background image

push-pull wiring of the coils
“reduces harmonic distortion
induced by the non-linearity of
the magnetic field.”

2

1

2

3 4 5

6

7

8

S

S

S

N

N

N

N

Figure 3.3: The magnetic circuit repre-

sentation of the MMC cartridge, show-

ing the diagonal pair of pole pieces for

one of the two audio channels.

1

2

3 4 5

6

7

8

Figure 3.4: The Micro Moving Cross MMC

4000 cartridge design. 1. Nude Pra-

manik diamond, 2. Low mass beryllium

cantilever, 3. Moving micro cross, 4.

Block suspension, 5. Pole pieces (4), 6.

Induction coils, 7. Mu-metal screen, 8.

Hycomax magnet

Figure 3.5: Large-scale models of the

MMC cartridges used for past demon-

strations.

3.2 Signal Levels

Every audio device relies on a rather
simple balancing act. The “signal”,
whether it’s speech, music, or sound

e

ects, should be loud enough to mask

the noise that is inherent in the
recording or transmission itself. The
measurement of this “distance” in level
is known as the

Signal-to-Noise Ratio

or

SNR

. However, the signal should not

be so loud as to overload the system
and cause distortion e

ects such as

“clipping”, which results in what is
commonly called

Total Harmonic

Distortion

or

THD

.

3

One basic method

to evaluate the quality of an audio
signal or device is to group these two
measurements into one value: the

Total Harmonic Distortion plus Noise

or

THD+N

value. The somewhat

challenging issue with this value is that
a portion of it (the noise floor) is
typically independent of the signal
level, since a device or signal will have
some noise regardless of whether a
signal is present or not. However, the
distortion is typically directly related to
the level of the signal.

In modern digital PCM audio signal
(assuming that they are
correctly-implemented and ignoring
any additional signal processing), the
noise floor is the result of the dither
that is used to randomise the inherent
quantisation error in the encoding
system. This noise is independent of
the signal level, and entirely
dependent on the resolution of the
system (measured in the number of
bits used to encode each sample). The
maximum possible level that can be
encoded without incurring additional
distortion that is inherent in the
encoding system itself is when the
maximum (or minimum) value in the
audio signal reaches the highest
possible signal value of the system.
Any increase in the signal’s level
beyond this will be clipped, and
harmonic distortion artefacts will
result.

Figure

3.6

shows two examples of the

relationship between the levels of the
signal and the THD+N in a digital audio
system. The red line shows a 24-bit
encoding, the blue line is for 16-bit.

The “flat line” on the left of the plot is
the result of the noise floor of the
system. In this region, the signal level
is so low, it’s below the noise floor of
the system itself, so the only
measurable output is the noise, and
not the signal. As we move towards
the right, the input signal gets louder
and raises above the noise floor, so the
output level naturally increases as
well. However, in a digital audio
system, we reach a maximum possible
input level of 0 dB FS. If we try to
increase the signal’s level above this,
the signal itself will not get louder,
however, it will become more and
more distorted. As a result, the
distortion artefacts quickly become
almost as loud as the signal itself, and
so the plots drop dramatically.

This is why good recording engineers
typically attempt to align the levels of
the microphones to ensure that the
maximum peak of the entire recording
will just barely reach the maximum
possible level of the digital recording
system. This ensures that they are
keeping above the noise floor as much
as possible without distorting the
signals.

-160

-140

-120

-100

-80 

-60 

-40 

-20 

0   

Input level (dB)

0

20

40

60

80

100

120

140

Level of Signal relative to Dist Noise (dB)

Figure 3.6: Two examples of the rela-

tionship between the levels of the sig-

nal and the THD+N in a digital audio

system. These are idealised calcula-

tions, assuming TPDF dither in a “per-

fect” LPCM system. The red line shows

a 24-bit encoding, the blue line is for 16-

bit.

Audio signals recorded on
analogue-only devices generally have
the same behaviour; there is a noise
floor that should be avoided and a

3

The assumption here is that the distortion produces harmonics of the signal, which is a simplified view of the truth, but one that is easy to measure.

