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Technical Paper 3. HD Audio: 'Ringing' why it is undesirable and how to address it

Definition:

AMR has spent the past few years judiciously testing, measuring and listening to each and every available HD DAC. Once we cherry-picked the chipset that was the musical, we 'threw out the textbook', literally.

We implemented the same exacting approach taken for the Philips TDA1541A in the CD-77 and the UDA1305AT in the CD-777. We discarded the textbook/datasheet schematic approach, which would have been far, far easier as it would have replicated known and tested circuitry that the DAC manufacturer guarantees will work. It is a route often taken, however it would have led to a mediocre quality of sound that was not in keeping with AMR's exacting standards.

    Ringing: verb (used without object)
  1. 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 sound.
  3. to have the sensation of a continued humming sound.

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 as "echos" near transients

BEFORE the transient it is called Pre-Ringing; AFTER the transient it is called call Post-Ringing. 

This only has Post-Ringing.

post-ringing
Source: AMR

This has both Pre and Post-Ringing.

pre-post-ringing
Source: AMR

Post-Ringing

Post-Ringing is found in mother nature, with most 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 leading edge of a pulse. The causes can be mechanical or electrical. Many moving coil cartridges when playing vinyl exhibit strong ringing (from the stylus tip/vinyl resonance). Moving Coil 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.

So the presence of some “Post-Ringing” in an audio component as such, does not necessarily disqualify it from offering high 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 Pre-Ringing, the situation is somewhat different; Pre-Ringing is something that appears to violate the fundamental law of the universe that an effect is preceded by its cause, called causality.  In Pre-Ringing, the effect (ringing) happens BEFORE the cause (the transient).

For those who fear the universe will fold in on itself and end tomorrow, now that the law of causality has been so rudely violated by 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 transient.

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:

  • 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.

graph
Source: AMR

  • 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

 graph1
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 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:

pre-ringing
Source: AMR

The clean and "square" green pulse represents the 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.

1The music signal is much more akin to a transient than a steady state sine wave often used in the laboratory.

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

Traditional Approach

New Technology

Improvements

  1.  

Phase

Linear Phase
(No phase distortion)

Minimum Phase
(Some phase distortion)

Eliminates Pre-Ringing

  1.  

Cut-off Frequency

Half-band
0.5 Sampling Frequency
(e.g. for 44.1kHz sampling rate, the cut-off frequency is at 22.05kHz).

Apodising
<0.5 Sampling Frequency
(e.g. for 44.1kHz sampling rate, the cut-off frequency is at ~19KHz instead of 22.05kHz).

Attenuates the Analogue-to-Digital Converter’s ringing embedded in recordings

  1.  

Roll-off

Sharp/Steep Roll-off
Brickwall

Soft Roll-off

Reduces ringing magnitude and length in time


Source: AMR

AMR has made the above new technologies available on the DP-777 within different filter settings.

 

Model

 

Filter Name

Digital Filter Technology

Minimum Phase

Apodising

Soft Roll-Off

AMR DP-777

Organic

tick

tick

tick

 

Apodising 808

tick

tick

-

 

MP Listen

tick

-

tick

 

Traditional

-

-

-

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.

 

Under the DP-777’s HD 32-Bit DAC’s ‘Organic’ Digital Filter, it is the only high-definition DAC chipset that addresses all three crucial areas of ringing.

 

Pat Wayne
Director – Engineering
e: patw@amr-audio.co.uk
t: +44 (0) 01704 227 204;

     
 
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