Re: how do different frequencies playing at the same time get represented?
Re: how do different frequencies playing at the same time get represented?
- Subject: Re: how do different frequencies playing at the same time get represented?
- From: Bob Lang <email@hidden>
- Date: Fri, 14 Jan 2005 17:01:32 +0000
Hi Ben
From the tone ;-) of your question, it's obvious that you are a
complete newby to this particular topic. That's OK - we all had to
start somewhere.
You need to appreciate that sound moves through the air as *waves* of
high and low pressure, moving at a speed of around 340 metres per
second outward from some source. If you imagine a large lake, then the
ripples on the water are another form of wave - throw in a stone and
waves move outward along the surface of a the water.
A microphone reacts to the high and low pressure waves in air in
exactly the same way as our eardrums. A microphone creates a voltage
which can be displayed as a trace on an oscilloscope. This shows the
voltage as it changes instant by instant and (surprise, surprise) this
trace looks just like ripples on water because it's also a wave.
If you play two different sounds into a microphone separately, you'll
see that each one has its own characteristic wave. If you play them
simultaneously into the mike then the pattern of voltages that comes
out will be very complex, but if you look at it on an oscilloscope
you'll make out that what's actually happening is that (instant by
instant) the two waves are added together to produce a wave that is a
sum of them both.
An analogue recording device, such as a tape recorder, records the
microphone voltage instant by instant over a period of time. When put
in playback mode, the recorded voltages are reproduced exactly (within
engineering limits) and sent to a loudspeaker. The loudspeaker creates
pressure waves in the air and we hear a copy which is identical to the
original sound.
If we have two tape recorders playing, we can arrange for the output
voltages from each to be added together at the speaker. When we do
that, we hear both sounds at once. This is the basic idea behind the
complicated multitrack mixing desk in a recording studio.
Digital recording simply replaces the technology of the tape recorder.
Voltages from the microphone are recorded at some "sampling rate"
(44100 times per second for CDs), converted into a digital form and
saved on a computer. This produces a stream of digital values,
representing the voltage coming out of the mike at successive sampling
times.
To play back, the digital values are converted back into voltages (at
the same sampling rate used for recording) and sent to a speaker.
So to answer your question: the waves of both sounds are added together
instant by instant and recorded. On playback, we hear the two sounds -
although it's really the lump of jelly between the ears which actually
works out that there were two sounds to begin with. Whether the
recording technology is digital or analogue is completely irrelevant.
Of course, there's much much more to it. I suggest you need to start
with a really basic text on physics to understand how waves work and
what happens when waves get added together. Waves are a fundamental
component of the universe so it's really helpful to get a good handle
on what they can do.
Bob
--
On 14 Jan 2005, at 15:11, Ben Dougall wrote:
hello,
what i want to know about sound on a computer is, how do two
continuous tones occurring at the same time get represented? it seems
similar to wanting to, with images, wanting to represent say red and
yellow in the same pixel (which would actually just end up as orange.
not red, and yellow, at the same time.) in fact colour is split up
into several elementary colours (like red green blue) and they are
treated with different values which are called channels, and a pixel
can be made up of a mixture of the channels, but as i say, those
various colours end up as one colour. and that isn't the way sound
channels are anyway, as each channel is a stream of audio (like one of
the two streams of stereo) -- all and multiple tones can be
represented in one audio channel, so channels in audio are irrelevant
to what i'm asking about i think. how does a mono, single channel
stream of sound manage to represent two tones occurring at the same
time without merging the two tones into one?
just to go on and try and illustrate exactly what i'm asking:
situation 1:
two different tones/frequencies playing at the same time. one who's
tone is, say, 10. the other who's tone value is 20.
situation 2:
a single tone playing who's tone is 15
how is the difference between those two situations
achieved/represented in digitised audio? how do the tones 10 and 20
playing at the same time not end up as the same as a single 15 tone?
any pointers to relevant info appreciated.
thanks, ben.
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