You cannot have talked about audio and computers any time in the last 15 years and not have heard of an MP3 file. MP3 audio files and websites, like the original Napster, started a shift in where, how and when people acquired music. If you are on the older end of the spectrum, like many of us in the mobile electronics industry, then you bought your CDs, cassettes and maybe even your vinyl at a record store. Computers and the Internet changed that. You could go online after dinner and download an illegal copy of a song in a few minutes. It was wrong, but people acquired tens of millions of songs this way.

In the 1990s and early 2000s, accessing the Internet was slow. We started connecting to the Internet using phone lines and modems. Each byte of information took time to transfer to your computer, so anything that would speed up the process was a treat. Downloading (stealing) music using the Internet is where the popularity of the MP3 audio file met its calling.

A Primer on Digital Audio

We could write 10 articles about digital audio – and we just might. For now, we are going to look at the basics and use the compact disc (CD) as our reference. CDs store digital audio sampled at 44.1 kHz with a resolution of 16 bits. These numbers mean each sample can have an amplitude that is a single value within a range of 65,536 different levels (2 to the power of 16). The information is sampled 44,100 times a second. Sampling at what is known as 44.1/16 allows capturing the audible range of audio (considered 20 Hz to 20 kHz) with good detail and accuracy.

To store 1 second of audio at this resolution, we need to store 1,411,200 bits of information. Anyone who has played with audio transcoding software may recognize 1,411 kbps as a standard data rate. This number is calculated by multiplying the number bits per sample (16) times the number of samples per second (44,100) times 2. The times-2 factor is because we record in stereo – which is two channels. So, a 3-minute long song is 254,016,000 bits or 31,752,000 bytes.

Let’s round it off to 31 megabytes of information. Can you imagine how long it takes to download that with a dial-up modem running at 14,400 baud? The answer is at least 3.5 minutes – without error checking, line noise and other factors that slow the real download time to about 5.5 minutes.

Data Compression

What if someone found a way to shrink the size of the audio file to speed up download time and reduce bandwidth usage? The caveat is that the audio still sounds essentially the same on most basic audio systems, such as a TV, computer speakers or a 1990s factory car radio. In 1991, a group of companies, including the Fraunhofer Institute, France Telecom, Philips, TDF and IRT, started working on a way to reduce file size while maintaining relevant information. That is the key to how file size is reduced using MP3 compression.

The MP3 file format is a “lossy compression” algorithm. Lossy compression means that information is thrown away to reduce file size. The development team worked on a compression method called perceptual encoding to decide what information to remove. Perceptual encoding is based on how we hear sounds relative to other information, and the limits of our hearing.

What MP3 Files Throw Out

We are going to analyze the information that MP3 files remove to reduce file size. One of the easiest ways to cut back on information storage is to reduce the highest frequency that will be reproduced. If we analyze a 128 kbps MP3 file, we see that the highest reproduced frequency is just below 16 kHz. If that were the only information that was removed, our new bitrate with 16-bit samples in stereo would be about 1,004,800 kbps instead of 1,411,200 kbps for 20.05 kHz.

The next part of the compression process analyzes content that is common to both channels. It is common for some parts of a recording to be virtually in mono. The encoding process removes duplicated information from the file and adds code to copy the opposite channel. If the audio track were purely mono, the file size would be divided in two. Few tracks are completely mono, but we can see more space saving from this process.

Subsequent processing looks at low-level information during high-amplitude passages. Let’s use the example of a song with a lot of bass in it and some very quiet harmonic midrange information. Perceptual encoding processes like MP3 will remove this low-level information from the audio track. This process is called audio masking. There is enough audio information at other frequencies to distract you from hearing what is removed.

Can You Hear the Difference?

Dozens – nay, hundreds – of tests have compared MP3 files to full CD-quality audio tracks. Are there differences? There most certainly are. One thing became apparent during our research: How an MP3 file is created is crucial to its subjective sound quality. Different encoders work in different ways with different results.

Perhaps the best way to describe the difference between a CD-quality recording and an MP3 file is to look at the difference between the two. I wish we could share some samples here for you to listen to, but that would break copyright laws. What we can do is visually show you the difference.

