e015020.PDF

READERS’ CORNER

P.O.Box 190

We can only answer questions or remarks of general interest to our readers, concerning projects not older than two

years and published in Elektor Electronics . In view of the amount of post and email received, it is not possible to

answer all correspondence, and we are unable to respond to individual wishes and requests for modiﬁcations to,

or additional information about, Elektor Electronics projects.

How much current

from the USB?

USB port is 500 mA, and no fur-

ther devices may be powered

once this current is reached.

Depending on their current

requirement, USB devices fall

into one of three classes:

1. low bus powered

2. high bus powered

3. self powered

Devices from the first two

classes draw all necessary cur-

rent from the USB port. Self pow-

ered devices, on the other hand,

have an internal power supply

and require little or no power

from the USB. A unit load of

100 mA has been deﬁned for the

USB. Devices from the ﬁrst class

(low bus powered) are only

allowed to draw one unit, i.e.,

100 mA. The same goes for high

bus powered devices (class 2)

when in the pre-configuration

state. Up to 5 load units are

allowed in class 2 devices when

conﬁgured.

The amount of current an USB

device is allowed to draw from a

port depends on ﬁve factors:

A. Device status

– power-on: max. 1 load unit

– conﬁgured: 1 load unit

(100 mA) for low-power

devices, or up to 5 load units

(500 mA) for high power

devices provided they have

been allowed to do so by the

– suspended: not exceeding

500 mA including current

through pull-up to D+ or D–.

signal of a CD player playing

Jean Michel Jarre’s ‘Oxygen 7-

13’. It was clear that the peaks

of the waveform were flat where

the signal was greater than

what could be reproduced by the

CD. This is also known as clip-

ping.

Using an oscilloscope in this way

can’t really tell us how much the

CD player affects the signal qual-

ity. Between the CD and oscillo-

scope will be an oversampling

digital filter, a digital-to-analogue

converter and finally an ana-

logue filter. The most accurate

way to check a CD is to extract

the track digitally and store it as

a WAV file. This will be an exact

digital copy of the CD, assuming

there were no read errors. The

creation of the WAV file shouldn’t

be a problem since it is a func-

tion that is found in most cur-

rent CD ‘rippers’. A computer

program can then be used to

analyse the contents of this WAV

file.

A WAV file consists of a header

followed by the thousands of

samples that make up the digi-

tised sound. The 16-bit format

used on CDs gives these samples

a range from -32768 to 32767. In

order to have a closer look at

these WAV files I have written a

program that can analyse them.

As the first step, the program

creates a histogram of the WAV

file. This counts how often each

sample value occurs in the WAV

file, and gives a display as shown

in Figure 1.

The values near zero occur most

often; the more you go to the

extremes of –32768 and 32767,

the less often they occur. If there

was any clipping at the record-

ing stage, all strong signals will

be rounded to the extremes of -

32768 and 32767, which results

in two peaks at the ends of the

histogram. This way you can see

if there is any clipping present

and at what level (it is possible

Dear Sir — in the article ‘USB

Audio-DAC’ (December 2000),

you state on page 45 that the

current consumption of the cir-

cuit (60 mA), is such that it was

not considered appropriate to

power it from the USB port,

which has a maximum spec of

100 mA in this respect. Accord-

ing to information I have avail-

able, a USB device may draw up

to 500 mA. For example, the

Canon CanoScan N650U gets its

complete power requirement of

2.5 watts (500 mA x 5 V) from a

USB port. This works just fine,

even if an additional USB camera

is connected. I would therefore

be interested to know where you

got the 100 mA spec from?

M. Radde (by email)

B. Hub type used to connect the

device. Up to 500 mA allowed

using a hub with internal power

supply. If the hub is bus pow-

ered, this is reduced to 100 mA.

A hub is a PC is normally con-

sidered self-powered, while a

hub in a portable device is bus

powered. When a device draws

to much current, it may happen

that the USB goes along by sup-

plying more current than is

strictly allowed. None the less, it

is recommended to provide for

an internal power supply.

C. Conﬁgured and running: cur-

rent consumption should not

exceed the level reported by the

device (by means of the enumer-

ation in the configuration

descriptor) during the conﬁgura-

tion phase.

The speciﬁcation ‘100 mA’ is the

current a USB port is always

capable of supplying. During ini-

tialisation, a higher current is

allowed. Once initialised, a USB

device requiring more current

may draw more current after

reporting its requirement to the

operating system. The limit of an

The same restrictions in respect

of current demand apply to

devices with an internal pow-

ered supply (self powered). Good

separation is essential between

the USB and the external power

supply. In the event of the inter-

nal supply failing, the device

may not present a heavier load

the USB than ‘promised’ during

conﬁguration (i.e., not more than

one load unit of 100 mA when

configured as self-powered or

low power bus powered).

