Audio Compression Tutorial Made Easy By Producer Karen Kane

Posted by Giacomo on February 10, 2012
Tips and Tricks / 1 Comment

Using a compressor on a microphone or on a recorded track may seem to be a simple task, however, there’s a lot to learn about the theory of compression.


Understanding this theory and why compression is useful will help you in using these devices more effectively. Compressors are “dynamic range processors” or “variable gain amplifiers” (VGA’s). They affect the dynamic range of sound by varying the “gain” (volume) of that sound.

If the dynamic range of music is potentially near 120db (that’s loud!) and the dynamic range of recording devices and live sound systems are only near 75-90db, we need a way to control the dynamic range so that those loud levels do not cause distortion. The dynamic range has to be gain reduced to a level appropriate for recording or a live concert. This “gain reduction” could be accomplished by manually riding the fader up and down but our hands are not fast enough. A compressor automatically controls the volume of a sound, leaving our hands free for other things.

Compressors are used on vocals, bass, guitar, drums and almost anything that fluctuates widely. Very simply put, a compressor is an amplifier whose gain decreases as the input increases. It makes big changes into smaller changes.

Most compressors have 4 main controls: input level, output level, threshold and ratio. Some compressors also have attack time and release time controls.

Input level: is exactly that, the amount of input level. When you set it to “O”, you get the exact level that you are sending from your mic pre-amp or from an already recorded track.

Output level: the amount of output after compression.

Threshold: A set point at which the compressor begins to work. Incoming levels below your set threshold are unprocessed. Incoming levels above your set threshold are compressed according to the compression ratio. The threshold on a compressor is similar to an air conditioner thermostat. When the temperature exceeds a certain threshold, the air conditioner kicks in. When the temperature drops below threshold, the air conditioner shuts off. This is the same idea for the threshold of a compressor.

Ratio: is the ratio of the input signal to the output signal after compression.

Example: 4:1 ratio means if the input is 12db, the output level is only 3db. So level input (12db) divided by ratio (4) is the output level. A ratio of 2:1 would give a 12 db input an output level of 6 db and a ratio of 8:1 would give an output level of 1.5db. So the rule of thumb is, the higher the ratio, the more extreme the compression is going to be. Therefore, ratios from 2:1 up through 6:1 are considered “gentle” and ratios above 6:1 are considered “hard”. “Hard” compressing is called “limiting”. A limiter is merely a compressor with a very high ratio, very fast attack times, fast to medium release times and high thresholds. Limiters are great for live sound systems as a safety device when very high levels are introduced into the system. Any signal above the threshold is “clipped” off. They protect speakers from blowing. 8:1,10:1, 20:1, 100:1 are common limiting ratios.

Attack time: The speed with which the device affects the signal. The time it takes to react to a signal above the threshold.

Release time: The rate at which the device lets the signal decay. The time the compressor takes to return the signal to normal (the way it was before hitting the threshold).

Stereo compressors come in one box with 2 channels and can be used in 2 different ways. You can put 2 individual mics or tracks into it and each channel will have completely independent settings from each other. Obviously, a mono compressor just has one channel. The second way to use a stereo compressor is to put an entire stereo studio mix or the stereo output of a live soundboard into it. This is called using the compressor as a Left/Right Stereo Bus compressor. When you use it as a L/R Stereo Bus compressor, you have to link both sides electronically so the same amount of compression happens on both sides at the same time.

A “Frequency Selective Compressor “(De-esser) is a special compressor that reduces the level of a very narrow band of frequencies. It’s very useful when a singer has a strong, sharp, sibilant “S” to their voice. Typically in the 5-8k range. When 5-8k exceeds the threshold, it reduces strong, sibilant “S’s” without affecting the rest of the word.

There are 2 types of compressors: Tube and solid state. They have a wide range of quality and price ($200-$4000). Upper mid price and expensive compressors can compress a signal heavily and you can hardly tell it’s working. The sound remains much more natural than when an inexpensive compressor is compressing.

