Compressors, Limiters, Loudness War, and Mastering Inception
by, 11-22-2011 at 07:00 PM (4553 Views)
Years ago, I was lucky enough to spend a decade as a live sound mixer and recording studio engineer in Western Canada. While most of my sound engineering experience was with rock bands, I had the opportunity to work with many talented musicians, recording/mixing many different types of music, from folk, country, jazz, choirs, and classical. I spent a quite a bit of time working with compressors and limiters, so I thought I would share some of my experiences with them, along with mixing and mastering. This summarized info may assist folks in their decision criteria for selecting high resolution downloads.
Here is a story to illustrate why compression and limiting is inevitable in rock (actually most multi-track recorded) music. I remember recording a bass player in the studio using a direct box. I wish it were this one: http://www.musicvalve.com/directbox.html This would be your audiophile direct box - a work of art and beautiful sound. I figure Computer Audiophile’s could appreciate that even in the pro sound category, some manufacturers are obsessed with sound quality. Back to the story, the point is when the bass player played his part and depending on how hard or soft he played on the strings, produced a wide dynamic range.
I was using a Sony 24 track analog tape recorder with 2" tape, Quantegy (Ampex) 456 brand: http://www.quantegy.com/specsheets/PDF/456.pdf . The general idea is to record as "hot" as possible on the tape so you get the best signal to noise ratio. The tape had quite a bit of headroom, (+10db) so it was popular to "saturate" the tape just a bit (ok sometimes slammed) that also gave the music a "hot" sound. A VU meter is used to measure the level of audio signal going to tape. Side note, that's the short version. The long version is that each strip in the mixing console had a VU meter and that was calibrated along with a corresponding VU meter on the tape recorder, x 24 tracks or more - the calibration took a long time). The idea is, on average, to record around 0db or +3db is hot, and the tape had over 10db of headroom.
The issue is that when the bass player played softly, the level on the VU meter was too low. So if I increased the gain on the input preamp on the console, I could not control the amount of headroom and sometimes when the bass player played harder on the strings, that would "pin" the meter and the result would be preamp and/or tape distortion. Further, given the wide dynamic range, the bass really did not "sit" well in the overall mix.
You may be surprised to learn how sensitive the VU meters are. It does not take a wide variation to swing from -20db to +3 on the meter. While I used the bass player to illustrate a point, recording music of any type, whether it be with a direct box or mics naturally have a wide dynamic range.
Even the most consistent musicians I have worked with, in the sense of even playing, still produced a wide dynamic range. So what choices are there to get the best signal to noise ratio, without distorting the tape (or direct box, input preamp, digital recorder, etc.)? I can't very tell the bass player to change his playing style or sound as you can imagine how nervous people get when they are being recorded in the first place. I have a lot of respect for muscican's to go on tape.
Enter the compressor. I think Wikipedia does an excellent job of describing how a compressor works: http://en.wikipedia.org/wiki/Dynamic_range_compression I am not going to rehash the operation or details here. From a critical listening perspective, depending on the compression threshold, ratio, attack, and release, the compressor will “shape” the waveform passing through it. Folks with an ear for the compressor can set it just enough to maintain good dynamic range, but not distort the tape or digital overload. Judiciously used, the idea is that the (critical) listener would not hear the "compression" working.
The point being, in order to get a good (and even) level on tape (analog or digital), with good signal to noise ratio, a compressor is more often times than not used on every single channel of a 24 track tape recording. Especially drums, voices, or instruments that have wide dynamic range and are closely mic’d typical of any studio/multi-track recording. High end mixing consoles like Neve and Solid State Logic (SSL) have compressor/limiter plugins for every channel "strip" on the console. There is usually a line/mic preamp on the strip, along with EQ, usually parametric with full range of adjustment over multiple bands, headphone mix level, effects level, and then the channel fader or volume control that then feeds the master buss or subgroup. A lot of electronics in the chain even before it hits the tape electronics and tape itself.
After spending many years recording and mixing sound, one of the side effects is that I can easily hear compression/limiting on virtually every single song I listen to. I am so intimate with it that I can probably estimate the compressor settings used and in some cases I can even tell the brand of compressor being used. Here is an example of someone that intimately knows the sound of UREI's line of compressor/limiters: http://www.gearslutz.com/board/568476-post2.html
How does this effect mastering?
So the compressor is not only used, most of the time, on a track by track basis for recording, but sometimes even on certain tracks during mix down. This happens when the individual track still has too wide a dynamic range to "sit" in the mix when it was put to tape. Additionally, there is usually a 2 channel compressor/limiter used on the mix down buss as well, that then is also going to a 2 track (i.e. stereo) recorder, whether analog or digital.
So 24 track mix down --> 2 track master
Even after the final mix has been created, sometime folks want to fiddle with the mix again and rather than mix down from the 24 track, they use the 2 track mix and feed it through the console again, apply whatever "processing" is required and generate another 2 track mix down (whether analog or digital). I am guilty of that myself.
2 track master --> processed 2 track master
That then goes to the mastering lab (or a copy for fear of the original being lost if the engineer did not mix down from 24 track to 2 masters) to be mastered onto whatever media for distribution. Even during the mastering process may repeat processing as described above. The Wikipedia article on mastering does a decent job on describing all of the steps: http://en.wikipedia.org/wiki/Audio_mastering As you can see by the time the music gets to you, the actual copy may be several generations away from the original master. This is especially true when discussing analog generations.
So what about remastering?
