and an audio amplifier to make the sound loud enough to drive the speakers. The audio from the CD-ROM drive is fed directly to the amplifier on the sound card. An A to D converter is provided for recording of analogue sound from, say, a microphone or other source as a digital recording generally in the format of a wave (.wav) file. A 15 way d-type socket is fitted to most soundcards, for connection of two joy sticks and for MIDI signals in and out, these being used to control MIDI compatible musical instruments, the majority of which are keyboards. There are others however, Guitars Sax etc., but these are rare. 4 stereo 3.5mm jack sockets are usually fitted. These enable connection of speakers, microphone, line in and an external amplifier.

Perhaps some explanation of the terms and techniques used in digital audio may be useful. Firstly lets look at digital recording; basically an audio signal is a constantly varying complex waveform, i.e. it is made up of many different frequencies and their associated harmonics (see any physics book!). In digital recording this waveform is quantized; this means that it's amplitude is measured at regular intervals and then it's value is stored as a number. The value of this number can be between 0 and 256 for an 8 bit recording and between 0 and 65535 for a 16 bit recording. The sound quality obtained from a digital recording is directly linked to the resolution (8bit 16bit 32bit etc.) and the sampling rate 8KHz to 44KHz. CD Quality is 16 bit @ 44.1KHz.

An A to D (Analogue to Digital) converter is used to measure the amplitude of the waveform and convert it to a binary number. Clearly, the higher the sampling frequency the larger the sound file will be for a given sound. This is also true for the resolution; 16 bits uses twice as much memory as 8 bits. So, although very high sound quality recordings can be made using a sound card you will need a large hard disk if you want to record in high quality for any length of time. Incidentally, you may be wondering why if you try to record a standard 640MB audio CD onto a hard disk using a sound card the resulting file is considerably larger than 640MB. The reason is that CD's use a special compression system which is patented by Sony & Philips and is not available for use on sound cards due to the high cost of licensing the system. This is also the reason why all audio CD tracks are read as 1KB by Explorer and DOS. You cannot copy the digital data from an audio CD directly to your hard disk because of this system.

When you playback a digital recording using a sound card the data is read at regular intervals and the digital numbers are fed to a D to A (Digital to Analogue) converter which produces the waveform which we started with. The signal thus derived is fed to the audio ampifier and onto the speakers.

The digital sound files produced in this way are called wave files and are standard between PC's; they usually have a file extension of .wav and can be in any of the above sampling rates and either 8 or 16 bit resolutions.

Finally a word about MIDI (Musical Instrument Digital Interface). This is a standard system used by many musical devices. It is, as it's name suggests, a purely digital system for connecting and controlling musical instruments. Data from, say, a keyboard, is fed digitally to a synthesiser and the synth plays the notes as if the keyboard was part of it. The note value, how hard the key was pressed and the time of note on and note off are all sent sequentially to the synthesiser in a standard format.

A soundcard can read these MIDI signals and display them on the screen for editing as well as storing them in a file for playing back at a later time. This is how a lot of today's music is made. The resulting files are called MIDI files and usually have the file extension .mid. Because the sound card has a basic synthesiser on board .mid files can be played without having to connect an external synthesiser. If you have a MIDI keyboard you can use it's internal synthesiser to play MIDI files instead of the soundcard's FM synthesiser. This requires a special lead, not supplied with sound cards, in order to connect the 5 pin din MIDI socket of the instrument to the 15 pin d type socket of the sound card. Although these leads are not particularly common they are not expensive and if you own a MIDI keyboard they are well worth the extra investment as they considerably increase the entertainment value of a sound card.

Some sound cards are now becoming available with Dolby Pro-Logic circuitry on-board to provide surround sound capability when playing sound files encoded with this, as used in films and some of the latest games.