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Mid-September 2014
What Is, And Is Not, High-Resolution Audio
Article By John Siau of Benchmark Media Systems
The music industry is struggling to define
High-Resolution Audio or "HRA". In doing so, most have focused on the
delivery formats - analog vs. digital, 24-bits vs. 16-bits, 1X vs. 2X and 4X
sample rates, PCM vs. DSD, uncompressed vs. compressed.
But, High-Resolution Audio is much more than the
delivery format. Delivery formats may limit
resolution, but they do not define the resolution delivered to your ears.
To understand this, it may be easier to define what is not
High-Resolution Audio.
High-Resolution
Audio Is Not
 | A
24-bit recording played through the headphone jack on a portable device |
| A
native 192 kHz recording played through laptop speakers |
| A high-quality
CD recording playing through a typical car stereo |
These first three examples are limited by the quality of
the playback hardware.
It is nearly impossible to build a low-power portable device
that achieves much more than 16-bit performance. 24-bit audio provides little
value when played through low-power, low-voltage portable devices.
Likewise, the bandwidth provided by a 192 kHz sample rate is
of no use when played through the speakers on a laptop computer. The 44.1
kHz CD sample rate is more than sufficient when the sound will be delivered by
the laptop speakers. High sample rates provide no value when speaker response is
very limited.
Similarly, the noisy car environment limits the
playback experience. In this small and noisy car environment, the CD format
is not even close to being a limiting factor in the playback resolution.
High-resolution recordings may provide no audible
improvement when played through small portable devices, through small speakers,
or in noisy environments.
High-Resolution
Audio Is Not
| A
192 kHz up-sampled version of CD |
| An
up-sampled and "restored" version of an MP3 |
| A
high-resolution digital copy of a vinyl record |
| A
high-resolution digital copy of an analog tape |
A Low-Resolution Source Cannot Be Enhanced to Make it High-Resolution
These four examples are limited by the quality of the source.
A 192 kHz up-sampled conversion of a CD will never be better
than the original CD. The added processing will actually decrease the
quality. In some cases, the decrease in quality may be noticeable.
MP3 compression is "lossy compression". Some of the
musical details are lost forever and cannot be recovered by some fancy
processing scheme. Lost is lost. The processing may change the way the recording
sounds (sometimes for the better), but it cannot recover the details that were
lost when the MP3 compression was applied. High-Resolution Audio is all about
details, and these details have been permanently removed by the MP3 process.
Vinyl records have a certain appeal, but they are not
high-resolution recordings. Vinyl records have very specific performance
limitations. They contain noise levels that are much higher than a CD, they have
limited stereo separation, and they impose constraints on the upper and lower
ends of the audio spectrum. High-amplitude signals cannot be recorded on vinyl
at either end of the audio spectrum. The standard CD format exceeds the
capabilities of vinyl in nearly all respects. Transfers from vinyl cannot
be considered High-Resolution Audio because they don't even approach the
measured performance of the CD format. However, a high-resolution copy will
capture everything that is recorded on a record without altering the sound.
The unique vinyl sound can be accurately captured and reproduced by a
high-resolution format. However, this does not mean that the end result is a
high-resolution version of the original performance.
Analog tape may exceed the frequency response of the CD, but
it cannot achieve the noise performance of the 16-bit PCM encoding used on the
CD. A high-resolution digital copy of an analog tape may provide a wider
frequency response than a CD, but it will contain more noise, distortion, and
time-base errors than an all-digital recording. These defects probably
disqualify tape from the high-resolution recording and playback chain.
Nevertheless, a high-resolution digital copy is valuable in that it preserves
and transmits everything that was captured on the original tape.
High-Resolution
Audio Is Not
| Robust |
Any single low-resolution device or process in the recording
and playback chain is sufficient to render a low-resolution result. Noise,
distortion, and frequency response accumulate with each processing step.
