If you imagine quite far back you started off [with] midi files.
[It] was one of the ways of transmitting music initially over the internet, then you added things called mod files,
these were midi files with a very small amount of audio -- normally 8-bit audio added to it.
You're talking about 1980s video games [of] the territory so [it] was slightly better sounding the Midi was but
considerably worse than audio was, and then
compression techniques started to come in, with sort of a ten-to-one, twenty-to-one compression rates and you could get a
mono audio track down to something you could easily download off the internet.
So, we're talking about digital audio,
fascinating topic,
I think you've done a piece on it before, just to recap taking in a
analog Audio Stream
and then sampling it both that way over time and in that way over amplitude -- slightly less relevant today,
but obviously when the internet first got it going you didn't have much speed so he wanted to have these files very much reduced.
You know so take an audio file
that was quite massive, squeezed it down so you can pull over a small connection that led onto the field of audio compression being studied
One of the studies was our understanding of how we hear,
so clearly we don't want to encode information [that] is irrelevant to the way we hear.
[It] requires us to understand the human ear, so let's have a look at that.
So if you imagine in your ear itself this one will be pretty apt you've got
a bunch of little bones, and then you've got what's called a "basilar membrane" so a piece
of
skin that's rolled up,
very much like that, and then alongside of it, you've got these little hairs
They're called "cilia" and what happens is all the sound comes in different bits, if you imagine this basilar membrane
unrolled vibrate at different points -- if it's a low frequency it vibrates at one end, if it's a high
frequency it vibrates to the other -- that with the little nerves
hairs underneath they make a little nerve firing up the brain which enables you to perceive pitch in another higher level sort of
concepts of sound.
Because of that rather limited nature of hearing, what happens is if -- if there's a loud sound
in a certain area, because [it is a] physical membrane this bit vibrates, but makes us desensitized to other
frequencies around it yeah
and that -- that's the basis by which we get to do some aspects of compression that particular bit is called the
psychoacoustic effects which is how we perceive sound
(Off camera:) Is it like the louder sounds kind of, like, obscure the lower sounds
and then we kind of hide the different frequencies?
The loud sounds obscure the less loud sounds around a certain frequency,
because of that physical membrane representation of the frequencies.
And then there's been huge amounts of studies to understand how much it affects it to what degree.
What you do is then you take your digital audio
and you can imagine doing this yourself if you were -- and if you have a done an audio multitrack
system, you know sort of --
all of them, there's Cubase -- whatever
pro tools --
take your audio track which would be a constant audio stream -- you can imagine filtering out into all its frequency bands let's say from
20 Hertz to
100 Hertz, 100 Hertz [to] 500 Hertz and so on and so on and then you can imagine
sampling them at different rates, so this one might be 8-bit, this one might be 16-bit, this one might be 24-bit
and you know by sampling things at different rates you effectively add a bit of noise to it, but you reduce the overall amount of
bits you need to represent it.
(Off camera:) So you lower the quality but obviously you're lowering the file size.
Yes but you're lowering the quality in such a way that because of the other sounds are going on you -- you can't perceive it,
and if you plotted that out, this would be called a psychoacoustic mask,
and that means you can reduce the amount of bits you need to describe something.
But,
you can make very small,
perceptible differences to the overall sound, and that -- that's one of the core tenants, so it's sort of using the -- the rather
oddities of the way our hearing system has evolved -- which is, you know, far from perfect obviously --
to take advantage of that and then reduce the amount of data, and then you can do your normal sort of
compression tricks which work for any type of data stream, like, I keep seeing this string of digits come up,
so I'm going to put those up in a lookup table and encode it more efficiently than just encoding those bits each time.
No different than, say, morse code does by saying we use certain letters more than you do others,
so I'm going to attribute a short key length to those and there's actually a whole
technique around that, called "Huffman trees" for anybody who wants to look that up --
fascinating things -- so you get, you know all of the other aspects information coding come into play,
but that sort of psychoacoustic bit is the bit that's very audio centric
So it's a bit that has quite a lot of relevancy.
So then there's other tricks, like if you've got stereo channels for comparing them in some way or
taking information out that way, but that's the sort of basic tenants of compression now that
absolutely, then opened up the internet music.
(Off camera:) Since compression is still working the same way now, if you -- somebody is, you know, listening to spotify and
streaming audio is it there, or are there any other tricks they use...
Well there are lots more tricks...
For me, sort of compare this to any sort of innovation, so -- so it's slightly obscure
but if you take the -- something like the steam engine revolution, there was major innovations, and then people filled in all the gaps.
In steam engine it was the notion that there was a steam engine, you drive a piston off and then you can
pump water out of mine, and then people put wheels on it and turned it into a train and all the other sorts of things,
but the -- the fundamentals of it stay the same
(hesitation)
People have then done significant innovations inside of that,
but the notion, there is an understanding of how you hear, that clearly gets better over time so that bit [has]
improved, then there's a notion of
all of the encoding tricks you can go do on information sources, so this is like encoding it as better
Huffman trees, or whatever the modern equivalents thereof, so you get those pieces, but I think the fundamental tenants stayed the same,
just the overall increases have got better -- interesting also obviously bandwidth constraint to come down,
so the pressure has come off it a little bit as well in terms of an overall need.