Technical Paper 3. HD Audio: ‘Ringing’
why it is undesirable and how to address it
Definition:
Ringing:
1.
Post-Ringing
– verb (used without object)
to give forth a clear resonant sound, as a
bell when struck: The doorbell rang twice.
2.
to
be
filled
with
sound;
re
echo
with
to
have
musical instruments (e.g. Piano, violin etc.) and is
quite
often
found
in
HiFi
systems.
It
is
quite
literally, the delayed release of energy from the
sound.
3.
Post-Ringing is found in mother nature, with most
the
sensation
of
a
continued
leading
edge
mechanical
humming sound.
of
or
a
pulse.
The
electrical.
causes
Many
can
be
moving
coil
cartridges when playing vinyl exhibit strong ringing
Ringing (electronics):
ringing artifacts are artifacts
that appear as spurious signals ("rings") near sharp
transitions
(transients)
in
a
signal.
Visually,
they
appear as "rings" near edges; audibly, they appear
the
transient
it
Step-up
Transformers
and
Transformer
Volume
Controls may also show appreciable ringing. Even
recording microphones have resonances that can
induce ringing when exposed to sharp transients.
as "echos" near transients
BEFORE
(from the stylus tip/vinyl resonance). Moving Coil
is
called
Pre-Ringing;
AFTER the transient it is called call Post-Ringing.
So
the
audio
presence
disqualify
This only has Post-Ringing.
of
component
it
some
as
from
“Post-Ringing”
such,
does
offering
not
high
in
an
necessarily
sound
quality
(though an excess of ringing at frequencies close
to the audible range can cause problems).
Pre-Ringing
Only
systems
with
digital
filters
(i.e.
digital
electronics) may exhibit Pre-Ringing. But with PreRinging, the situation is somewhat different; PreRinging is something that appears to violate the
fundamental law of the universe that an effect is
Source: AMR
preceded
by
its
cause,
called
causality.
In
Pre-
Ringing, the effect (ringing) happens BEFORE the
This has both Pre and Post-Ringing.
cause (the transient).
For those who fear the
itself
and
causality
end
has
universe
tomorrow,
been
so
now
rudely
will
that
fold in on
the
violated
law
by
of
the
designers of digital filters, rest safely. This violation
is only apparent, not actual; causality as such is
preserved in time. In actual fact, the bulk of the
“cause”,
namely
the
complete
transient
is
DELAYED and only a small fraction of its energy is
leaked
at
the
instant
the
transient
should
have
been, the rest progressively “oscillates up” to the
Source: AMR
transient.
1
To illustrate this more clearly, imagine a film (with
sound) of one guitar string being plucked and the
trace of the signal on an oscilloscope, shown
below is the oscilloscope trace:
Pre-Ringing and Post-Ringing at a frequency that is
just outside (above) the audible range (22.05KHz
or a little lower depending on the precise filter
design). This then gives this picture:
At the instant of plucking the string there is sharp
peak in the signal, the characteristic sound of the
string being struck, followed by the sound (ringing)
of the string’s tuned frequency which slowly decays
into nothing.
Source: AMR
Source: AMR
The clean and “square” green pulse represents the
1
original signal/music transient , the red is the
output from the digital filter. Notice how the
ringing starts at the instant of the pulse and how
the signal from the digital filter slowly “rings up” to
a substantial fraction of the signal before the
transient, which has turned into a spike, not a
clean pulse.
To understand Pre-Ringing, just run the film in
reverse (ignore the dotted lines). Our string starts
to vibrate slowly and builds up in level and only
then comes the sound of the string being struck,
basically as shown above
Source: AMR
Reverse Guitar Pluck?
Can you imagine how a guitar would sound like
that? Pre-Ringing is literally counter to nature and
it is severely different from our experience of
natural sounds, which may include Post-Ringing,
but NEVER Pre-Ringing. Pre-Ringing is just plain
bad news for audio playback and has long
contributed to the “Digital” sound of the CD
medium. Never has the phrase “wrong note at the
wrong time” been more appropriate.
Moreover, the common “Brickwall” digital filters
for CD have absolutely MASSIVE amounts of both
1
The music signal is much more akin to a transient than a
steady state sine wave often used in the laboratory.
2
New Digital Filter Technologies
Since the early days, Digital Filter Technology has
come a long way and within the last 10 years, three
relatively
new
and
innovative
technologies
developed for the digital filter have come to the
fore.
Signal
Characteristic
1. Phase
Traditional Approach
New Technology
Linear Phase
Minimum Phase
(No phase distortion)
(Some phase distortion)
<0.5 Sampling Frequency
0.5 Sampling Frequency
(e.g. for 44.1kHz sampling rate, the
(e.g. for 44.1kHz sampling rate, the
cut-off frequency is at ~19KHz
cut-off frequency is at 22.05kHz).
Attenuates the Analogue-to-Digital
Converter’s ringing embedded in
recordings
instead of 22.05kHz).
Sharp/Steep Roll-off
3. Roll-off
Eliminates Pre-Ringing
Apodising
Half-band
Cut-off
2.
Frequency
Improvements
Reduces ringing magnitude and
length in time
Soft Roll-off
Brickwall
Source: AMR
AMR
has
available
made
on
the
the
above
DP-777
new
within
technologies
different
filter
settings.
Digital Filter Technology
Model
Minimum Phase
Filter Name
Apodising
Soft Roll-Off
2
AMR DP-777
Organic
Apodising 8083
-
MP Listen
-
Traditional
-
-
-
Source: AMR
Under
the
Digital
Filter,
chipset
that
DP-777’s
it
is
HD
the
addresses
32-Bit
only
all
DAC’s
‘Organic’
high-definition
three
crucial
DAC
areas
of
ringing.
Pat Wayne
Director – Engineering
e: [email protected]
t: +44 (0) 870 420 550
2
3
DP-777’s default Digital Filter for
HD Audio.
This Digital Filter is available only on 44/48kHz with the
HD 32 bit DAC, not available for 88.2/96kHz or higher.
3