[TNT-Audio Vintage column] Suggested technique for removing pre-emphasis from ripped files

Welcome back from recess; I hope you all wore your masks and stayed at least six feet apart during your rugby game. To continue where we left off, now that we've outlined the problem with pre-emphasis, we need to figure out how to solve it. Wouldn't want ol' Procrustes having the last laugh, now would we?

As a preliminary point, what follows is directed to Microsoft Windows users who, like me, have already copied, or "ripped," their CDs to hard drive, perhaps over a span of years (in my case, since 2007 or before), and only belatedly learned of the pre-emphasis problem. Now we're faced with possibly large numbers of files already stored but in need of correction. To address them, you'll need your original CDs; if you are one of those folks who say "copyright be damned" and sell or otherwise dispose of your discs after making copies, you're probably out of luck. I myself am able to proceed without major inconvenience only because just before the COVID stay-home orders came down I closed out an off-site storage locker and brought home boxes and boxes of CDs that I'd put there in bulk storage after ripping them. I sometimes say, “I never disposed of a record that I didn't end up regretting it”; in this case, I'm certainly glad I didn't listen to certain family members who advised, “Oh, now that you've copied the discs you can get rid of them and save some space, right?” Oh, my apologies to Apple adherents; I have no experience with Apple products and can't really help much there.

OK, with that admonition out of the way, we can proceed. The first step to solving a problem, the saying goes, is to admit that you have it. The first step to addressing pre-emphasis is determining whether a particular CD incorporates it. Some early CD players actually had an indicator that would light up if they detected pre-emphasis (see example below), but as far as I know that feature was never widespread and hasn't appeared on any player made in this century. That leaves us with computer software, and it isn't all that well-attuned to the issue, either. One old beta implementation of Exact Audio Copy (“EAC,” for those in the know) would pick up on pre-emphasis and show a telltale, and I gather the ripper in iTunes will detect and compensate automatically, but I've also read it degrades stereo separation in the process.

Do you need to check every CD in your collection? How widespread is this problem, anyhow? The answers to these questions are “no” and “not pervasive, but more than you might think,” respectively. Pre-emphasis faded from the scene for the most part by the early 1990s. While not perfect, a sensible strategy is to check discs from or earlier than maybe 1992 or 1993 and particularly those from the early to mid-1980s, recognizing that even during that period the practice was far from universal. As we shall see, you can encounter pre-emphasis in later issues, but for the most part it's an “early CD” problem. That said, if you have a late issue disc of recordings actually made during the suspect period, further investigation may be in order. On the other hand, you can safely ignore those made after the mid-1990s unless they are on a label known to have continued the practice after the early days, such as Supraphon, Denon, and BIS.

A good, but somewhat cumbersome, tool for chasing out which discs have pre-emphasis is software called CueRipper, part of a package called CueTools available for download here. In theory, every CD with pre-emphasis is supposed to have a flag in its table of contents and also warning signals embedded in its so-called “subcode,” meaning little silent warnings to the playback machinery mixed into the disc's audio datastream. In practice, sometimes discs fail to include the table of contents flag. According to the Hydrogenaudio website's thorough discussion of pre-emphasis, most software that addresses the issue at all reads only the flag in the table of contents; CueRipper, by contrast, looks there and at the disc's subcode. Although good, it's not the best software for actual ripping, but for detecting pre-emphasis it may well be the only practical choice.

I've been experimenting with it for a little while, and the workflow I've developed is as follows: place a suspect CD in the computer's CD drive and start a rip with CueRipper. As a first step, it will read the disc checking and logging gaps between tracks; if you look in the lower left corner of your screen, you'll see a notation to that effect. Once that notice changes from “detecting gaps” to “ripping,” unless you actually need to rip the disc, click the “abort” button at the lower right corner of the screen to stop the process. Then, in File Explorer, go to the folder in which the output is stored (on my Windows 10 machine, a subfolder of c:\users\david\music) named in a way that evokes the title of your disc--it may be the name of one of the performers, it may be the name of the album itself, it may be something else, but it will be recognizably related. Go down that chain of folders as far as you can, and you'll find a like-named file with the extension .cue. If you stopped the process after the gap check, it will be the only file there; otherwise, it will share the folder with your ripped tracks. Although actually a text file, it may well show as being a media player file. However it shows up, open the file with a text editor like EditPad Lite and skim through the report looking at each track for “FLAGS PRE.” If you don't see that notation, you're safe: the disc has no pre-emphasis. If you do, however, you have pre-emphasis and will want to address it.

