Hi-Fi 2.0

Hi-Fi 2.0: Let’s do the math!

by David D. on September 15, 2011

Part 3 in a series: After decades of continual improvements in audio recording and playback technologies, the average 21st Century listener will hear most music at lower fidelity than before the millennium. How did this happen, and can anything be done about it?

In Part 1, we reviewed some of the reasons for the general decline in audio quality.  Part 2 explored the differences between analog and digital audio.  Now it’s time to take a closer look at the various digital formats, do some math, and find out what high fidelity means in the 21st century.

The resolution of a digital audio file is based on three factors: the sample rate, bit depth, and bit-rate.

Sampling Rate (sample rate, or sampling frequency) – the number of times audio is measured (or sampled) per second.  The standard sampling rate for a CD is 44.1 kHz, which means that the audio is sampled 44,100 times per second.

An analog signal (light bue) measured over time at a fixed sampling rate (red).

The sampling rate determines the frequency response — the range of sounds from low to high that a file is capable of reproducing.

Imagine a felt hammer striking a piano string.  A string that is thinner, shorter, or under more tension will vibrate more rapidly, producing a higher frequency or pitch.

The lowest note on a piano (A0) has a frequency of 27.5 Hz, and the A above middle-C (A4) has a frequency of 440 Hz, or 440 cycles per second.  The highest note on a piano is C8,  at 4186.01 Hz, and the normal range for human hearing is from 20 Hz to 20,000 Hz.

According to the Nyquist-Shannon sampling theorem, a sampling rate needs to be twice the frequency of the signal being sampled in order to accurately capture the sound.  So the 44.1 kHz sampling rate used for CDs should be sufficient to capture audio frequencies up to 22,050 Hz, beyond the range of human hearing.

However, some maintain that inaudible frequencies above 22,000 Hz can “color” the sound and affect the lower-range frequencies we do hear.  And the digital recording process can produce distortion through the aliasing of these higher frequencies that requires filtering to correct.

There is no theoretical frequency limit for an analog signal, but the physics of audio reproduction place a practical limit on what can be achieved.  Testing on some analog systems has shown evidence of frequencies up to about 50,000 Hz.

Most professional digital recordings are made at a sampling rate of 96 kHz, so they can capture the high-end audio frequencies that might be found in an analog recording but would be missing from a CD.  And if you absolutely must go higher, you can use a sampling rate of 192 kHz.  At this rate, you are slicing each second of audio into 192,000 pieces, and capturing frequencies up to 96,000 Hz (ouch!).

Bit-Depth -the number of bits used to record each slice of audio.  Think of this as the number of levels available to capture each slice.  Every bit doubles the number of levels: the resolution for 16-bit audio is calculated as 216, giving you 65,536 possible levels.  24-bit audio is calculated as 224, providing over 16 million levels.

Most professionally-recorded digital audio is 24-bit.  Recording at a higher resolution allows for a greater dynamic range (the difference between the softest and the loudest sounds in a recording) and a better signal-to-noise (S/N) ratio (i.e., more signal, less noise).

Sound levels are commonly measured in decibels (dB), and the normal range of human hearing is from 0 dB (threshhold) to 120 dB (hearing damage).  The dynamic range for 16-bit digital audio is 96 dB, and the range for 24-bit audio is around 144 dB. The best most analog formats can offer is a dynamic range of around 60 dB, and there will almost always be more noise present.

By the time all of these calculations end up as air moving from your speakers, compression may have obliterated some of the differences in dynamic range between the formats.  A lot depends on the type of music you listen to — look for an update on the “Loudness War” in an upcoming article.

Bit rate – the number of bits processed per unit of playback time.  For an uncompressed digital audio file, this can be calculated as:

Sample Rate x Bit-Depth x Number of Channels = Bit Rate

Let’s do the math for a CD: 44,100 x 16 x 2 = 1,411,200 bits per second (or 1411 kbps, or 1.4 Mbps).  Compressed audio, such as an MP3 file, is a different story.  The sampling rate for an MP3 file can vary, and there is no equivalent bit-depth, so the bit rate is an indicator of how much compression was applied to the original signal.  A higher bit rate results in a larger file size and greater fidelity to the original sound.  Since a CD has about 11 times the bit rate of an MP3 file, does that mean it sounds eleven times better?

How high is up?

Let’s think about this for a minute.  Higher sample rates and greater bit-depth will result in more information being captured for each sound.  Higher resolution means better sound, but there are limits.  Our ears impose limits: the highest frequencies we can hear drop with age, and some ears are better-trained and more discerning than others.