We took a 3-second sample from Daft Punk’s “Give Life Back to Music.” We chose this track because of Daft Punk’s clear and conscious effort to make a high-resolution version of the album commercially available. We want to thank them for that! The sample is from 31.5 seconds to 34.5 seconds into the song.

This Spectrogram shows the frequency content of the sample. The horizontal scale is time. The vertical scale is frequency. Finally, the color intensity shows the amplitude.

You can see that there is frequency content up to 30 kHz, clearly demonstrating the high-resolution nature of this track. Each vertical color band represents a drum machine beat – more or less.

128 kbs MP3 File Analyzation

It is clear that audio information above 16 kHz has been removed. Infrasonic frequency content is clearly different as well. There is more information in the MP3 file below 30 Hz compared to the original. This increase in information will, however, present itself as less-dynamic range.

MP3 Vs Original File

We inverted the MP3 file and added it to the original sample to make the image you see here. The net result is the difference between the two tracks. You can see the high-frequency content that was removed above 16 kHz. In fact, information was removed at all frequencies, and that information follows the intensity pattern of the audio file.

The original file has a peak amplitude of -0.1 dB for both channels and an average amplitude of about -14.2 dB. The removed information has a peak level of -10.9 dB and an average amplitude of -37.01. The removed information is buried deep below the peak amplitude information.

What does the removed audio sound like? We would describe the clip as the sound of a distant marching band. The audio is mostly high-frequency information. The track has a decidedly warbled texture to it as well: The drum machine beats are clear and present, but they sound like distorted cymbal hits.

Even with a high-end headphone preamp and studio grade headphones, the difference is hard to perceive when switching between the original track and the MP3 file. In a listening environment with a larger soundstage, it may be more apparent.

Conclusions about MP3 Files

Purists will tell you that you should have the highest-quality recordings available. There is no fault to this logic. Why skimp when you can have it all? High bitrate MP3 files, like those at 320 kbps, for example, are excellent in quality. Repeated testing has shown that when created with quality compression algorithms, the sound difference between a CD-quality recording and a 320 kbps MP3 file is almost impossible to detect. Lower bitrate MP3 files start to dispose of more information, and the differences become bigger.

The latest source units on the market are capable of playing WAV and FLAC audio files of great resolution and bit depth. Shortly, we will see units that will play MQA files over digital connections. Almost every source will handle MP3 and WMA files.

Drop into your local mobile electronics specialist retailer today, and bring along some music to enjoy. We think you will be impressed – no matter what format you choose.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Amplifiers have a very tough job. They have to take a very low voltage signal and increase it in amplitude so it can drive a speaker. In this transformation, we expect the signal to remain pure – no distortion or no noise should be added. We also want significant amounts of power to drive our speakers, even though we only feed our amplifiers with a measly 12 to 14 volts of electricity. The laws of physics seem to want to work against us at every turn – but we prevail! Modern car audio amplifiers are amazing feats of engineering and design. This article looks at the two main types of amplifier classes used in the car audio industry and the benefits and drawbacks of each. Welcome to Class AB vs. Class D.

The Math behind how Amplifiers Make Power

No matter how we configure the components inside an amplifier, the goal is the same: Increase the voltage of the preamp audio signal so it can drive a speaker. Because the speakers we use are low in impedance (2 or 4 ohms for most midrange speakers), we need to be able to provide a significant amount of current to the speaker as well. This delivery of current to the speaker is the second task an amplifier has to undertake.

By way of some quick math, if a 4 ohm speaker is getting a 12V RMS signal, we can make a few calculations. To calculate the current flowing through the speaker, we divide the supplied voltage by the impedance of the speaker. In this example, we have 12 divided by 4, so 3 amps of current are flowing through the speaker wires and the voice coil. An easy way to calculate the power going to the speaker is to multiply the supplied voltage times the supplied current. The product of 12 times 3 is 36. This speaker is receiving 36 watts of power.