CORRECTIONS &UPDATES

Free Downloads

now supplied as zip ﬁles

MCS BASIC-52 V1.3

On page 22 of the article, the

sentence ‘The PROG commands

are also redundant, so they

have been deleted’ should read

‘The FPROG commands…’

Having obtained a licence to

distribute .zip files via the Free

Downloads section of the Elek-

tor Electronics website, we are

pleased to announce that all

previously supplied .exe self-

extracting archive files have

been replaced by .zip equiva-

lents. The program needed to

unpack these files is called

Winzip and may be downloaded

free

MIDI Lights & Slide Control

The SAB80C535 used in this

project may is not easily

obtainable and may be replaced

by the more commonly avail-

able and fully compatible type

SAB80C515-N.

CD Recording Quality

Dear Editor — roughly a year

ago you had an article in Elektor

Electronics about CDs that were

supposedly recorded with over-

driven signals. An oscilloscope

was used to look at the output

charge

from

http://www.winzip.com .

Elektor Electronics

5/2001

READERS’ CORNER

From this I conclude that the

original mix, which was recorded

perfectly, was remixed with an

amplification of 1.5, causing clip-

ping in the new recording.

The track “Gloria, lonely boy” on

the CD “Metamorphose” by Jean

Michel Jarre was then analysed,

in order to see if his latest CD

had signs of clipping. Here too it

seemed that the recording was

overdriven. But once more it was

the histogram that returned

some fascinating information

(Figure 3).

The smallest and largest sam-

ples found in the recording were

-32022 and 32021 respectively

(0.977 of the maximum range).

These were also the levels at

which clipping occurred. From

the histogram it can be seen

that every fortieth value is twice

as large as its neighbours are.

From these observations you can

conclude that this CD is a remix

of an original CD recording. The

samples have been multiplied by

0.977 before being converted

back into CD format.

This raises the question why this

meaningless conversion was car-

ried out on the recording. The

only reason I can think of is that

the remix was made to hide the

fact that the recording was over-

driven. Anybody who uses an

oscilloscope to look for clipping

would set the trigger level at the

largest possible signal level,

about 99.9% of the maximum

signal possible. The remix would

cause the signal to remain below

this level so the oscilloscope

would never trigger and the clip-

ping would remain undetected.

The person making the remix

would have known that clipping

occurred and was determined to

hide this fact. The correct proce-

dure would have been to make a

slightly softer remix from the

master tape and avoid any clip-

ping that way.

The record industry is currently

promoting the new Super Audio

CD (SACD), which has an

improved quality. They would be

better off stopping the messing

around with the recordings.

These fudges also sound bad in

SACD format and making careful

recordings in CD format will

result in bigger improvements

than the change to SACD format.

A. Kappert, Netherlands

We’ve said it before and we’ll

say it again: We think that the

price of a CD is high enough to

expect the best quality record-

ings. Initiatives such as these by

quality conscious music lovers

should therefore be applauded.

Mr. Kappert has been kind

enough to make his analyser

program available to Elektor

Electronics readers free of

charge. The program can be

downloaded from our website at

www.elektor-electronics.co.uk . It

can be found as ‘CD-analyser’

on the Free Downloads page,

May 2001 items.

for it to occur at different values

than -32768 and 32767, due to

badly set up A/D converters).

Next, the program displays the

actual waveform stored in the

WAV file. This window has a

search facility that searches for

clipping values, as determined by

the previous histogram function.

I have used this program to

analyse the track “...Baby One

More Time” by Britney Spears,

from her CD “...baby one more

time”. It was obvious that there

was a lot of clipping in this

track, but the display returned

by the histogram was the most

surprising.

This is shown in Figure 2. It con-

tains the number of occurrences

in the WAV file for samples with

values from 4 to 515. After every

two values that occur over a

1000 times, there follows a value

that rarely or never occurs at

all. You could only get such a his-

togram when a 16-bit recording

is re-recorded after amplification

by a factor of 1.5. Samples with

values of 1, 2, 3, 4, 5, 6, 7 etc.

are multiplied by 1.5 to give 1.5,

3.0, 4.5, 6.0, 7.5, 9.0, 10.5 etc.

These values then have to be

rounded to integers before being

written to the CD. This gives the

result 2, 3, 5, 6, 8, 9, 11 etc.

You’ll see that every third value

is missing from this series of

numbers. By looking at the wave-

form where clipping occurs, it is

possible to deduce by how much

the recording of the CD was over-

driven. I estimate this to be a

factor of about 1.5.

5/2001

Elektor Electronics

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