Audio Dynamic Range Compression: Limiting and Expansion

Free Download Mp4Gain

Audio Dynamic Range Compression: Limiting and Expansion

Audio Compression: What It Is and How It Works

As an audio engineer, I’ve spent countless hours working with dynamic range compression. At its core, compression is a tool used to control the dynamic range of an audio signal. This means that it can be used to make quiet sounds louder and loud sounds quieter, resulting in a more consistent overall volume. Compression is an essential tool in the world of audio production, and it’s used in everything from music to film and television.
One of my favorite quotes about compression comes from the legendary producer Quincy Jones. He once said, “Compression is like a good pair of eyeglasses. You don’t notice them, but they make things look better.” And it’s true. When used correctly, compression can make an audio signal sound more polished and professional.

Dynamic Range Compression vs. Limiting

While compression and limiting are often used interchangeably, they are actually two different processes. Compression is used to control the dynamic range of an audio signal, while limiting is used to prevent the signal from exceeding a certain level. In other words, limiting is a more extreme form of compression.
One of the most common uses of limiting is in mastering, which is the final step in the audio production process. During mastering, the goal is to make sure that the audio sounds consistent across different playback systems. Limiting is used to prevent the audio from distorting or clipping when played back on a variety of systems.

Audio Expansion: The Other Side of the Coin

While compression and limiting are used to control the dynamic range of an audio signal, expansion is used to increase it. Expansion is the opposite of compression, and it’s used to make quiet sounds even quieter. This can be useful in situations where you want to bring out the details in a recording.
One of my favorite examples of the use of expansion comes from the film “No Country for Old Men.” In the film, there’s a scene where the main character is walking through a hotel room. As he walks, you can hear the sound of his footsteps echoing off the walls. The sound of the footsteps is very quiet, but it’s still audible. This is a great example of the use of expansion to bring out the details in a recording.
Final Words:
In conclusion, dynamic range compression, limiting, and expansion are all essential tools in the world of audio production. Whether you’re working on music, film, or television, understanding how to use these tools can make a huge difference in the final product. And while there are many different techniques and approaches to using compression, limiting, and expansion, the most important thing is to use your ears and trust your instincts.
audio compression, dynamic range, audio engineering, audio processing, audio mixing, audio production, audio mastering, audio software, audio plugins, audio effects, audio equipment, audio gear, audio technology, audio science, audio theory, audio education, audio tutorials, audio tips, audio tricks, audio hacks, audio workflow, audio techniques, audio aesthetics, audio quality, audio fidelity, audio perception, audio psychology, audio philosophy, audio history, audio culture, audio art, audio creativity, audio innovation, audio trends, audio industry, audio business, audio marketing, audio branding, audio advertising, audio monetization, audio revenue, audio distribution, audio streaming, audio downloads, audio formats, audio codecs, audio standards, audio compatibility, audio hardware, audio drivers, audio firmware, audio troubleshooting, audio repair, audio maintenance, audio optimization, audio performance, audio testing, audio measurement, audio analysis, audio visualization, audio synthesis, audio sampling, audio recording, audio editing, audio mixing, audio mastering, audio post-production, audio restoration, audio enhancement, audio noise reduction, audio equalization, audio filters, audio modulation, audio synthesis, audio synthesis

Free Download Mp4Gain

Mp4Gain Main Window

Mp4Gain Features

Free Download Mp4Gain

Dynamic compression

Dynamic compression

dynamic compression

Dynamic Compression (DRC): narrowing (or expanding in the case of an expander) the dynamic range of a soundtrack. Dynamic range is the difference between the quietest and loudest sound. Sometimes the lowest sound on the soundtrack will be a little louder than the noise level, and sometimes a little lower than the loudest. The hardware devices and programs that perform dynamic compression are called compressors, distinguishing four main groups among them: the compressors themselves, limiters, expanders, and gates.

dynamic compression

Up and down compression

Creep Compression lowers the volume when it exceeds a certain threshold, leaving quieter sounds unchanged. The extreme downward compression option is the limiter. Increase compression (upward compression), conversely, increase volume if it is below a threshold value without affecting louder sounds. In this case, both types of compression reduce the dynamic range of the audio signal.

Expander and gate

If the compressor decreases the dynamic range, the expander increases it. When the signal level rises above the threshold level, the expander raises it further, thus increasing the difference between high and low sounds. These devices are often used when recording drums to separate the sounds of some drums from others.