Let’s look at the same example as though this were a subwoofer amplifier. In this second example, we will assume we have a Dual 2 Ohm voice coil subwoofer with both coils wired in parallel to produce a 1 ohm load. If we supply this speaker with 12 Vrms of signal, then 12 amps of current flow through the speaker wire and the subwoofer. To calculate power, we multiply 12 times 12 to get 144 watts. 144 watts is a lot more power and current for the same amount of voltage.

General Amplifier Function Overview

Most amplifiers are composed of three or four key sections (or stages), depending on their design and complexity. The input stage is the portion of the amp where the low-level preamp audio signal enters the amp and receives any processing in the form of equalization or filtering.

An amplifier has a power supply. The power supply converts the supplied 12 to 14 V of direct current to positive and negative rail voltages. Let’s say, for example, a theoretical amplifier has +25 and -25V rails, relative to our ground reference. Depending on the size of the amp, there will be a driver stage. The driver stage is responsible for increasing the low-level audio signal to a higher voltage. How much the driver stage increases the voltage depends on how much power the amp will be making.

Finally, we have the output stage. The output stage is relatively simple – it does not significantly alter the signal coming from the driver stage, but the devices (MOSFETs or transistors) used to provide the output signal with the current the load requires. The power supply and the output stage are the two portions of the amp that do the most “hard work.” That is to say, they are the stages that pass a lot of current.

In almost all amps on the market, we use dedicated devices for the positive half of the waveform and separate devices for the negative half of the waveform. To clarify , if we measure the output signal of the amplifier about the vehicle ground, we will see that it swings back and forth above and below 0V. Think back to our +25 V and -25 V power rails. Speakers don’t care about the value of the signal being sent to them; all they care about is the difference in voltage from one end of the voice coil to the other end.

Class AB Amplifiers

For this article, we are going to generalize Class AB amps into an Analog, Amplifier model. In our analog amplifier, we have large transistors in the output stage of the amp. When we want half of the positive rail voltage at the output, we feed half the voltage to the positive output device. When the signal goes negative, we turn off the positive device and start using the negative device only. Looked at a different way, the audio signal from the driver stage controls the resistance of the output devices and, subsequently, how much current can flow to the speaker.

In an Analog Amplifier, the output devices can be “turned on” in varying amounts about the audio signal. This means the output devices are often acting as resistors. Power is wasted as heat when we pass current through a resistor. Keep this in mind as part of our comparison later in the article.

Class D Amplifiers

In a Class D amp, the output devices receive a control from a Controller Integrated Circuit (IC). This controller sends out a variable duty cycle square wave. The square wave amplitude is high enough that it turns the output devices all the way on or off. The output devices spend very little time operating as resistors and act more like switches.

The logical question is, how in the world do we get music out of a square wave? If you thought that, good for you! The frequency of the square wave is much higher than the maximum frequency of our music. In fact, some modern Class D amplifiers switch the output devices at frequencies as high as 600 kHz.

To recreate music, the Class D controller sends out a signal that is Pulse Width Modulated. The amount of “on” time about the “off” time determines the output level of the signal. As a very general analogy, if the positive output devices were sent a square wave with a 50% duty cycle (on for as much time as it was off), then the average of the output would be 50% of the positive rail voltage. If the square wave is on for 75% of the time, then off for 25%, then we would get 75% of the rail voltage at the output.

As you can imagine, the signal from the Class D controller is quite complex. It has to modulate the duty cycle of the square wave going to the positive and negative devices fast enough to accurately recreate the audio signal. It also has to control both the positive and the negative output devices separately.

Benefits and Drawbacks of Analog Amplifiers

Because the audio signal in an analog amplifier is never chopped up into tiny pieces, analog amplifiers can remain faithful to the original signal. The best-sounding amplifiers in the mobile electronics industry are analog. Analog amplifiers are, historically, given a reputation for accurate high-frequency response.

The drawback of an analog amplifier is its efficiency. Efficiency describes how much energy is wasted as heat as compared to the energy sent to the speaker. Because of the output devices in an analog amplifier work as variable resistors, they get hot. Typical analog amplifiers operate in the 70-80% efficiency range regarding total efficiency, while operating at full power. That missing 20-30% is released as heat. At lower output level, the efficiency drops even more.

