Welcome, this is your short video on compression. Compression is a very useful tool. It is often used at the end (of editing) for mastering. It is one of the universal tools that is used in mastering, that is the final process for putting things together for CD recordings, DVDs. Particularly in a car you get a lot of background noises so your quieter passages in music can be covered up by that background noise. Compression is a way of boosting the low audio levels without distortion in the high audio levels. I have taken some information from Wikipedia. I gave you the address up above. Let’s take a look at this. Here is an example of what we call the “knee.” This is compression. You can see this is the input. This is the output. The compression, the threshold has been lowered. There is the input of decibels and the output is lowered through compression. This gives you an example of the different ratios. Here is a 1:1, where nothing is happening. 2:1 is where you have reduced the sound in half. 4:1; infinity to 1 is just a gate or a filter, where compression is immediate and nothing comes beyond that level that you have set. It is important to understand that we have “hard knee” and “soft knee” in compression. The hard knee is where the point of compression starts immediately. Sometimes that is very audible to the ear. Often the soft knee is preferred compression. So in review, our reasons for using compression: 1. Use compression to avoid distortion with peaks in the file, where the sound overwhelms the speaker system. And also to boost the quieter portions of the track.
OK, so let’s have a look at Audacity and the use of compression. I want to preface this by first addressing the Electronic Media and Film students or those of you that have had experience with digital audio workstations. Certainly in this course for the later assignments such as the ones we find in units 6 and 7 you can use your own software. You will find that Audacity is lacking in many features. For instance, you cannot route tracks to a master track in Audacity. Many of you would use compression on the master track rather than individual tracks. You will also find that the compression tool lacks many features in Audacity. That aside, for this course the tools will be fine for our minimal uses here for our digital audio workstation. So I have loaded here my sound file. You can see that it exceeds, it clips the volume here, above 0 dB. Let me cancel that out here and let’s play this and see what happens. [Music] When we get the red appearing here in this area and when you see it exceeding 0, that means that the file is too loud—you are getting distortion. So we need to use compression on this file. Go to effectàCompressor and this gives us our simple compression tool. Let’s examine some of the features here. So threshold is where compression is applied. I have it starting at -12 dB. You can slide it back and forward depending upon where you would like the compression to start. The noise floor is the lower part of the compression. It is the lowest sounding, the quietest part. The ratio is the knee here. This compressor does not have a soft knee it only has a hard knee, which we talked about earlier. You can see that as I make compression more harsh, the knee changes. I will leave it around 2:1. That is just fine there. The attack time deals with how quickly the compression is applied. The release time is how quickly it is released and you go back to the original (uncompressed) sound. There is a nice feature—preview—here. So if I can move my compressor over here, move that out of the way a little bit. Alright, so I have cancelled this out. Let’s do this preview and see if we like what is going on here. [Music] |Well I somehow stopped it. Let’s do that preview again. [Music] OK, so it only allows a little bit in the preview. What we were seeing in this area was that it didn’t go above 0. That was just fine. What if we do a steeper ratio. Let’s preview that. [Music] OK. That allows it to be louder until it has a very steep cut off. Let’s go back here to the 2:1. Let’s say I like that and then apply the changes. Oh, you can see immediately that the visual representation of the sound file has changed showing that these peaks are smoothed out a bit. Let’s have another listen to this. [Music] OK, so for this file where I have a continuously sustained sound it is harder to hear how quickly the compressor comes in and out. However, if you are using drums or percussion instruments with sudden surges in the sound, then this attack and release time will be very important. You will need to play around this to make sure that it sounds as natural as possible. Very good. So be sure to use compression on your future sound files so that you do not have sounds that peak over 0dB. Sound File example: http://www.freesound.org/people/bigmanjoe/sounds/348275/