Tube compressors have such a great sound that sometimes a signal is put through it just to get the sound of the tubes while hardly compressing it at all.

Be careful not to overuse a compressor. When overused, it creates a very unnatural sound. The trick is to learn to use a compressor in a subtle manner.

Related Links
Karen Kane Website

Tags: , , ,

Recording LP Albums On Your Computer

Posted by Giacomo on February 10, 2012
Tips and Tricks / Comments Off

Most of us born before the 90s have an analog record collection that can best be described as being somewhere between ‘Spartan’ and ‘Now What Am I Going To Do With All These Albums?’. With the miniscule cost of digital media these days, the desire seems to be to transfer your analog record collection to digital format on CD-R disks or possibly to an MP3 library on your computer or maybe even both. How is this accomplished?


I’m glad you asked that question! Here is a list of the items you need to accomplish the transfer of your LPs to your computer: some albums, a turntable with a reasonable cartridge, a phono preamp, a pair of RCA cables, a computer with sufficient hard drive space, a software application that can record audio directly to the hard disk in the computer, a hardware interface (AKA, a soundcard), a software application that can record digital audio files to the CD burner in the computer, and some blank digital media. Let’s briefly discuss each one of these items.

Before you start any recording, you might want to clean up your albums so that they will sound their best for the recording process. If the albums have been sitting in a box for several years, you can bet they have dust all over them (especially sitting deep down in the grooves). Invest in a commercial record cleaner such as the Bib Groov-Kleen Audiophile ($20), the Discwasher 1006 ($15) or the Radio Shack 42-117 Pro Record Cleaner ($9). Heavily thrashed albums may require a professional cleaning if you are really serious about this process.

Any turntable that can hold a fairly constant speed and has a phono cartridge in reasonable shape can be used for this task. If the turntable speed is a problem, try replacing the belt if it is a belt-drive turntable. If the phono cartridge is shot, it is best to replace it with a new one to achieve the best results. Make sure the cartridge is properly aligned and that the tone arm counter-weight is set correctly for proper tracking with that particular cartridge.

The problem most people encounter with a turntable is that its output can’t be plugged directly into the line level input of any recorder. Most turntable cartridges put out a signal on the order of just a couple of milliVolts (mV). The turntable also can’t just be plugged directly into the mic inputs on a mixer, recorder or computer, even though they are setup to handle a signal with an amplitude of only a couple of mV. The reason is that when LP vinyl records are recorded, they have a special equalization curve (called the RIAA EQ curve) applied to the signal. The RIAA is the Record Industry Association of America. This special EQ curve is used to limit low frequencies and accentuate high frequencies when the disk is made to account for the limitations of the vinyl LP medium. Then when the LP disk is played back, the opposite EQ curve is applied to flatten the signal out again (i.e., accentuate the low frequencies and reduce the high frequencies). This special EQ resides in what is known as a phono preamplifier.

You will need a phono preamplifier to record LPs from a stereo turntable, but many new receivers and amps do not have a phono preamp built in. The least expensive standalone phone preamp I have seen is the MCM Electronics P/N 40-630 for $13.95. This will fit the bill nicely in allowing you to get your audio from LP to computer. Call them at 800-543-4330 or go to mcmelectronics. Another low-cost phono preamp is the Rolls VP29 for about $55. I recommend this one for the best price/performance ratio. You can find it at several places, including bswusa.

Generally, most turntables come with their own set of stereo cables, and you will connect these to the phono preamp inputs. If there is a ground wire coming from the turntable, connect it to the ground screw on the phono preamp (if there is one) or connect it under a chassis screw on the phono preamp. The sound card you have in your computer dictates what type of cable you need to go from the phone preamp outputs to the sound card input jack(s). Take a look at the sound card audio input jack(s). It may be a stereo 3.5mm jack or it may be a pair of RCA jacks. Buy a set of cables that will allow you to connect from the phono preamp RCA output jacks to the particular input jack(s) on the sound card. Make sure you use the Line In jack on the sound card, not the Mic In jack. If possible, try to keep these audio cables 6 feet long or less.