Benefits and Drawbacks of Digital Amplifiers

Modern digital amplifiers switch at extremely high frequencies. We see amps capable of audio frequency response beyond 50 kHz, and some that exceed 70 kHz. This performance is a long way from the first Class D amps that were only for subwoofers and struggled to produce audio above 5 kHz. That said, because digital amplifiers require filter networks at the end of the output stage, they still cannot quite match the performance of a premium analog amp. With this information in mind, consider that there are some good digital amplifiers that sound better than many poorly designed analog amplifiers.

Because the output devices of a digital amplifier rarely operate in their resistive range, these amplifiers can be very efficient. A well-designed Class D amp can have an efficiency around 92%.

Another problem with Class D amplifiers is noise. Because the output devices are driven by a square wave, there is a lot of high-frequency energy in the output signal. The filter network we talked about removes much of that from the output signal, but that energy can still have detrimental effects on other systems in the vehicle. An unfortunately common trait for many Class D amps is that they cause interference with radio reception when in operation.

Choosing Between Amplifier Classes

It would be nice if we could formulate a set of hard-and-fast rules for choosing the right amplifier for your system. With so many variations on each kind of amp at so many different price points, that is truly impossible. We strongly suggest that the only way to pick an amp is to compare one to another under controlled conditions: Use the same music and the same speakers, and listen at the same volume. You will hear differences in frequency response and dramatic differences in imaging and staging capabilities.

Is one kind of amp better than the other? For an installation dedicated purely to sound quality, the choice is clear. For an installation where power delivery is limited or massive amounts of power are required, the choice is clear there as well. In the middle, it depends on your application and budget.

Drop in at your local mobile electronics specialist retailer to find out about the latest amplifiers on the market. They would be happy to help you choose one that meets your application and works with your budget.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Among the most under-appreciated components in any audio system are the speakers. You could be using the best source unit on the planet, an amazing amplifier, and the most esoteric of interconnects and speaker cables – but if you don’t have great speakers, you still won’t have great sound. Your speakers are the only link between your electronics and your ears, so choosing great speakers is critically important to reproducing great sound. This article discusses what you get when you spend more money on speakers.

More Power Handling

Several factors limit the ability of a speaker to convert an electrical signal into speaker cone motion. One such limit is thermal capacity – just how much heat the voice coil, tinsel leads, former and cone can handle before one of them fails. Heat is the number-one enemy of speakers. Most of the power you send to your speakers is converted to heat rather than to sound – in fact, probably 95% or more is converted to heat.

As the quality of speakers increases, you often encounter a larger voice coil and better cooling technologies. These features, coupled with tighter tolerances around the voice coil, result in a speaker that can absorb more energy without failing.

More Excursion

Often tied directly to how much power a speaker can handle is the distance the speaker cone can move forward or back. This specification is called excursion. For most speakers, the specification for excursion is called Xmax. Xmax is the distance the cone can move in one direction without the voice coil moving out of the magnetic field, and the measurement is typically provided in millimeters.

A second and often equally important specification is Xmech or Xsus. These specifications describe the physical limit of how far the cone can move, based on the design of the suspension (spider and surround). A speaker can be pushed past the Xmax limit, but as this happens, distortion occurs. When a speaker cone reaches its physical excursion limit, distortion will spike even higher. When it comes to speakers, there is no good distortion, so operating them within their physical limits is critical to producing accurate sound.

Why is excursion important? When combined with the ability to handle more power, a “better” speaker will play louder when provided with more power. The same features also help to reduce a phenomenon called power compression. Power compression is a reduction in the efficiency of a speaker as its components heat up. The key drawback to power compression is that when the music gets quieter, you turn the volume up, thereby sending more power to the speaker and accelerating the heating effect. In no time, the speaker will fail.

Excursion Linearity

It’s one thing for a speaker to have good power handling and excursion characteristics. It is equally important that the speaker remains linear throughout its excursion-based operating range. All speakers change in their characteristics as excursion increases – a great speaker minimizes these.

