Audio Compression Techniques

Sound is nothing more than a vibration in the material (usually air or water) surrounding us that we can recognize as such. These vibrations need to happen between 20 times and 20,000 times a second for us, as humans, to hear them. To make it easy for us to speak about, we refer to these vibrations in either cycles per second (CPS) or Hertz (Hz). The fact that humans are sensitive to this range of frequencies is where the golden yardstick for audio equipment comes from; we can hear audio between 20Hz and 20,000Hz.

For years, audio was recorded in its original analog format in the form of records and, later, magnetic tape. By the early '80s, there was a real push to produce a more portable and reliable format for audio storage and, through the miracle of digital, we have that in the form of audio CDs.

For a more in-depth look at the state of audio, read through this book by Ken Pohlman.

In the jump from analog to digital, certain things must happen. The first is to sample the analog audio and then digitize it. With CD audio, this sampling takes place 44,100 times every second with each sample being represented with a 16-bit digital word. The resulting digital file tends to be pretty large; a one-minute stereo sound file, at CD-quality, eats up about 10MB of storage. Upon the release of the original audio CD, there were rumors that it was designed to hold 74 minutes of hi-fidelity music which corresponded to Wilhelm Furtwängler's recording of Beethoven’s 9th Symphony from the 1951 Bayreuth Festival. In later years, this rumor was squelched, but it still makes a pretty good story. With later updates to the audio CD format, present audio CDs hold 80 minutes of audio.

For those of us who lived through the early days of digital audio, we remember what it was like to try and carry a portable CD player as gently as possible. We needed to make sure the player was not subjected to any bumps because that would cause certain skips and pauses in the music. For that and several other reasons, portable digital audio started to take shape as a purely silicon-based item. Some of the digital audio players from the late '90s only had 32MB of storage on-board. Without extra help, those little players could only hold a little over three minutes of audio; enter the idea of data compression.

With data compression, there is an attempt made to reduce the size of the digital data. Grossly oversimplified, there is lossless compression and lossy compression. Lossless compression looks at digital data that is repeated and removes that data after having made a note that it exists. This type of compression can reduce an audio file 50% to 60% in size. When reconstructed, a lossless file will be mathematically the same as the original file. FLAC files are an example of a lossless compressed audio file.

Lossy compression goes a step or two further and, with the help of psychoacoustics, tries to predict what sounds would be either outside our hearing range or those masked by other sounds happening at the same time. For example, many people cannot hear frequencies about 15,000Hz, so the MP3 format simply throws that data away. As I mentioned in an earlier post, and demonstrated with photos, the MP3 compression format really negatively affects an audio file. For a much more meaningful explanation on compression, have a look at this research paper: Introduction to Data Compression, by Guy E. Blelloch .

Psychoacoustics is a process best explained with a demonstration. Assume for a moment that two people, Phil and Bob, are standing within 20 yards of a Civil War era cannon. Just as Bob starts speaking to Phil, the cannon is fired producing a deafening BOOM! As is predictable, Phil is not able to hear what Bob says. This same theory is used with psychoacoustics to predict what sounds in an audio file would not be heard and, therefore, could be removed from the data file. Typical sound files, e.g. MP3 and AAC, use psychoacoustics to create files that are approximately 1MB in size for every minute of audio present.


An inexpensive way to find out more about the sound quality differences between lossless and lossy files would be to purchase a SanDisk Sansa Clip+.

NuForce NE-600X Final Impressions

In January 2012, Nu-Force introduced their new NE-600X earphones. Roughly a month later I took delivery of a pair of these very affordable phones and here were my initial thoughts about them.

The first thing that surprised me was the hard plastic case in which the earphones were packaged. It protects the item very well, but was extremely easy to open, unlike most packaging these days.

Right out of the box and into my ears… I am using my iPad to feed music to these earphones since that is a situation in which these 16 ohm devices are most at home. My initial reactions are that the sound is very bright, almost metallic and a bit forward, constrained, and nasally. There is very little warmness to the sound. I would go as far as calling them “clinical” at this time. Honestly, I would say they work very well on jazz music at this time, but not on much else.

For good measure, I will allow these earphones to play-in for several hours before making my next critical listen. From past experience, I expect this NuForce item to develop a much warmer demeanor. We shall see.

Specifications as listed on NuForce website:
Earphones
Driver Size: 11mm
Impedance: 32 Ohm (16 Ohm according to package)
Frequency Response: 20 to 20kHz
Rated Power: 10mW (1mW according to package)
Max. Input Power: 40mW (3mW according to package)
Sensitivity: 100dB+/-3dB
Connector: 3.5mm 3-pin stereo plug
Mechanical
Weight (without packing): 12.5 grams
Length: 110 cm (43.3 inches)
Package Contents Include
S,L spare tips (medium tips already on earphones)
Warranty
1 Year

NuForce NE-600X Product Page

NuForce NE-600X Available from Amazon

Comply T-400 Foam Tips Available from Amazon

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After 12 hours of play-in, the NE-600X earphones were already sounding much less metallic. Much of the harshness of the upper-mid frequencies had reduced, but there was still a pronounced sibilance.

