Sound Off!
This week's article takes a look at sound files - namely, the different types there are and how you can create and manipulate them as there seems to be a lot of confusion about this topic.
How is sound stored?
There are basically two types of sound file that you get.
· Sampled sound. This is real sound such as speech. Stored in .WAV files
· Digitised sound. Created via MIDI (Musical Instrument Digital Interface) instruments such as synthesisers. Stored in .MID.
But what about MP3 files? What are they? More on them in a moment.
Sampled Sound
Have you ever seen an oscilloscope? Where you get the crazy lines across a small screen - similar to those heart machines in hospitals? Sound can produce such readings. If you took a measurement of each "dot" as it passes a fixed point you end up with a (complicated) set of numbers. These numbers when fed into a sound chip will produce "noise" - that is - the sound that the dots refer to. Play these numbers one after the other (known as streaming) and you end up with sampled sound.
The reason it is called "sampled sound" is that the computer takes a sample (reading) at different times (frequency). The higher the frequency, the closer in time each sample is taken and therefore the clearer the sound is. CD quality is 44Mhz, which means 44000 samples per second! Speech can sound reasonable at 11Mhz but music sounds dreadful as it is a much more complicated sound and requires higher frequency (sampling rates) to sound good.
Imagine someone speaking to you. You hear a constant stream of sound as the person speaks to you. Now imagine that in a 1-second period, you could only hear for half a second. The sentence:
"The cat sat on the mat"
Would sound like
"T.. ca. sa. o. th. ma."
Complete rubbish yes? This is because you would only hear some of the sound - not all of it. Increase the frequency (say you could hear for three-quarters of second - losing a quarter of second). The sound would be clearer because you can hear more. That's the principle behind sampling.
There is one major drawback here. The higher the frequency the more numbers the computer has to store in a sound file. The more numbers mean a larger file - a much larger file. 30 seconds of CD quality music could take up megabytes of space. Not convenient for the Internet! The great thing with sampled sound is that everyone will hear the same thing providing they have a reasonable set of speakers.
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