Sampling and recording

When a sound card records analogue audio, it is converting the sound waveform into digital information and then copying this in real time onto the hard disk . Essentially, it is using the disk as a digital tapeless recorder. To hear what's been recorded, the sound card takes the digital information off the hard disk, converts it back into analogue, and then feeds it to loudspeakers, headphones or a conventional sound recorder.

The process of converting analogue to digital is known as digitising or sampling . With audio, the analogue waveform is chopped into a number of slices per second. At each slice, the amplitude is measured and rounded to the nearest available value. Clearly the more chops per second ( sampling rate ) and the finer the values assignable to the amplitude (dynamic range), the better the representation of the original.

CD digital employs a sampling rate of 44.1kHz and a 16-bit dynamic range. That is, 44,100 chops every second, each one describing the waveform amplitude at that moment in time with a 16-bit number; 16-bit itself offering 65,536 steps from which to choose. Of course, CD is a stereo system so that means two 16-bit words every 44,100th of a second. That works out at around 160 KBps, 10.5 MB/min or 630 MB/hour. The most common file format used to store digital audio on PCs is WAV .

All sound cards should offer up to 16-bit resolution and sampling rates of 44.1kHz or 48kHz, although hey will also operate at lower quality settings for less demanding circumstances. Superior sound cards boast lower noise levels and higher-quality analogue-to-digital and digital-to-analogue converters .

Recording and editing audio uses a large amount of hard disk space, with ten minutes' CD quality requiring over 100MB. The faster the disk and I/O sub-system the better when working with such large files. Modern hard disks and PCI controllers are capable of sustaining a transfer of at least 4 MBps. Serious practitioners will want to ensure that there are no interruptions in the audio stream. Many hard disks pause, to thermally recalibrate, which can result in a short but undesirable pause. Some AV drives are specifically designed not to thermally recalibrate, thus eliminating this effect.

For those wanting nothing but the best, nothing can match the virtually loss-less quality a completely digital audio processing system can offer. However, since the digital-only market is small and specialised and with few competitors, this currently comes at a price. The principal components of an all-digital system are a sound card equipped with S/PDIF (Sony/Philips Digital InterFace) format Digital In and Digital Out sockets and software to transfer digital audio onto a hard disk. The CDGrab Professional utility is an example of the latter capable of copying digital information directly off a CD-ROM drive ontoa hard disk in 16-bit stereo 44.1kHz WAV format.


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