Bass took on a much fuller, heavier feel that had a certain amount of texture in the upper-bass registers. Some of this might have come from either the placement of the speaker armature inside the earphone body or, perhaps, there was a bit of body resonance showing through.

NuForce NE-600X Available from Amazon

Comply T-400 Foam Tips Available from Amazon

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Replacing the standard rubber tips that come with the NuForce NE-600X earphones totally changed the overall demeanor of these phones.

With some effort it is possible to install the Comply T400 foam tips. These helped create a much better seal in the ear canal which dramatically warmed the upper bass frequencies and created a tauter, more full lower bass sound. Mid-range frequencies became a bit more recessed.

Sibilance was no longer a problem, maybe because of the additional warmth in the bass or the play-in period.

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As the earphones continued to play-in, I had the following to say:

The earphones have been playing-in for about 36 hours now and the overall sound quality has taken a turn from being brash and shouty, to being warm and laid back throughout all the lower end and up through the upper-midrange frequencies. There still appears to be a bit of texture in the higher frequencies, perhaps around the 8KHz level.

Overall these earphones are sounding more like $100 devices than the sub-$25 level at which they sell.

And, again, at the 48-hour point:

After 48 hours of play-in, the NE-600X earphones sound almost identical to their older, more-expensive, and discontinued relation, the NuForce NE-7M. To me, that is a quite a compliment.

The sound quality is very warm, but detailed. After this much burn-in time, nearly all the harshness is gone, but the shimmer of the high frequencies is still intact. One area these earphones differ wildly from the NE-7Ms is in their sensitivity. These phones are much more sensitive; to the place that instead of running my iPhone or iPad at three-quarters volume, I can only go half-volume before the sound pressure starts to become too much. This all but eliminates the need for a headphone amplifier.

NuForce NE-600X Available from Amazon

Comply T-400 Foam Tips Available from Amazon

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In order to get a better idea of the abilities of these earphones, I subjected them to some quantitative testing as evidenced below.

The NE-600X earphones have now fully played-in.

For clarification, my standard play-in cycle is this: 48 hours playing music at normal listening levels, 24 hours resting period, 48 hours playing a complex synthesis of tones available below, and then into service.

Burn-In Sound File Download Link

A couple notes about this sound file… This is a highly compressed and shortened version of the original file. I developed the original file several years ago with the intention of creating a single sound that would have several layers including a background of white noise, a low-pitched thump intended to cause the driver to travel a great distance, and a varying signal that is very difficult to hear but which spans the full spectrum from 20Hz to 20,000Hz. This sound should never be played very loudly as it includes frequencies down as low as 1Hz which will not be well-tolerated by most audio equipment. This is nearly the equivalent of holding the speaker terminals to a battery and letting it stay there. Warning given…

The version of this file that I have available here does not extend much beyond 15,500Hz as evidenced by a comparison of the compressed file to the original file.

The above capture shows a relatively smoothed image of what the original signal looks like. Notice that the high frequencies are clearly emphasized. One reason I designed the sound this way is that most speakers are very capable of recreating the frequencies above 1KHz but then lose their abilities starting around 15KHz. Also, I have heavy emphasis on the bass and mid-bass frequencies because I think one characteristic that really ruins an otherwise good driver is muddiness. By burning in a speaker with a lot of activity in the lower ranges, I believe the muddiness works out quickly.

In this capture of the MP3 compressed file, it is easy to see the effect the compression has on frequency distribution. Notice the overall flattening of the sound pressure and the brick wall cutoff above about 15,500Hz. If you ever wondered why MP3s do not sound like the original, here is a clue. In an upcoming post, I will do more comparisons and thoughts on different audio compression algorithms available.

Back to the NE-600X earphones, after all the play-in, I can say that the overall sound is warm, somewhat laid-back, and very tube-like. These are not clinical phones and they do not over-emphasize faults in the music source. In fact, I would say they do an excellent job covering up imperfections in the source material.

Would I buy these earphones again? Absolutely! Remember, I am in no way affiliated with NuForce and get nothing for saying nice things about their product. I just like to tell people when I have a good experience with a product or service.

NuForce NE-600X Available from Amazon

Comply T-400 Foam Tips Available from Amazon

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After two years of use, I am still extremely happy with the NuForce NE-600X earphones. One issue that is important to note is that, when you adopt the use of the Comply foam tips, it is imperative that they be replaced every month or so. The foam simply breaks down and then the acoustic isolation ceases to exist and the overall sound signature changes.

For less than $35 for the purchase of the earphones and foam tips, this is a steal of a deal!