We recently talked about how the conversion from analog to digital works and explained some of the basic terminology associated with digital audio files. In this article, we are going to look at how different digital audio compression algorithms work to reduce file size so that files can be shared more easily.

The Basics of Digital Audio

As we explained previously, sampling an analog audio waveform at CD-quality resolution requires 44,100 samples per second with a resolution of 16 bits for a pair of stereo channels. This results in a data stream that is 1,411 kilobits per second. For a one-minute long song, you’d need to store 84,672,000 bits of information. That’s about 50 megabytes for a five-minute song.

When we store audio in an uncompressed format, the information in the file doesn’t affect the size of the file. We could have a track containing a recording of a symphony orchestra, an audio test track or the last, last, last performance of the Rolling Stones. The size of the file will be the same if the track length is the same.

Lossless Audio Compression

Reducing file size has always been a concern when it comes to transmitting a file. Back when we had dial-up internet service, it would take hours to download a whole song in an uncompressed format. Acoustic modems had an optimal transfer rate of about 300 bits per second. High-speed analog modems reached a peak of 48 kilobits per second using data compression algorithms. To transfer high-quality audio, it would take about 30 seconds for every second of music. See the problem?

One of the most popular and well-known file compression methods is to zip a file. Zipping a file analyzes the content of the file and replaces repeated strings of data with a shortcut to identical information. When you unzip the file, you get the original back without any modification.

In terms of audio compression formats, the most popular lossless formats are Free Lossless Audio Codec (FLAC), Apple Lossless Audio Codec (ALAC) and the Monkey’s Audio (APE) format in a distant third place.

In terms of compression, converting our 70.69 megabyte audio file to FLAC results in a file size of 37.5 megabytes. This is a reduction of about 50 percent with no loss of sound quality, accuracy or detail.

Lossy Audio Compression

Whether you are trying to shrink an audio file or a photograph, one of the easiest ways to reduce the file size is to throw away some of the detail in the original file. For audio files, this often means limiting high-frequency information and reducing the detail of or eliminating low-level signals.

If we convert our original audio file to a 320 kbps MP3 file, the file shrinks to an amazing 16.0 megabytes. We do so by throwing out audio information that is difficult to hear. For example, if there is a loud guitar riff in one channel, the compression algorithm can dramatically reduce the detail of the relatively low-level audio information in the other channel without much change in the perceived quality of the playback. This is called perceptual audio encoding because the algorithm specifically affects information that is more difficult to hear (or perceive).

At a compression rate of 320 kbps, most listeners can’t tell the difference between the original file and the compressed version. As the compression increases, the differences become much more apparent. We start to lose high-frequency information and detail.

If we want to shrink the file further, we can convert it to a 128 kbps MP3 file. The benefit of extreme data compression is that our audio file now has a size of 6.59 megabytes. At this size, the song can be attached to an email without much concern for bandwidth or download time on a modern broadband internet connection.

The most popular lossy file compression formats are MP3 (formerly MPEG-1 Audio Layer III or MPEG-2 Audio Layer III), Adaptive Transform Acoustic Coding (ATRAC), Advanced Audio Coding (AAC) and Windows Media Audio (WMA).

Does File Size Matter?

As mentioned, audio compression algorithms were created to allow us to transmit audio over limited-bandwidth connections. Today, data storage is incredibly inexpensive. You can buy a 128 GB USB memory stick for less than $25. You can store about 400 hours of high-quality FLAC audio on a 128 GB memory stick. At the same time, internet bandwidth speed is at an all-time high. Most smartphones with LTE can download data at 150 mpbs. That’s faster than most people’s high-speed internet at home. Downloading a 37 megabyte file over a connection like that takes about five seconds.

Unless you are bandwidth-limited, you may as well download your music in at least CD-quality FLAC or an uncompressed WAV format. That way, you get the best sound quality possible from your audio system. If you have questions about how many tracks you can store on a memory stick or what digital media file formats are compatible with your car radio, visit your local mobile enhancement retailer. They should be able to answer any questions you may have.

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.

 

These days, it seems like there are more vehicle technology options than ever that can plug into the power outlet in your car or stick to the windshield. The problem with using the cigarette lighter outlet in your vehicle is that you end up with a lot of wire clutter and components attached to your windshield that can obstruct your vision and be very distracting. In this article, we are going to talk about keeping the interior of your car looking original while still adding great technology that will make your drive more efficient, entertaining and safe.