Again, I feel Wikipedia does a good job here: http://en.wikipedia.org/wiki/Remaster and I am not going to rehash. But this is the point of Mastering Inception, it is not clear as to the source of the material, where did it come from? How many generations is it? What was done to remaster it? Is it a different re-mix direct from the original 24 (48 track, whatever) track tape? Or is it a direct transfer from an analog 2 track master with no processing? Or maybe direct from 24 tracks to digital. Or is it all digital? Or did it get killed by the loudness war? These are excellent write ups on “The Death of Dynamic Range”: http://www.cdmasteringservices.com/dynamicdeath.htm and “What Happened to Dynamic Range”: http://www.cdmasteringservices.com/dynamicrange.htm
What is really disappointing to me is that even as far back as 1982, digital recording pioneers like Peter Gabriel and his all-digital album Security, that was recorded in his home http://en.wikipedia.org/wiki/Peter_Gabriel_(1982_album) has awesome dynamic range.
Mastering inception? Remaster? Master? Premaster? You may wake up one morning and come to Computer Audiophile to find the thread on HDTracks have come clean and they were are all CD copies that have been upsampled ;-) Just kidding of course. However, I searched all over HDTrack's site and if you read the wording in their Mission Statement: "It is our purpose to allow our customers access to the largest online library of DRM-free CD and DVD-Audio quality downloads complete with liner notes in a PDF format." And read from their About page, "Finally, audiophiles take note. HDtracks offers select titles in ultra-high resolution 96khz/24bit files. This is true DVD-audio sound quality for music lovers that demand the very best! " It almost sounds that their sources are from CD's and DVD-A and who knows, have they been upsampled?
As a side note, it would be nice to see Fleetwood Mac Rumours remixed from the 24 track master. http://www.soundonsound.com/sos/aug0...racks_0807.htm But if you read closely, given what was going on, and the tricks used by assembling bits and pieces from multiple 24 tracks is likely never to be reproduced again. Also consider that produce/engineers use track sheets to keep track of all the eq, effects, compressors, etc., being used. SSL consoles and others used computers to automatically keep track of fader positions, mutes, and the like so that the mixing engineer had more hands. Finally, consider that most pre- 90’s recordings were recorded, mixed on gear that simply does not exist or exist in a working fashion. So most material from the past, is likely to have come from an analog 2 track source. But on the other hand, it is a snapshot of history, never to be repeated again, but can be played over and over. Pretty cool if you ask me, provided of course that I am getting the best possible transfer.
Recently, I downloaded Tom Petty and The Heartbreakers - Damn the Torpedoes. Along with the download was a note from the Engineer and Producer. I have attached the note to this post, but here are the magic words I like to hear:
“We’re committed to finding the highest quality way to get music from Tom Petty and the Heartbreakers to you. We want you to hear it at home the same way we hear it in the studio.
We think the best audio option for your computer or media server is FLAC. FLAC is a high quality file that is the best sounding format for downloads. Unlike mp3, which discards elements of the audio to make the file size smaller, FLAC is a “lossless” format, which sounds the same as the source files it was created from. We made the FLAC files from the same high-resolution uncompressed 24-bit 96K master stereo files we used for the vinyl and Blu-Ray versions of Damn The Torpedoes: Deluxe Edition. When we compared those files to the FLAC’s, the waveforms tested out to be virtually identical.
FLAC captures the full dynamic range of the music from the quietest to the loudest sounds. Because of this (and because we are adding no digital compression) it will not sound as “loud” as a standard CD or mp3. To compensate for this, turn up the volume!”
So not all is lost. If we the consumer keep putting pressure on hi-resolution publishers of music to include where the source of the material came from and what the remastering process was, the quality should improve. And that really is the trick to discerning the sound quality of a new hi-res download – where did the source come from for the hi-res version and how was it remastered. Sounds simple enough (no pun intended), but trying to get that info is like pulling teeth and entering Mastering Inception.
A couple of caveats. I glossed over a LOT here. Hopefully, it is just enough info to give a you a flavor of compressors, limiters (the latter I really did not touch on, but virtually every LP that was mastered always had a limiter in the chain to prevent over modulating the disc while cutting) and mastering, premastering, remastering, and in a lot of respects, all the same thing, but at different stages between the 24 track analog/digital tape and you.
Another caveat is that Classical or other 2 channel recordings are typically recorded/mastered without compressors, limiters, or as little as possible in the signal chain and using as high-end mics and components as possible. I used to own a Sony PCM F1 Digital 2 track recorder when it first came out along with a pair of Crown PZM microphones (plugged directly into the unit) that I got to use recoding a number of choirs, orchestras, chamber music, including a cappella. Lots of fun and most of the time was spent getting the mics positioned in a stereo array and watching nervously at the level meter in hopes of getting a good signal to noise ration without clipping or having to ask the artists to perform yet another take.
Finally, Digital Audio Workstations (DAW) has revolutionized the recording industry. http://en.wikipedia.org/wiki/Digital_audio_workstation It has dramatically reduced the cost of having a “recording studio”. I have even heard marvelous uncompressed sound out of Garage Band http://www.apple.com/ilife/garageband/ with some good DI boxes and one or two decent mics. To a large degree, DAW’s being digital, don’t suffer near as bad when it comes to copying as it is all in the digital domain. Theoretically the same. However, I have heard of mastering engineers that will put in an analog loop so they can use their favorite tube compressor to get “that sound”.
My point in all of this is that it really makes a difference to find out where the source of the hi-res copy came from and how it was remastered. Those two decision criteria should assist in your hi-res download choices. Easier said than done :-)