The recording method and storage media impose another set of limits.  And the playback system comprises a long chain of limiting factors: the playback unit, audio circuitry, DAC, amplifier, wiring, speakers, and more.  The rooms we listen in, and where we sit in those rooms can have a dramatic impact 0n the quality and accuracy of the music we hear.

T Bone Burnett prefers analog, but maintains that if we have to listen to digital audio, we should do so at a minimum resolution of 96 kHz/24-bit. There is a fair amount of controversy over sampling at higher rates, with some engineers and audiophiles claiming that 192 kHz audio is a gimmick, overkill, or “just stupid”.  From one detractor:

Sampling audio signals at 192KHz is about 3 times faster than the optimal rate.  It compromises the accuracy which ends up as audio distortions.  There is an inescapable tradeoff between faster sampling on one hand and a loss of accuracy, increased data size and much additional processing requirement on the other hand.

The optimal sample rate should be largely based on the required signal bandwidth. Audio industry salesman have been promoting faster than optimal rates. The promotion of such ideas is based on the fallacy that faster rates yield more accuracy and/or more detail. Whether motivated by profit or ignorance, the promoters, leading the industry in the wrong direction, are stating the opposite of what is true.

~ Dan Lavry, “Sampling Theory for Digital Audio”

While looking at the above chart, remember that we are comparing apples (uncompressed audio files such as those on a CD) and oranges (compressed files).  While a CD track may contain 11 times the information in a 128 kbps MP3 file, it’s not really a fair comparison.  The compression algorithm is designed to throw away the unimportant and mostly inaudible parts of the music, it doesn’t just randomly remove 90% of the data.

What is HD Audio, and how do I get it?

So the CD might not sound 11 times better, but it definitely sounds better — MP3 files are a step backwards from CD-quality audio.  There are a few competing definitions and formats, but for our purposes, High-Definition (HD) audio will be defined as audio formats that exceed the sampling rate and bit-depth (44.1/16) of the Red Book CD Standard.

There is a high-definition audio specification from Intel for PC audio up to 192 kHz/32-bit for two channels, and 96 kHz/32-bit for as many as eight channels.   But this spec supports sample rates as low as 6 kHz, as well as 8 and 16-bit audio, so it falls outside of our definition.  Oh, and then there’s HD Radio, which has nothing to do with high-definition audio.  HD originally stood for “Hybrid Digital”, and now is just part of the HD Radio trademark and stands for nothing.

We will take a deeper dive into the competing formats for HD Audio in our next installment, and look at the various ways and means to get high-fidelity in the 21st century.  Onward!

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Hi-Fi 2.0: Analog vs. Digital – Everybody Wins! (Loses)

by David D. on September 1, 2011

Part 2 in a series: After decades of continual improvements in audio recording and playback technologies, the average 21st Century listener will hear most music at lower fidelity than before the millennium. How did this happen, and can anything be done about it?

In Part 1, we reviewed some of the reasons for the general decline in audio quality.  Here we will explore the differences between analog and digital audio and sort out the winners and losers.  Let’s start with the basics: what is the difference between analog and digital audio?

The analog world is built from atoms, and the digital world is composed of bits.  This distinction plays out when music is recorded, stored, distributed, and played back.

Recording

All sound waves are analog: an instrument or voice produces a disturbance that moves atoms through a medium. Here on Earth, that medium is usually air.

In an analog recording, these waves are typically picked up by a microphone that converts them into an electrical signal.  Variations in the voltage of the signal are then converted into a continuous physical representation of the sound on lacquer or magnetic tape.

For digital recordings, an analog-to-digital converter (ADC) samples the signal at a specified frequency, and converts the results into bits ( a series of ones and zeroes).  The bits are recorded onto a digital storage medium, usually a computer disk.  Most professional studios record digital audio at 96 kHz, or  96,000 samples per second.  When transferred to a CD, the rate is reduced to 44,100 samples per second.

Storage

Analog recordings are most often captured on tape.  They are then transferred to another physical medium such as vinyl for distribution and storage.  (They can of course be transferred to a CD, at which point they are no longer analog.)  Digital recordings are usually captured on a computer disk, and stored on a CD, DVD, or in a computer file.

Distribution

There’s no way around it: tapes and vinyl records  have to be shipped and then carried into your home.  Digital recordings can be distributed in a similar manner by CD or DVD, but can also be transferred as files around the globe at the speed of light.