Bear in mind that some discs, particularly compilation discs drawing on material from various times and sources, may have some tracks with pre-emphasis and some without. One example, also an example of a suspect recording in the current catalogue, is a three-disc set of Mendelssohn's youthful “string symphonies” by Ross Pople leading a group billed “The London Festival Orchestra” on the Hyperion label. Recorded in sessions spanning the years 1985 to 1990, it's still in the Hyperion catalogue, available from the label's website on CD or as a download, although with a new number; I don't know if the current issue has been remastered to eliminate pre-emphasis. The set was also issued on CD under license by the Musical Heritage Society, and I don't have it in that form to check, either. Be that as it may, the original Hyperion issue had pre-emphasis in tracks 13 to 15 of the first disc; all tracks of the second; and tracks 1, 7, 8, and 9 of the third. The other tracks on discs 1 and 3 were free of pre-emphasis. A quick glance at only the first entries of the disc 1 cue sheet would miss that the disc is mixed.

Having found pre-emphasis, one way to remove it is with a program called Sound eXchange, or “SoX,” available for download here. Earlier I promised a little bit of vintage computer tech, and here we go. SoX is what's known as a “command line” program, one invoked and run in the way everything was before graphic user interfaces like macOS and Windows became the dominant model for personal computers. Rather than clicking on an icon, you type a command in bare text to the right of a little arrowhead. Don't worry about that, though--once the program is set up, there's a way to make it do its stuff with the familiar graphical interface by drag and drop. Bear with me!

Once you have run the SoX installation routine, as usual you'll have a folder for the program filled with an array of curiously named files. One is the Sox executable, but because Sox is a command line program, double clicking it will not launch it in Windows but rather will open a command line (DOS lookalike) window with a mess of text that scrolls through too fast to read and then closes again. You can run the software by manually invoking a command line window, but there's a much easier way once you do a small amount of setup. Here's how.

First, move the Sox files to a folder with an easy name--say, “Sox” in the root directory of your c: drive. Presumably because it's a command line program, Windows doesn't seem to care where you put it, and later you'll want easy access to it through File Explorer, without needing to go down a long chain of folders and subfolders. Now, open that folder and find a file called “batch-example.bat.” Open it with a text editor (not a word processor), or right click it and select “edit,” and you'll see a line reading as follows:

Exit the now-edited file if you're in “edit” mode or, if you're in a text editor, save the file with a new name, say “batch-deemph”; if your text editor is like mine, it will pretty much insist on giving the new file a .txt extension. Before you can make use of the file, you'll need to change that to .bat. By default, Windows File Explorer won't show you extensions, so you'll need to turn them “on”; take the following steps:

Having made extensions visible, in File Explorer navigate to the Sox folder and find your modified batch file. Right click and rename, changing the extension from .txt to .bat. Ignore the dire warnings from Windows about how doing so may bring the Second Coming of Donnie and Marie and select OK. You now have set up a way specifically to address pre-emphasis in graphical Windows and need have no more interaction with old-style command lines.

The last step, optional but strongly recommended for reasons I'll explain shortly, is to put the directory in which you installed Sox into the system's path statement. Rather than detail the process here, I'll refer you to this helpful website, which gives nice, step-by-step instructions with screen shots.