GPS Portable Navigation Systems

Everyone has seen a driver with a Garmin, TomTom or Magellan navigation system stuck to their windshield. While these devices are a great way to make traveling safer and more efficient, they are one of the biggest and most obtrusive additions you can make to your field of vision. Yes, having navigation information with voice prompts can reduce the potential for sudden, last-minute lane changes, and they are certainly better than having to look at a paper map while driving. On the downside, a bright display right in front of you can reduce your long-distance vision at night, and the constant stream of information is distracting.

A great solution is to install a new multimedia receiver that includes built-in GPS-based navigation. Modern receivers replace the radio in your car and provide a fully integrated solution that knows when to adjust the radio volume to let you know well ahead of time what lane to be in and when to make a maneuver. If the voice prompts aren’t enough, the screen is still available to give you additional information while staying out of your line of sight.

Smart Phone Mounts and Brackets

Pretty much every smartphone has access to a navigation application like Google Maps or Waze. These programs and the associated access to the internet for real-time traffic flow information offer a dramatic improvement in accuracy and efficiency over a portable navigation system. Voice recognition systems like Google Assistant and Apple’s Siri help you plan a route without having to touch the screen. The downside to using your phone is that you need to plug it in and place it somewhere that’s easy to see. Windshield, console, air vent and cup holder brackets are convenient but add clutter and wires running around the car interior.

A truly elegant solution is to choose a multimedia receiver with Apple CarPlay and Android Auto built into it. Not only can you get turn-by-turn directions, but you can also make phone calls, send text messages and choose from at least a dozen music or streaming audio services to keep you entertained.

Many of the latest multimedia receivers include wireless connectivity for Apple CarPlay and Android Auto. If you choose one of these solutions, you can keep your phone in pocket or purse while still being able to communicate as you’ driving down the road.

Phone Chargers

It seems like everyone has purchased at least one USB charging plug at one time or another. They are handy for topping up the battery in your phone while you drive and, in most cases, provide more current than the USB port built into the factory entertainment system. The downside, again, is clutter. Cables wear out and you still need somewhere to store your phone while you drive.

Many of the latest smartphones from Samsung and Apple support Qi (pronounced “chee”) wireless charging. Your local mobile enhancement retailer can source and install a vehicle-specific or custom wireless charging base in your vehicle. Once it’s integrated into your car or truck, all you have to do is place your phone in your center console, armrest or glovebox and the high-strength neodymium magnets in the Qi system will hold it in place while it charges wirelessly.

Another option is to ask your local mobile enhancement retailer to add a high-current USB port to your glovebox or center console so you can charge the phone while keeping it safe and secure.

Custom-Installed Radar Detectors

If you are a driving enthusiast or want to avoid the wasted time and money associated with speeding tickets, then you have likely considered, or already own a radar detector. The most cost-effective options are portable units that stick to your windshield or rearview mirror. While these solutions offer good radar performance, they can be a visual distraction and don’t offer protection from police laser systems.

If you want to beat police lidar, then a custom-installed radar system from Escort, K40, Radenso or Stinger is your only choice. These systems include laser transmitters that send a scrambled signal as soon as they detect that a lidar gun is being used. This scrambled signal prevents the police officer from taking a reading until you slow to a legal speed and cancel the transmission. Once canceled, the officer can try again, see that you are close enough to the speed limit and let you carry on your way. Of course, custom-installed radar detectors stay out of your line of sight, with no cables or wires visible in your vehicle.

As we’ve established throughout this article, it should be no surprise that a custom-installed radar detector and laser defense system will be practically invisible in your vehicle. Some detectors use a tiny control panel that’s about the size of a stick of gum while others use a wireless remote to enable you to operate the system. In most cases, a hidden speaker provides voice prompts to warn you about police activity.

SiriusXM Satellite Radio

If you travel a lot or live in a rural area, then finding music you enjoy on a radio station can be tricky. Adding satellite radio to your car or truck is a perfect solution, but using a solution like the Onyx or Stratus adds clutter and a “tacked on” look to your dash. Dozens of aftermarket radios are available from such companies as Sony, Alpine, Kenwood, JVC and Pioneer that offer integrated satellite radio solutions. Once installed, these radios blend nicely with your vehicle interior and in most cases, you can change stations using the steering wheel stereo controls that came with your vehicle.