Playback

Analog recordings can be played back on tape or vinyl.  In a reversal of the recording process, a tonearm moves a needle along the grooves of a record, and sends the signal through analog cables to an amplifier.  The amp powers speakers that disturb the air to reproduce the sound waves.  It’s atoms all the way through.

Digital recordings require an extra step for playback.  A digital-to-analog converter (DAC) takes the bits from the file and turns them back into an analog electrical signal.  This conversion can take place at the source (e.g., CD or MP3 player), within the amplifier, or in an outboard device that sits between them.  After conversion, the signal is amplified and sent through the speakers.

In practice, the analog and digital realms are almost always intermixed.  Vocals may be sent through a digital signal processor (DSP) before they are recorded on an analog tape deck.  Digital recordings might be mixed on an analog console and then converted back to digital for mastering.

Prior to the introduction of digital recording equipment, all recordings were “pure” analog.  When compact discs (CDs) first became available, they were often labeled with a three-letter SPARS code using A (analog) and D (digital) to indicate the type of equipment used for recording, mixing, and mastering.

So theoretically, a DDD disc would be “all digital”.  This system has been largely abandoned because of the confusion caused by overlapping technologies at various stages of the recording process.  It’s worth noting that there was at least one quadruple D disc.  The 25th Anniversary Edition of Switched on Bach* from Wendy Carlos was labeled DDDD, since the sounds were produced by a digital instrument and then digitally recorded, mixed, and mastered.

Do CDs sound better or worse than the LPs they replaced?

Yes.  As should be obvious by now, there are a lot of variables going into the signal that ends up as sound waves emanating from your speakers.  How well was the original recording engineered?  How carefully was it mixed, mastered, and pressed?  After that, there are even more obstacles to high-fidelity:  how good is your your turntable or CD player?  What about your amplifier and speakers?

The LP was introduced in 1948, so the art of analog recording had matured considerably by the time the CD was introduced in 1982.  The digital arts were in their infancy, and CDs created a huge spike in demand for catalog titles that resulted in thousands of discs being quickly, and sometimes carelessly, mastered.

Some early CD releases sound just awful compared to their analog predecessors — I know, I bought them (many for the third time: LP, cassette, CD).  There were a few cases where CDs sounded harsh because the master tapes had already been equalized for vinyl, and others where the wrong (i.e., inferior) master tape was used.1 Over time, digital techniques and equipment matured and sound quality improved dramatically.

With well-produced source material, you can get excellent sound out of either a digital or analog system.  The digital system will probably be less expensive, and digital formats are more portable, less prone to degradation, and easier to distribute.  The advantages of analog are harder to quantify, and advocates usually end up making emotional appeals instead of technical arguments.

Broadcasting from his basement, BadEditPro concedes that a CD “smokes” the LP in just about every specification that can be measured, but prefers LPs because of their “warmth, crispness, and depth, and reality. ..you feel like you are listening to the artist and not a representation of what the artist recorded in a recording studio.”  The typical arguments are nicely summed up by the writers below:

When I get a chance to hear vinyl after long bouts with zeros and ones analog always surprises me. It just sounds better–nicer–and more, well, musical. You folks who love music and have never experienced vinyl, you literally don’t know what you’re missing. No one’s saying analog’s perfect, there are distortions, scratches, noise, and dirt that dig-o-philes never deal with. It’s just that digital seems to miss the natural warmth that analog seems to capture so well. Maybe we’re “designed” for analog and digital is just too unnatural to fully enjoy.

~ Steve Guttenberg, “Intelligent Design vs Science, analog vs digital, CD vs LP–and the winner is?“

Hey, it’s one thing to subjectively prefer vinyl’s “warmth” and “richer sound” (what others might call “muddy bass” and “rolled-off highs”) to CDs. But that subjective judgment shouldn’t be equated with better “audio quality,” which implies a more rigorous technical standard of measurement.

Everything else being equal – and admittedly, that isn’t always the case – I know of no technical criterion where vinyl is in any way superior to CD (with the possible exception of upper frequency range, but only in some special cases). This of course doesn’t suggest that all CDs sound better than all LPs, or vice versa, or that CDs are perfect – only that CDs offer a (far) greater potential for accurate sound reproduction.

~ Rich Pell, “Audio myth: Vinyl better than CD?”