8

Содержание Beogram 4000c

Страница 1: ...Beogram 4000c Technical Sound Guide Bang Olufsen A S This manual is for information purposes only and is not legally binding November 27 2020...

Страница 2: ...ross 7 3 2 Signal Levels 8 3 3 Tip shape 10 3 4 Bonded vs Nude 12 3 5 Tracking force 12 3 6 E ective Tip Mass 12 3 7 Compliance 13 3 8 Soundsmith SMMC20CL 13 4 Audio Specifications 14 4 0 1 Magnitude...

Страница 3: ...Emil Berliner was awarded a patent for a sound recording and reproducing system that was based on a groove in a rotating disc rather than Edison s cylinder the original version of the system that we k...

Страница 4: ...are the same at Time 0 ms However it is also evident that when this is true they have very di erent amplitudes in fact the amplitude would have to double for every halving of frequency a drop of 1 oc...

Страница 5: ...ns of the RIAA equalisation filter define the transition points as time constants instead of frequencies So instead of 50 Hz 500 Hz and 2122 Hz as shown in the response plots the points are listed as...

Страница 6: ...down left up right for example This means that signals that are identical in both channels move the stylus laterally exactly as in earlier monophonic discs 4 Figure 2 10 An over simplified depic tion...

Страница 7: ...eophonic Transducer Cartridge In 1963 Erik R rbaek Madsen of Bang Olufsen filed a patent for a cartridge based on the Moving Iron principle In it a cross made of Mu metal is mounted on the stylus Each...

Страница 8: ...be encoded without incurring additional distortion that is inherent in the encoding system itself is when the maximum or minimum value in the audio signal reaches the highest possible signal value of...

Страница 9: ...here is a relationship between the total playing time of a vinyl disc and the modulation velocity In order to have 20 minutes of music on a 12 LP spinning at 33 1 3 RPM then it the standard method was...

Страница 10: ...here are a number of options when choosing the shape of the playback stylus 3 3 Tip shape The earliest styli were the needles that were used on 78 RPM gramophone players These were typically made from...

Страница 11: ...k deeper into the groove making it more di cult for it to move independently on the two audio channels The second is that the point of contact between the stylus and the vinyl becomes smaller which ca...

Страница 12: ...pin which is in turn connected to the cantilever the long arm that connects back to the cartridge housing This bonded design is cheaper to manufacture but it results in a high mass at the stylus tip...

Страница 13: ...the compliance of the stylus suspension Compliance is the opposite of spring sti ness the more compliant a spring is the easier it is to compress and the less it pushes back Like many other stylus pa...

Страница 14: ...of 70 7 mm sec 4 0 3 Rotational speed Every recording playback system whether for audio or for video signals is based on the fundamental principle that the recording and the playback happen at the sam...

Страница 15: ...changes in the speed that are higher than 100 Hz This is typically only a problem with analogue tape decks caused by the magnetic tape sticking and slipping on components in its path and is not often...

Страница 16: ...nal relative to the average frequency however the e ect of very slow and very fast changes have been reduced by the filter Finally the standard deviation of the variation from the average is calculate...

Страница 17: ...mith SMMC20CL Stylus Nude 0 12 mm square Radius of curvature Contact Line Recommended tracking force 1 3 g Compliance 28 m mN E ective tip mass 0 32 mg General information Automatic speed selection Ye...

Страница 18: ...nditions in the Cutting and Playing of Stereo Disk C R Bastiaans Vol 11 Issue 1 Jan 1963 Factors A ecting the Stylus Groove Relationship in Phonograph Playback Systems C R Bastiaans Vol 15 Issue 4 Oct...

Страница 19: ...commendations for Lateral Cut Commercial and Transcription Disk Recordings 98 Processed Disk Records and Reproducing Equipment 386 Method of Measurement of Speed Fluctuations in Sound Recording and Re...

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