The computer you use can be a PC or Mac. All that is important is that you understand how to work the computer and that the computer has enough horsepower RAM memory and hard disk space to run the software application you have chosen for recording your albums. For reference, every stereo minute of uncompressed digital audio requires 10MB, so 1 hour of digital audio will take up 600MB on your hard disk drive.

The software application you use is a matter of choice and must be compatible with the computer platform (PC or Mac) you have. If you bought a decent sound card for your computer, then they usually come bundled with some sort of sound recording program that will allow you to record external audio from the Line In jacks and digitize it to your hard drive. If you don’t have a sound recording application on your computer, you can get one off of the Internet. Wave Repair has an audio recording application that you can download as Freeware at waverepair. Total Recorder is available at highcriteria for $12. Audio MP3 Sound Recorder can record any audio streaming through your sound card to your hard drive for $15 at mp3-recorder. CD Wave is available at cdwave for $15. LP Recorder is available at cfbsoftware for $50. For the Mac, I really like Micromat’s SoundMaker for $50 because it also includes a powerful audio editor (micromat soundmaker).

As far as PC sound cards go, I would avoid the no name El Cheapo models. Stick with the name brand models that come with powerful bundled software such as the Creative Sound Blaster Live 5.1 ($40). If your wallet is bigger, try the Creative Sound Blaster Audigy or Audigy 2 family of products. Other sound cards can be found at nextag. Macs are all already set up from the factory with their own built-in sound card capabilities, but you can add higher quality audio interfaces via USB if you desire (all it takes is money). For the Mac, you can add USB interfaces from Griffin Technology ($35 at griffintechnology) or from MacAlly ($49 at macally), however, you will still need a RIAA phono preamplifier.

Hook up all the connections, fire up the computer, launch the sound recording software application, put on an album and monitor it with your computer speakers. If you are getting hum, make sure that all the grounds are connected and that all the audio cables are seated properly in their jacks (and that they are free from oxidation). Other things to try are making sure everything uses the same AC outlet. If you still have the problem, try disconnecting other cables from the computer system that are not being used in this recording session (such as cable modem connections, TV cable connections, and other digital bus connections).

Find the loudest part on the album (if possible) and set the recording level so that this section of audio is a few dB below the 0dB level on the recording level meters of the audio recording software application. If the incoming audio goes over the 0dB level on a digital recording, it will definitely result in ugly distortion and ruin your recording. (Digital audio recording is not like analog audio recording on a cassette deck in this regard.) Now you can begin recording the album.

If the recording software you are using also has an editor associated with it (or if you have a digital audio editor as part of another program), you can record the whole album side and then split the tracks up into separate audio files later with the editor. This is quite a bit less tedious than recording one album track at a time and then creating a digital audio file of just that one track, however it can be done this way. Continue recording until you have converted the whole analog album into one or more digital audio files.

Once you have converted your album tracks into WAV (on a PC) or AIFF (on a Mac) uncompressed digital audio files, then you can burn an audio CD-R and/or you can convert those uncompressed audio files to the MP3 compressed audio format. To burn an audio CD-R, you will need a CD-R or CD/RW drive in your computer and the software that came bundled with it to perform the actual creation of the audio CD-R. Note that this CD Burning software is not the same as the Sound Recording software you initially used to record the analog audio into your computer. You could also buy one of the commercial programs available now such as Roxio’s Easy CD & DVD Creator 6 to accomplish this task. To convert an uncompressed audio file to the MP3 format, you will need the proper conversion software to accomplish this, such as MP3 WAV Converter (americanshareware), M3’s Encoder ( mthreedev), AudioConvert (AudioConvert), or any of the MP3 encoders for PC, Mac and Linux machines offered at jumbo..

Copyright 2003 Pacific Beach Publishing (John J. Volanski is an electrical/audio engineer who has recently written the book Sound Recording Advice. It is available at or at fine bookstores everywhere in the USA.)

Tags: , , ,