How does your portable music

actuallyabandonedElectronics - Devices

Nov 15, 2013 (3 years and 8 months ago)

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How does your portable music
player work?

J.D. (jj) Johnston

Chief Scientist

Neural Audio

Kirkland,
Wa

So, how do we carry music around?


We need a few things:

1.
Some way to store music recordings

2.
Some way to control what you want to play

3.
Some way to play it back

4.
A level control

5.
Some way to get it to headphones

6.
Some way to power 1 to 3.

Graphically Speaking

Storage

Mechanism

Reproduction

Mechanism

Control

Mechanism

Power

Source

Headphones

Yes, it’s that simple.

But:


The bit rate for PCM CD storage is 11
megabytes per minute. (well, 10.5)


Storage takes space (although a few orders of
magnitude less than it did 10 years ago)


You need power to drive headphones


Batteries are still not very good, in terms of
energy density vs. volume


It has to fit in your pocket

Let’s get Power out of the way


Power is the basic problem in any portable player.



CPU uses power



Amplifiers use power



Discs use power



Memory uses power when you read it.



POWER IS THE BIG PROBLEM


That wasn’t always the case, though. See the storage issue coming up.

Storage Issues


At first Storage of 22 minutes (a2b music) was
acceptable


Disc storage of many GB was the next step


Flash storage of several GB is now common.
(8,16,or 32 for one common player)



At each step the memory power has been
reduced.


So, what is the relationship of time vs.
storage for a standard rate?

Bit rate (kb/s)

Storage Size

Minutes of Music

128

100 MB

100

192

100 MB

70

128

1 GB

1000

192

1 GB

700

128

32 GB

32000

192

32 GB

21000

320

32 GB

12000

Note: Minutes are rounded

Reproduction stages:

Digital Processing

Digital to Analog
Conversion

Power

Amplifier

The Digital Processing


The digital processing must do at least the
following steps


Figure out where the music to be played is stored


Read the storage medium


Convert whatever (MP3, AAC, FLAC, PCM) is on the
storage medium to PCM


Do whatever other processing is desired


Level compression


EQ


In some cases, volume control


Send the PCM to the Digital to Analog Convertor

What does this entail?


An operating system


A file system


An audio decoder (or decoders)


Signal Processing


PCM transmission to the DAC

What? An Operating System???


Well, yes, you need an operating system in
order to:


Tell the user what is available to play


Get user input


Figure out what that means


Find the data


Play it, decoding it and processing it if necessary


and a file system


The operating system has to know


Where to find the list(s) of songs


Where the music is stored


How big it is


Where to read it from


Any metadata (album art, whatever)



That’s what file systems are for

The “decoder”


Most (nearly all) music is stored on portable
players in a compressed form. There are two
kinds:


Lossy perceptual coders


Lossless coders



Both of these require decoding in order to
recover the PCM data.

What’s the difference?


Lossless coders are simple, they reduce the bit rate in a
fashion that preserves the PCM data exactly.



Perceptual coders, on the other hand, attempt to remove
parts of the signal that are not PERCIEVED


They are
lossy
, you can never get the PCM data back exactly.


Done right, they sound very similar, if not identical, to the
original.


They have a much lower data rate than lossless coders



MP3, AAC, WMA (and Pro), AC3, AC2,
Ogg
-
Vorbis
, and DTS are
all
lossy

coders of various rates, qualities, and designs.

What’s the implication here?


Typically, the more efficient the encoding, the
more complex the encoder and decoder


This is not quite exact, due to differences in
entropy coding methods


These coders take CPU.

Why does CPU matter?


Physics.



The more you use the CPU, the more you use the
battery


The more you use the battery, the more often you
need to charge/replace it.


More about Coders


Nope, not here.



I have a 3 hour basic tutorial on coding.


I haven’t given it here, but I don’t sense a huge
desire from the audience


I hope!




That’s for another day

Some notes on coding


Many players will set a default bit rate for encoded audio
that maximizes the amount of music you can put on your
device.



This may not be the ideal setting for your own listening
pleasure.



Always check the default rate, and if you can notice audio
quality problems, increase it to the maximum your player
can accept.


This cuts down storage.


With modern storage capacity, this doesn’t matter a whole lot
any more.

EVIL Coding mistakes


Using more than one layer of perceptual
coding, i.e.


First, code into MP3


Then, to get it into your device, use WMA


OR VICE VERSA



Using codecs sequentially is wrong. It is evil. It
is not to be done. Don’t do that.

Signal Processing


Wait! Won’t that use more CPU and more
battery?


Yep. Hence the rudimentary DSP in most players.



The most common device is an equalizer, with
some number of fixed profiles.



Some Chinese players also have:


Externalization (i.e. “not in the head”) processing


Loudness adjustment (level compression based on
loudness)

Level Control


In many players, the level control is performed
digitally on the PCM signal. This is simply one
more multiply/sample on the PCM output
samples.



This means that when the volume control is low,
the SNR out of the DAC isn’t very good.


Some players use a control integrated into the
DAC.

PCM Output


Because of the way the disc/memory fetching
and processing work, PCM is usually
generated in “blocks”.


A final process (often hardware) spreads out the
blocks into individual samples to send to the DAC.


Various other hardware
-
related things happen
here, exactly what depends on what kind of
hardware is in the player.

The Control System (User Controls)

Screen and
controls

Operating system

More on the control issue


This is a major marketing point


I’m not even going to try to cover the various
methods


Some players can now decode video


Some have no display, and only “forward” and
“reverse”



To each his own

The Power Amplifier


Now, recall:


The biggest problem in portable players is POWER


Headphones require power


Not much, but even sensitive modern headphones
require many milliwatts in order to get to levels
that can cause hearing damage


Most players will put out that much, too.


FOR SHAME!

The Power amplifier


Many are “switching” amps, some are linear.



They all use many tricks to get power
dissipation down.


Low idle current


i.e. crossover distortion


Low current capability


Problems with low impedance (32 ohm) phones

The output capacitor


Nearly all power amps are also single
-
ended
amps that run from a single power supply


They have a DC blocking capacitor per channel


This is expensive, and requires volume


These capacitors are often, well, to put it not too kindly,
cheaped out on.


This means that LF response varies with headphones.


Digital equalizers can only fix this to some extent. As
you ‘correct’ peak level goes down


Bass is the most energetic part of most audio

More on the capacitor


They don’t
match,either

capacitance or ESR.


Frequency response mismatches


Level mismatches



When running into 10K loads, this improves
gigantically.


Then the crossover distortion that sometimes
exists will get you.


Still, using line
-
level outputs is usually better.

Some measurements on devices.

(thanks to Bob Smith)

Frequency

Response

IMD

THD

Crosstalk

Red:
Headphones

Black: No load

And, some more measurements

(thanks to Rick C)

Headphone

Line (100k)

To summarize

1.
Portable players usually use lossy coding


so
they generally cause at least some audio
impairment

2.
Portable players are generally not intended
for “critical” uses. N.B. a few, special,
expensive ones may be.

3.
The headphone amp vs. power issue also
reduces quality.

So, what are they good for?


Playing music in an easily portable fashion.




What problem can this cause?


When you listen too loud, you go slowly deaf.


You might not be paying attention to that siren
behind you


Or the car honking its horn

(((

Be Careful

Out There!