With one possible caveat, now all you need do to de-emphasize ripped files is to select them as a group in their accustomed folder and drag-and-drop them onto the icon for the batch file as modified above. It will open a command line window and display, line by line, the commands it executes as it converts the file(s). Be patient, as it can take a while between files. When done, it will display “pause” and instruct you to press any key to continue. Press the key of your choice and the window will disappear. Your de-emphasized files will be saved with the same file names in a folder called “converted,” most likely appended to the folder in which you placed SoX (and from which you ran the batch file) but just possibly appearing in the folder where you originally stored your pre-emphasized files; check both places. Whichever proves to be the case, from there you can move your de-emphasized files to wherever you prefer to keep them. Be sure to clear the “converted” folder before running another conversion.

I promised an explanation for adding the SoX directory to the path statement. If you don't, you'll need to copy the target files to the SoX directory before dragging them onto the batch file icon; otherwise the batch routine won't find and run SoX to apply the de-emphasis. If playing with the path statement is too much fuss, or if you need to de-emphasize only a few files, that's certainly doable, but it adds an extra pesky step to each conversion. Also, I mentioned the possibility of a caveat: pay close attention to the file types supported by SoX. SoX handles a wide array of file types--including most or all of the most popular ones, such as .wav, FLAC, and on and on--but if your files are stored in a format not listed in the documentation, you'll need to convert them separately to a supported format before you can run them through the batch de-emphasizer. Unfortunately, that's my situation: nearly all my files are all stored in Monkey's Audio .ape losslessly compressed format, and SoX is not compatible with .ape files. Therefore, I must decompress every file I convert. Had I adopted FLAC 'way back when I started my voyage into mass computer storage of my music, I wouldn't face that issue.

Below are a couple of examples to demonstrate the effects of uncorrected pre-emphasis. Underneath each image (and obviously I chose these recordings for their audio, not their distinguished cover art!) are buttons, appropriately labeled, for playing the music with pre-emphasis and after de-emphasis with SoX. Click a button to hear the audio extract and click the “stop” button at any time to turn off the audio then playing.

The first example is about the final minute of Beethoven's seventh symphony performed by the NHK Symphony Orchestra of Japan under its then-Honorary Lifetime Conductor, the Croatian Lovro von Matacic. The disc is on the Denon label, the products of which frequently were pre-emphasized, and bears a production date of 1995 but a recording date of 1984. Pay particular attention to the high trumpet line and how pre-emphasis unduly increases its prominence. Also note how even the applause after the final note sounds unnaturally bright.

The second example is about the first minute of J.S. Bach's Prelude and Fugue in C, BWV 531, performed by French organist Michel Chapuis on the Schnitger organ at the Church of St. Michael in Zwolle, Holland. I believe this recording, dating to the late 1960s and part of a largely complete cycle of Bach's organ works, originally appeared on a Telefunken LP, but this CD issue is on the French Auvidis Valois label and bears a production date of 1987. Although the series ran to 14 CDs, only volumes 2, 5, and 6 had pre-emphasis. Within the past few years, the set was reissued in a single box on a label called United Archives. I don't know whether that release removed the pre-emphasis. As an aside, the first 25 seconds or so of this bravura fanfare, including the trill at the end of the passage, are played entirely by foot on the pedalboard! From the end of the pedal fanfare onward note how pre-emphasis exaggerates the already high registrations chosen by the organist.

Conclusion

Pre-emphasis may be a relic of a bygone audio day, but it lingers on to bedevil those of us who choose to store digital audio as data on computer hard drives for playback rather than playing individual physical discs. It also is a problem for those whose CD players do not conform to Redbook standard by automatically detecting and compensating for it. I hope the foregoing has given you a working understanding of pre-emphasis and one solution to the problem. It should be especially helpful to those of us who prefer to store our music on computer drives, and I see a crestfallen Procrustes glowering out on the playground. Note, however, that even those unlucky souls who prefer to play physical discs but who own non-conforming players may wish to copy affected CDs, de-emphasize, and then burn the results to home-generated audio disc for physical playback. The techniques outlined here will work just as well for that purpose, too. OK, class dismissed. Hey, but I hope you took good notes--we'll have a short quiz next period!

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On an Overgrown Pathé

Procrustes Strikes Back

Part II, in Which We Solve the Riddle (of Pre-Emphasis, not the Sphinx)

Conclusion