Backup Camera Systems

Many big-box and auto parts stores offer backup camera systems that include a stand-alone monitor to display an image of what is behind your vehicle. While having a backup camera is one of the best safety systems you can add, having even a small display on your dash or console takes away from the tidiness of your vehicle interior.

Your local car audio specialist retailer has many options for adding a backup camera to your vehicle while making it look as though it belongs. In some cases, the image from the camera can be displayed on the factory-installed color screen in your dash. If you don’t have a screen, a variety of aftermarket rearview mirrors with built-in LCD displays will show what’s behind your vehicle. If you have upgraded to a new multimedia receiver, that screen is also an option.

Seat Heaters

In the northern U.S. and across Canada, commuters dread getting into their cars on a cold winter morning and sitting down on a seat that’s essentially frozen. Heating pads that plug into your power outlet offer some help, but even the best look like the afterthought that they are.

Original equipment-style heating elements can be installed under the cloth, vinyl or leather upholstery in most vehicles. If you have a remote car starter, your seats can be nice and warm when you get to the vehicle or, at the very least, will warm up quickly once you start the car.

Visit Your Local Specialist Mobile Electronics Retailer Today

If you are interested in adding a feature or technology to your car, truck or SUV, drop by your local specialist car stereo shop today and ask them about options for integrating that technology into your vehicle.

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.

Next, on our look through the common specifications included with modern car audio amplifiers, we want to take a look at amplifier efficiency. With the electrical systems in modern cars shrinking in capacity with every passing year, getting power from your amplifier without taxing the wiring, battery, and alternator in your car is a genuine challenge and concern. Modern Class-D amplifiers have quickly become the standard to drive our speakers. Read on to find out why.

What Is an Amplifier Efficiency Specification?

The efficiency specification of the amp you have chosen will let you know how much of the electricity fed into your amp is converted to an audio signal and how much is wasted as heat. A typical specification would look like 75.1 percent at full power into a 4-ohm load. This information tells us that 75.1 percent of the power going into the amp is converted to audio and that 24.9 percent is used to process the audio signal and is converted to heat.

While comparing maximum power ratings is fun, the music we listen to is quite dynamic and its levels vary a great deal. We set up a pair of amplifiers in our lab and took a series of measurements to graph the efficiency of the amp relative to its power output capabilities.

As you can see, the Class-D amplifier is, more often than not, at least twice as efficient as this particular (very low quality) Class-AB amp. Many reviewers list amplifier efficiency at two levels: full power and 1/3 of rated power. The two amps in this test delivered 23 percent and almost 71 percent efficiency at their 1/3 of maximum power rating. Indeed, you are reading that correctly. The Class-D amp would draw less than 33 percent of the current required to produce the same amount of power as the Class-AB amp. Since we operate our amplifiers in this range most of the time, even with the music quite loud, the effect on the vehicle’s electrical system can be dramatic.

Where Does the Heat Go?

As mentioned, the energy that enters an amp that is not sent to the speakers is converted to heat. To illustrate this effect, we fired up these same two amplifiers and let them run at an output level of about 21 watts for 10 minutes. The thermal images below give you an idea of how they differ.

Where Efficiency Really Matters

In a motorcycle, UTV or side-by-side where the current production capabilities of the factory electrical system are quite limited, choosing an amp with excellent efficiency is significantly more important than, say, in a pickup truck that is equipped with a 180-amp alternator. For these applications, look for an amp that offers the highest efficiency number you can find. Several motorcycle-specific amplifier solutions exceed 90 percent efficiency at full power.

Idle Current Specification

Another specification you will see listed in reviews and some owner’s manuals is idle current. Idle current describes how much current the amplifier draws when it’s turned on but not playing any music. A relatively high or low number doesn’t necessarily mean the amplifier is worth avoiding or is better than another solution. For example, amplifiers with onboard microcontrollers or signal processors consume a little more current than an amp without these devices and subsequent features.

If you drive a vehicle with an adequate electrical system, then considering amplifier efficiency isn’t a huge concern. If you drive a compact to mid-sized car, a hybrid or any kind of type of powersports vehicle, keep an eye on those efficiency ratings. Your local mobile electronics retailer can help you choose a solution that will sound great.

