What High-Resolution Audio is NOT

by

John Siau

 

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

Low-Resolution Hardware Cannot Deliver High-Resolution Audio

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

High-Resolution Audio Requires High-Resolution Performance at all Stages of the Recording and Playback Chain

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.

Additional Reading

"Creating a High-Resolution 5.1 Music Server"

This application note provides a guide for setting up a 5.1 music server that can play the lossless high-resolution audio tracks found on DVD and Blu-ray disks.

 

 John Siau




John Siau
John Siau

Author

John Siau is VP and Director of Engineering at Benchmark Media Systems, Inc.