A type of expander that is used not to amplify loud sounds, but to drown out quiet sounds that do not exceed the threshold level (for example, background noise) is called a noise gate. In such a device, as soon as the sound level falls below the threshold, the signal flow stops. Normally, the door is used to suppress noise during breaks. In some models, you can make sure that the sound does not stop abruptly when the threshold level is reached, but gradually fades away. In this case, the decay rate is set with the Decay knob.

The gate, like other types of compressors, can be frequency dependent (that is, handle certain frequency bands differently) and can operate in side chain mode (see below).

Compressor working principle

The signal entering the compressor is divided into two copies. One copy is sent to an amplifier, in which the degree of amplification is controlled by an external signal, the second copy forms this signal. It goes into a device called a side chain, where the signal is measured and based on this data, an envelope is created that describes the change in its volume.
This is how most modern compressors are organized, this is the so-called feed-forward type. In older devices (feedback type), the signal level is measured after the amplifier.

There are several analog variable gain amplification technologies, each with its own advantages and disadvantages: tube, optics with photoresistors, and transistor. When working with digital sound (in a sound editor or DAW), you can use your own mathematical algorithms or emulate the work of analog technologies.

Basic parameters of compressors
Limit
The compressor reduces the level of the audio signal if its amplitude exceeds a certain threshold value. Typically specified in decibels, a lower threshold (eg -60 dB) means that more sound will be processed than a higher threshold (eg -5 dB).

Proportion
The amount of level reduction is determined by the ratio parameter: the 4: 1 ratio means that if the input level is 4 dB higher than the threshold, the output signal level will be 1 dB higher than the threshold .
For example:
Threshold = −10 dB
Input signal = −6 dB (4 dB above threshold level)
Output signal = −9 dB (1 dB above threshold level)

It is important to note that the suppression of the signal level continues for some time after it falls below the threshold level, and this time is determined by the value of the release parameter.

Compression with a maximum ratio of ∞: 1 is called limiting. This means that any signal above the threshold level is suppressed down to the threshold level (except for a short period after a sudden increase in input volume). For more details, see Limiter below.

Attack and release
The compressor provides some control over how quickly it responds to changes in signal dynamics. The Attack parameter determines the time it takes for the compressor to reduce the gain to the level determined by the Ratio parameter.

Dynamic range normalizer

Dynamic range normalizer

Dynamics Processing

The audio quality of compressed formats such as MP3 or AAC is often perceived as better or at least equivalent to so-called lossless formats or audio CDs even by trained listeners. This is the conclusion reached by the TrustedReview test platform in a blind test with several people.

TrustedReview points out that this is not an experiment with strictly scientific parameters. However, what was remarkable about the results was that the self-reported hi-fi enthusiasts among the test subjects were wrong to recognize the uncompressed piece of music.

Dynamic Compression

Already sophisticated coding

“I am not surprised by this result, because we reached a similar conclusion in our internal experiments. Encoding technologies are now so sophisticated that most people no longer hear any difference even when compressing at a lower data rate,” he explains. Bernhard, MP3 expert at Fraunhofer. Grill in conversation with press release.

“It would even go so far that a properly encoded 192 kbit / s AAC sound file is the best alternative to today’s audio CD, as the 16-bit resolution of the CD can be far exceeded.” For people who are particularly sensitive to audio and want to be absolutely sure, Grill recommends a data rate of 256 kbit / s for AAC and 320 kbit / s for MP3.

The development trend goes in two directions

According to Grill, the current trend in audio encoding is bi-directional. For one thing, it’s about getting even smaller files with low-quality compromises. “This currently plays an important role, especially with streaming services over cellular networks, as bandwidths are still low despite UMTS and retrieval of large amounts of data is still expensive in most countries. “explains Grill. The most promising encoding is the HE – AAC audio codec, also known as AAC +, which is a further development of the AAC standard used by iTunes, for example.

On the other hand, the developers are also targeting the best of music listeners and want to show that with the right encoding technology, even the last noticeable differences from Audio CD can be eliminated. “Meanwhile, record labels and sound engineers have also understood how to encode for excellent sound quality. Another advantage of existing encodings, such as MP3 or AAC, is that the processes behind them are constantly being improved,” he says. Grill. MP3 encoded at 128 kbit / s sounds much better today than 15 years ago.