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.

As the next topic in our series explaining amplifier specifications, we will look at the frequency response information that manufacturers provide and explain how to interpret this information. In the simplest of statements, the frequency response spec will tell you about the low- and high-frequency limits of the amplifier based on its design. As with all the specs we have looked at, the information provided is as telling as the information that may be missing from the spec page. Let’s dive in and have a look.

Understanding Amplifier Frequency Response Specifications

Let’s take a look at a good amplifier with a specification of 4 Hz to 50 kHz. In this particular example, there is no tolerance provided, so we don’t know if those low- and high-frequency limits represent a 1dB or 3dB tolerance. Let’s fire up the amp and see what we can find out.

Connected to our digital interface and bank of load resistors, the amp shows a -1 dB frequency response of 8.21 Hz on the bottom end and 48.7 kHz on the top. Allowing for a tolerance of 3 dB, the measurement is 4.36 Hz on the bottom and above the 96 kHz measurement limit of my equipment on the top.

In short, this information tells us that this amp won’t dramatically affect the response of your audio system anywhere in the audible spectrum, and well beyond.

Speakers Are Not Resistors

Several factors govern the frequency response of an amplifier. Outside of a discussion of the circuit design and components used for the amp, what most people realize is that the speaker system you connect your amp to can affect its performance. In the lab, we use resistive loads. In the real world, speakers add a level of inductive reactance that opposes AC current flow and affects frequency response. When you add a passive crossover network, the load now includes capacitive reactance. Ultimately, even in a simple two-way passive crossover network, the load the amp sees varies a great deal depending on frequency.

I contacted John Atkinson, editor at Stereophile magazine, and asked permission to recreate his reactive speaker simulation network. His use of a reactive load for amplifier response testing was the result of an Audio Engineering Society paper by Eric Benjamin titled, “Audio Power Amplifiers for Loudspeaker Loads.” Atkinson consulted with Ken Kantor of NHT and International Jensen on the passive network, and the result was a version of the network you see below.

The purpose of this network is to present different impedances to the amplifier at different frequencies to evaluate its performance. The network replicates what an amplifier would see when powering a two-way, sealed-enclosure bookshelf speaker with a nominal impedance of 8 ohms. I created this network with the help of Frank Fabian at The Speaker Shop in Toronto. His store has an impressive supply of capacitors, resistors and inductors in stock. If you have a home speaker that needs repair or reconing, he’s the man to talk to!

Amplifier Response into Reactive Loads

The next step was to repeat the frequency response measurement of our reference amp using a 4-ohm load, a 2-ohm load and our reactive load to demonstrate just how much effect there is on the response.

As you can see, there is a small change in high-frequency response from this amp depending on the impedance of the load. The amp includes some filter chokes on the outputs as part of its variable voltage power supply design. The difference between the 4-ohm and the reactive trace is 0.85 dB at 20 kHz.

What About Inexpensive Amplifiers?

Our reference amp is just that – a high-quality amp that sounds amazing. So, what happens when you perform these same tests on an inexpensive amp? Let’s look and see!

Our cheap amp does a fair job with the resistive loads, rolling off by 1dB around 16kHz on the top and below 10 Hz on the bottom. The red trace shows that there is some emphasis between 2 and 3 kHz caused by the inductive characteristics of the passive filter network. Would that emphasis be audible? That would depend on your level of obsession. You can hear the difference of a few tenths of a dB when adjusting an EQ.

How About Our Class-D Amplifier Friends?

As we mentioned, the small filters on the output of our good amp resulted in a measurable change in frequency response between the varying loads. What happens when we measure a Class-D amplifier that uses large filters on the outputs?

Here we can see that there is a half-dB bump around 3 kHz and more than 2 dB of additional output at 20 kHz as compared to the 1 kHz reference level. Compared to a purely resistive load, the bump at 20 kHz is 3.5 dB more than a 4-ohm resistive load and about 7 dB louder than 2 ohms. If you’ve ever wondered why Class-D amplifiers sound different than a high-quality Class-AB, this is one of the reasons.