It’s hard to call this fight, and the results of most listening tests seem to be inconclusive.  Of course, when there is a conclusion, the losing side will claim that the test was biased, or flawed.  Digital may be a winner on paper, but analog has won many a heart.  And there is something appealing about natural sound waves creating analogous grooves that are used to recreate the same waves in another space and time.  It’s, well…groovy.

Here’s a theory.  We know that sound doesn’t exist in a vacuum.  In fact, since we read MMT we know that it can’t exist because there is no medium for its transmission.  Do you remember the first time you used a digital voice circuit and there was a pause in the conversation?  It sounded funny, right?  Like, um, dead.  Maybe it’s the imperfections in analog audio reproduction: the wow, the flutter — the hiss, crackle and pop that make it sound more “real” and  “warm” and “life-like” to us.

But just because analog is our friend, we don’t need to make digital our enemy.  One thing’s for sure: digital is here to stay.  In part 3, we’ll take a closer look at the various digital formats, do some math, and find out what high fidelity means in the 21st century.

In the meantime, check out what it takes to cut and press a vinyl record below, and get the full story here.

*all Amazon links are affiliate links

1 David Gerald, “Future Tense: Digital vs. Analog“

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Hi-Fi 2.0: Getting good audio in the iPod age (part 1)

by David D. on July 17, 2011

After decades of continual improvements in audio recording and playback technologies, the average 21st Century listener will hear most music at lower fidelity than before the millennium.  How did this happen, and can anything be done about it?

While audio technology continues to advance, the listening experience for most consumers, especially those just entering the market for recorded music, has devolved for a number of reasons:

  • compressed audio from mp3 files, YouTube, and other digital sources
  • playback through low-quality earbuds and computer speakers
  • low quality components in portable music players
  • a shrinking market for audiophile gear

This series will consider the factors contributing to the general decline in audio quality, and consider the following questions:

  1. Why does music from my iPod sound so bad, even when played through a high-quality sound system?
  2. Do CDs sound better or worse than the LPs they replaced?
  3. Why have premium audio formats such as SACD and DVD-Audio failed?
  4. What is HD Audio, and how do I get it?
  5. MP3, FLAC, LAME, lossy, lossless: I’m lost – what does all this mean?
  6. Where did all the audiophiles go?

Along the way, we will translate all the acronyms, define important specifications, and let you know how they affect what you hear.  Let’s start with the first question:

Why does music from my iPod sound so bad, even when played through a high-quality sound system?

Well, it may sound bad because it was poorly recorded, incompetently mastered, or horribly butchered when converted to an MP3 or other digital file.  But assuming it made it to your iPod in decent shape, the relatively cheap electronic components — especially the DAC (digital-to-analog converter) and headphone jack — will suck out any remaining quality left in the music. The only way around this is to bypass the iPod’s internal circuitry completely, and there are several solutions available.

Each of the products below will take the digital bit-stream from the iPod’s multipin connector and feed it to an external DAC, which may be integrated with the product, available as an outboard component, or contained within your amp or A/V receiver. Not every product is compatible with every iThing: some will not work with iPhones or iPads, and none will work with an iPod prior to the iPod Touch or iPod Classic.  You will also need to make sure that the output format and connections are compatible with the rest of your gear.  [Note: all Amazon links are affiliate links.]

Editor’s Choice: Cambridge Audio iD100 Digital iPod/iPad Dock

HRT iStreamer Outboard DAC for iDevices

Wadia 171iTransport iPod/iPhone Dock

Peachtree Audio iDecco Gloss Black Stereo Integrated Amplifier With Built-In DAC and iPod Dock

So is it worth $200 to $1,000 to improve the sound you get from your iPod?  That depends on a number of factors, including how and where you listen to music, your available audio sources, and the quality of your home hi-fi system.  Here’s a simple A/B test:

  1. Take a CD and rip a high-quality (320 kbps MP3 or lossless) audio track to your iThing.
  2. Get a 3.5 stereo male to RCA male Y-cable like this one (should be around $3 at Radio Shack), and connect your iDevice to your hi-fi system.
  3. Start playing back the CD through your stereo speakers, then switch to your iPhone. (Or better yet, have someone else switch back and forth for you, without identifying the source for each test.)

If you don’t hear any difference, or if the loss in quality doesn’t bother you, you’re done.  In my listening tests, it almost doesn’t matter what resolution you use to rip your audio files – the iPhone’s internal DAC and headphone jack circuitry will degrade the sound of everything – even lossless files.




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