In most cases, noise cannot be removed once it is added. Every
component or process in the signal chain adds some noise. Long signal chains
require very good noise performance at each processing step in order to achieve
a noise performance that exceeds capability of the 16-bit CD. This can and is
being done, but it is not easy. Most consumer playback systems cannot even
achieve the equivalent of CD-quality performance.
Likewise, distortion cannot be removed once it is added. Every
component and process is important in order to deliver a clean and accurate
reproduction of the original performance.
Frequency response is lost if any portion of the signal chain
has a hard-limit on the high-frequency response. Digital sample rates impose
absolute upper limits on the frequency response. If a 44.1 kHz sample rate is
used anywhere in the chain, the high-frequency limit of the system will be
22.05 kHz. Under such circumstances, nothing above 22.05 kHz can be recovered.
MP3 compression imposes lower limits than the CD format.
Lossy compression systems (such as MP3) may entirely remove
some low-level details from the audio.
High-Resolution
Audio Is Not
| Perfect |
High-Resolution Audio may
not have Audible Defects, but it is not Perfect
When the CD format was introduced, one reviewer called it
"perfect sound forever". We have since come to understand that the CD
format is a nearly-transparent delivery format with some slightly audible
defects. At high playback levels, the noise floor of the 16-bit encoding can be
audible. Likewise, the 22.05 kHz upper limit of the frequency response is close
enough to the limits of human hearing, that it may have some audible impact on
the listening experience.
In contrast, 24-bit encoding offers an SNR that is far higher
than the difference between the threshold of hearing and the threshold of pain.
While 24-bit encoding is not perfect, it is actually better than necessary.
Likewise 96 kHz sample rates have a usable bandwidth of almost
48 kHz - more than double the limit of a normal ear. The extra margin
between the 22.05 kHz limit of the CD and the 48 kHz limit of 96 kHz
high-resolution systems is more than adequate to transmit anything that we can
hear.
High-Resolution Audio is Not:
| The
only way to enjoy music |
Fortunately Music is Still
Enjoyable through Low-Resolution Systems!
We have all had the experience of being emotionally moved by
music that was played through poor-quality playback systems. Car radios, MP3
players, and low-resolution streaming audio all have a place.
For example, many people became fans of the Beatles while
listening on cheap AM transistor radios. They rediscovered their favorite tunes
when they purchased albums and upgraded their stereo systems. With the improved
systems they discovered details that they had never heard while listening to the
same song hundreds of times on a cheap radio. Today we have 16-bit and 24-bit
digital releases of classic Beatles recordings dubbed from carefully restored
master tapes. These digital releases may or may not fully meet the definition of
High-Resolution Audio, but they capture far more detail than the original vinyl
releases. It is hard to listen to one of these newer releases without
discovering wonderful details that were overlooked in many years of
low-resolution playback.
24-bit high sample-rate recordings are capable of delivering all
of the details captured in the studio while low-resolution formats make music
available in more places.
Will The
Public Embrace High-Resolution Audio?
Given an opportunity to hear the difference,
most listeners appreciate improved resolution. But High-Resolution Audio is
overkill for products that barely achieve CD-quality playback. The same audible
improvements could be made by transitioning from MP3 delivery to loss-less
CD-quality delivery. In the short term, CD-quality or near CD-quality
delivery has a greater chance of success than High-Resolution Audio delivery.
Even so, most consumers will only move to CD-quality if it is no extra cost and
no extra hassle. Convenience and cost trump quality for most consumers.
High-Resolution
Audio From An Audiophile Perspective
Audiophiles have eagerly embraced High-Resolution Audio
formats, but many have not experienced the full capabilities of these new
formats. They would do well to focus on acquiring playback equipment with true
high-resolution performance. 24-bit audio provides no benefit if the power
amplifier can only deliver 17-bit (103 dB) signal to noise ratios. Likewise,
high sample rates are useless when played through speakers having an 18 kHz top
end.
A good CD played through a high-resolution system easily
outperforms a High-Resolution Audio recording played through a low-resolution
system.
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