Working with Frequency Response Specifications

For most applications, you can ignore the frequency response measurements of the amplifiers you choose. The majority will be adequately flat from 20 Hz to 20 kHz. If you plan on driving a low-impedance load (low-impedance drivers or many drivers wired in parallel), the added impedance will dramatically reduce the high-frequency performance of a Class-D amp.

If you are planning on building an audio system that is truly high-resolution audio-ready, and capable of playing audio signals beyond 20 kHz, you are going to need to do some homework. Odds are, you’ll want a Class-AB amp for the tweeters, at the very least.

Finally, designing an audio system that uses active filtering will help reduce the variations in impedance caused by passive crossovers.

If you need help choosing an amplifier for your car audio system, drop into your local specialist mobile electronics retailer and talk to one of their product specialists.

Please check out other articles in our series on Understanding Specifications.

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.

The next topic in our look at car audio amplifier specifications is input sensitivity. This specification is easy to understand but is important to choose an amp that will work with the system your local car stereo shop has designed. In the simplest of terms, the sensitivity specification determines how much voltage is required on the input of the amplifier for it to produce full power. Let’s look at it in more detail.

Understanding Amplifier Input Sensitivity Range

Using our high-quality two-channel amp, our measurement equipment can show us just how much, or little, signal is required to produce a specific signal from the amp. We are going to use an output level of 10 watts for this example to keep our bank of load resistors cool and power supply happy.

With the sensitivity controls set to their lowest level, our amplifier produces 10 watts of output (6.32 Vrms) from about 1.4 V of input signal. This ratio equates to a system gain of 12.34 dB.

With the sensitivity controls on the amplifier turned to their highest setting, the amp amplifier is capable of producing 10 watts of output (6.32 Vrms) from a mere 54 millivolts rms of signal. This ratio equates to a gain of 41.41 dB.

Why Does Amplifier Sensitivity Matter?

Being able to get full power from your amplifier from a variety of signal sources is important to ensuring that your installer can make that amp work with any source. If you have a high-quality aftermarket source unit, the preamp outputs should provide 2 or 4 Vrms of signal with the volume at maximum and a recording at 0 dB.

If you are trying to power an audio system from something like an iPod, you may find that the signal from the headphone jack peaks around 1 Vrms. I tested one of my iPod Nanos at 1.03 Vrms. This lower maximum level means you need more gain from your amplifier.

At the other end of the scale, you may want your installer to connect your amplifier to the speaker outputs of your factory radio or factory-installed amplifier. The voltage from these sources may be as much as 8 V from a radio and could be as high as 40 V from a high-powered factory subwoofer amp. In those cases, you need to choose an amp that has dedicated speaker-level inputs or implement some sort of level converter to reduce the signal to something that the amp can accept.

System Tuning with Input Sensitivity Controls

When it comes to having a fully active audio system installed in your vehicle, unless you choose to implement a stand-alone digital signal processor (DSP), you will want to choose an amp with a lot of adjustability so that your installer can use the sensitivity control to reduce the output of the amp for the tweeters and midrange speakers, relative to the subwoofers and mid-bass drivers. You may find it useful to choose an amp for your tweeters that doesn’t produce a lot of power. Less maximum power output capability will reduce the amount of gain designed into the amp and result in a system that is easy to balance.

We would strongly recommend using a DSP since it enables you to configure crossover and output level adjustments quickly, but we understand that every audio system upgrade has budget limits. You can always upgrade later.

A Comment on Background Noise

If you have understood this article fully, then you realize that more signal from your source unit doesn’t necessarily represent an ability for your amplifier to produce more power, assuming the input sensitivity control is adjusted properly.

Years ago, when aftermarket source units with high-voltage preamp outputs were introduced, some companies marketed them as allowing stereo systems to play louder. If you didn’t adjust the sensitivity controls on your amp, this was a true statement.

The real benefit of a strong preamp signal is that you can turn down the gains on your amp and subsequently reduce the background noise in your system. Look at the difference in background noise of our high-end audio amp with the gain set at minimum and at maximum. Not all amps perform this well. As long as you can get full power from your amp, less gain means less background noise.

Choose an Amp That Works with Your Car Audio System

In most cases, the name-brand amplifiers available on the market today have the input voltage flexibility required to work in almost any application. Your local mobile enhancement retailer would be happy to work with you to pick a solution that will maximize the performance of your system.

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.