The term “compression” is misleading

The Fraunhofer MP3 and AAC developers find the term “compression” misleading anyway. “This should not be confused with dynamic compression, that is, the weakening of noisy passages and the elevation of soft passages, which has always been a technological necessity, especially with famous records and in FM broadcasting.” In this regard, MP3 and AAC with dynamic range up to 24 bits are even better than audio CD, which is limited to 16 bits. This restriction is not removed with a lossless copy of an audio CD.

Mp4Gain offers a dynamic range normalize, that is, the weakening of the noisy passages and the elevation of the soft passages, which is the most modern.

Much more important for good sound quality or an attractive sound image is not the medium or file format used, but the speakers used. “These are still the worst adulterers in the entire chain,” says Grill.

Loudness War

In 1973 came Raw Power, an album that punk icon Iggy Pop wanted to be the strongest in the world. But what exactly does the “strong” qualifier mean for a sound? How is this subjective sensation of acoustic power measured? How can we create it and, above all, what are the sacrifices? Why has music increased in volume in recent decades? Arm yourself with your most beautiful helmet and follow us in this report into the heart of the war for volume, a frantic race for the power of sound among producers, directors, musicians and engineers.

Loudness War

The origins of the volume war.

“How did it happen? Who is to blame? Of course, there are those who are more responsible than others and will have to account for it, but … If you are looking for a culprit, just look in a mirror. I know why Lo I did. I know you were scared. ”

The main argument of this decibel competition is taken from a psychoacoustic phenomenon: the human ear, if it is subjected to two identical pieces in all respects, will find that what is even a stronger decibel “sounds better”. The famous Fletcher-Munson curves partially explain this phenomenon: the weaker the sound, the less bass and treble are perceived. Therefore, a louder sound will not only appear linearly more powerful, but also richer “frequently”.

Loudness War

From there, to see an equivalence between “sounding louder” and “sounding better,” there is only one step, which music producers and radio directors in particular cheerfully crossed: the same tubes that loop through each other. All the Main stations, playing them higher was a way to stand out and attract new listeners. But holding them exclusively liable would be a mistake. In fact, like Iggy Pop, the artists themselves wanted to sound louder than their competitors. Sound engineers weren’t left out either: A well-known tip from mastering engineers who lacked time or inspiration was simply to assemble the mix a few decibels and go. Finally, the new millennium saw fans participate in the race, helped by the emergence of home schooling and the democratization of digital accessories.

“Too strong? Eat THIS.”

The popularization of mobile devices in the 1980s was also an important factor. Aboard the car radio and portable audio player, the radio and K7s come out of their living room and venture into the kitchen, take to the streets, take the car or plane, and rest on the beach. The sound must be more powerful in order to cover the background noise, and its dynamics (difference between the weakest and loudest sounds) is reduced to prevent the listener from doing the yo-yo with the volume button.

But frankly, basically, what would perpetual increase in noise be a problem? If the level increases, we just have to lower the knob! In fact, this oversupply would not mean any, provided it has an unlimited margin. Unfortunately, as with children, there are limits: the maximum dynamic range of a CD is 96 dB, radio levels around 60 dB, the speakers are separated above a certain sound pressure. , the consoles are saturated, not to mention our ears whose danger threshold is 90 dB, the pain threshold at 120 dB and the destruction threshold at 140 dB. How, then, to sound “stronger”, respecting the limits of the different formats and materials?

Stronger !

Here we are in the belly of the beast: how to offer a subjective sensation of greater power, without exceeding the maximum allowed sound level? And what exactly does it mean to sound “louder” when you can’t turn up the volume anymore?

Dynamic compression

Here’s the waveform for Norah Jones’ song Turn Me On: on the abscissa (horizontal), time, on the ordinate (vertical), amplitude, or volume. The peaks we see represent the loudest sounds, and the finest parts represent the weakest passages. The difference between the high amplitude signals and the others visually well marked, one does not even need to listen to the song to know that its dynamics are excellent. The mixture breathes and nuances are free to express themselves, in power as well as in softness. If we wanted to raise the level of this song in a classic way, by turning the knob or raising the faders, the amplitude of the peaks would quickly reach the maximum level, after which the signal would begin to distort. To increase the subjective level, therefore, it is necessary to decrease the difference between the peak of these peaks and the average level, then increase the overall volume – this is called dynamic compression.