Let’s look at a common problem with “simple speakers.” One of the characteristics that limit the high-frequency response of a woofer or midrange driver is inductance. The voice coil of the speaker has some natural inductance because, well – it is a coil of wire. This inductance is increased when we place the voice coil around the T-yoke of a speaker.

This is where the problem occurs: As the coil moves forward, some of it may leave the T-yoke, which will reduce inductance. As the coil moves rearward, inductance may increase. The result is that the speaker has different frequency responses depending on where it is within its range of excursion. Truly excellent speakers use technologies to minimize these effects. The result are speakers that sound just as good at low to moderate drive levels as they do when being pushed hard.

More Frequency Response

This is a bit harder to describe, because sometimes the improvement from a basic to a mid-priced speaker, or a mid-priced speaker to a high-end one, is extended frequency response. More often, it’s smoother frequency response. Better-quality speakers are, ideally, designed to reduce distortion caused by cone resonances, suspension nonlinearities and magnetic field imbalances. As you spend more on speakers, you will see that peaks in their frequency response, especially in the midbass region, are tamed.

Many great speakers still exhibit some cone resonances at high frequencies. As long as the speaker you are using in the adjacent band will play low enough, those peaks are not critical.

Reduced Distortion

When a speaker designer combines an appropriately damped and rigid cone with a carefully designed motor and suspension, the result is a speaker that produces less distortion. Some distortions are very subtle, while others are quite pronounced. Speakers exhibit both even- and odd-ordered harmonic distortions, depending on what component or design issue is to blame.

If you have read our article on distortion, then you know that it is the effect of adding content that was not in the original program material. Harmonic distortion results in even or odd multiples of a specific frequency.

When you hear an amazing speaker, the difference between it and an average speaker is clarity. Smooth frequency response and a lack of distortion make each sound the speaker produces more faithful to the recording. Voices will sound more realistic. Instruments will sound more natural. Complex passages will become easier to understand.

It is also critical to point out that distortions cannot be removed from a signal or sound once created. No amount of equalization can extract those harmonics. Yes, you can reduce their level, but you also reduce the level of original signal information at those same frequencies. The only solution is to use a speaker that does not introduce distortion.

The Right Tool for the Job

Another item that differentiates a good speaker from a great one is its design in terms of application. Let’s use a typical 6.5” midrange/midbass speaker as an example. If the speaker is designed to work in a typical door or rear deck-style installation, the Thiele-Small parameters for that transducer will be optimized for that application. If you attempt to put that speaker into a small kick panel pod that has only 0.1 cubic feet of air space, you will choke it. The low-frequency response will be dramatically reduced; typically, a bump in the midbass region will occur, and it will likely not sound very good. Most 6.5” speakers need an enclosure volume of about 0.6 cubic feet, and some want closer to 1.0 for smooth frequency response.

How do you look for a speaker designed for the right application? That is product knowledge that a properly trained mobile electronics retailer can provide. Do you need a midrange that will work in a small A-pillar pod? Do you need a woofer that will work in an infinite baffle application? Will your midbass driver be used with a subwoofer, or do you need one that will be the sole source of bass in the system? Knowing the intended application for a speaker ensures that the system designer will use it appropriately.

Are More Expensive Speakers Always Better?

Does price always determine performance? In the upper echelon of any audio industry, the rewards for spending exponentially more money diminish. That is to say, the improvement you get from spending $500 on a set of speakers as compared to $200 may not be as dramatic as the difference between $2,500 and $2,200.

Every speaker manufacturer wants you to buy their speakers. They all work hard to come up with great marketing programs to make you want to buy their products. Your goal in choosing an amazing speaker is to ignore the story and listen.

The best way is first to establish a reference. Listen to the best speakers you can find. Don’t worry about the price. You simply want to understand what is available. Then, choose your price point and listen to something in that range. If you can accept the differences, then proceed. If you can’t, re-evaluate your needs or your budget. You can’t get that extra level of clarity, dynamics and detail any other way.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.