the chances are it sounds terrible, and sometimes it just dissolves into static.

Telephone signals are not designed to carry music, and to explain why,

we have to start way, way back with old telephone equipment like this.

Early telephone calls were put through by a human operator

who would be literally connecting the two phone wires into one circuit

at a big board of switches.

Or a "switchboard".

Any sound at one end, whether it's speech, music, whatever,

would be converted to an electrical signal, sent down the wire,

and converted back at the other end.

You send music, you receive music --

although it'd be limited by the abilities of the tiny microphone and speaker.

But telephone exchanges like this only work for small numbers of connections.

Because, you know, you can't do this for every call.

[PHONE RINGS]

Phone companies quickly realised that they could save

on setting up thousands of wires between exchanges miles apart

if they were able to put several conversations down one wire at the same time.

So rather than transmit every frequency of sound in the original signal,

they would cut off the high and low frequencies,

the ones we don't absolutely need to understand speech,

and then they would shift each call into a different frequency band.

At at the other end, like tuning in a radio,

they would shift each call back to the right frequencies so you could hear it.

I mean, the quality wasn't great, but it was good enough.

And hey, we got cheap long distance calling.

We got used to that phone-quality sound.

[EXCHANGE CLICKS]

And it became the standard.

So already [PHONE RINGS] music was going to sound a bit dodgy,

with all the high and low frequencies cut off.

But while we were still running on analogue circuits

[EXCHANGE RESETS]

it wasn't going to sound like static.

That was the fault of computers.

Phone companies realised that they could send even more calls down the same wire,

or into the same airspace on a cellular network,

if they used computers to compress the audio.

And the more they compressed it,

the more calls they could fit at the same time,

and the cheaper it'd be for them.

These days, there is enough power in pretty much everyone's phone

to compress 4K video in real time, but back then, not so much.

MP3s had only just been invented, and these do not have enough power

to compress those.

The standards for phone calls, even today, rely on quick-and-dirty compression

built decades ago, designed to run on very little computing power,

and optimised for speech.

That compression is great at encoding one clear voice --

but chuck a load of complicated music at it, with all its instruments

[COMPRESSED MUSIC] and it's going to sound bad.

But not quite enough to break down into static.

There is one last piece of the puzzle.

Someone else cutting corners.

Because in the worst case, that hold music isn't just compressed

by the phone company on its way to you.

It's already been compressed by a completely different system

as your call bounces around a company's internal phone network.

An internal phone network that has probably been built by the lowest bidder.

So the phone company takes that already-mangled signal

and mangles it in a different way as it travels to you.

And in a really badly designed system,

maybe there's an entire third layer of terrible compression

from whichever other company runs the call centre

that you're actually talking to, off in some other country.

[MUSIC RISES] Speech can survive that chain

But music...

[MUSIC TURNS TO STATIC] ...music can't!

Thank you so much to all the team at the Milton Keynes Museum

and their Connected Earth gallery,

where pretty much all this telephone exchange equipment

is just open to play around with.

Pull down the description for more about them.

This is not a TARDIS.

This is literally just a police telephone box.