It's easy to forget that some of the technology we use is, to paraphrase a great author, nearly indistinguishable from voodoo. One reader wrote in asking for some clarification about lossless compression, and I explain the magic in this week's column. I also look at a question about iOS device backups, and one about missing audiobook chapters in iOS.

It must be magic

Q: I read your column regularly, and really appreciated your recent explanation of AIFF, WAV, and Apple Lossless formats. But I don't get it; how can the file size of Apple Lossless be half that of AIFF without some voodoo going on?

I received this email with the subject: Apple Lossless, Magic?. And I can understand that it can seem like there's some voodoo in this process, but it's actually pretty simple. (At least the concept is simple; the math behind it is a bit above my pay grade.)

Imagine that you have a text file with, say, the complete works of William Shakespeare. This text file contains 908,774 words, and takes up 5.6 MB on disk. If I compress the file using OS X's built-in Zip compression, the same file takes up just over 2 MB, or about 36 percent of the original file size.

Lossless compression for audio works in a similar way. Unlike, say, AAC or MP3 files--where psychoacoustic models are used to determine which parts of the audio can be removed without affecting what you hear--lossless compression formats simply compress all of the data in a file. When played back, these files are decompressed on the fly, so the compressed data becomes audio data again, in a bit-perfect equivalent to the original. Nothing is lost, just as none of Shakespeare's words are lost when I decompress the zipped file.

Bad backup

Q: The lock button on my iPhone broke, so the Apple Store replaced it. I had backed up the iPhone to my MacBook before exchanging it, but when I attempted to restore it to the new phone, iTunes said the backup was corrupt or had software not compatible with the new phone. Is there a way to get that backup onto the new phone?

Alas, no. If a backup is corrupt, then there's not much you can do. And because you can't it's worth thinking about your iPhone backup strategy for the future. If you sync your iPhone often, then each time you do so, iTunes will store a backup. It deletes the previous backups, though, so if the latest one is corrupt, you won't be able to work with it.

However, if you also use Time Machine to back up your Mac, you might be able to restore an older backup. iTunes stores these backups in /Users/yourusername/Library/Application Support/MobileSync/Backup, and each one is in a folder, with a name such as 4e6854637fb74a5a3d47e2bca56eacd7fb46197c3. (This name is your iOS device's UUID, or Universally Unique Identifier.)

If you go to that folder, then enter Time Machine, you'll be able to see older backups. You can try restoring one prior to the most recent one and see if it's usable. Make sure to replace the current backup with an old one or, if you keep the current backup, change its name so that Time Machine restores the older one with exactly the same name. You should also think about backing up your iOS device to iCloud every now and then. I discuss iCloud backups in this article from last year. Having both a local iTunes backup and an iCloud backup may seem redundant but in a case like this, it might be just what you need.

Where are my chapters?

Q: I recently downloaded an audiobook ofMoby Dick. It's rather long, and it's split into over 100 chapters. While these chapters display in iTunes, they do not appear on my iPhone when I play the audiobook. I loaded the audiobook onto a five-year old iPod nano, and it shows the chapters. How can I get access the chapter information when playing back the audiobook on my iPhone 5?

This can be confusing for people who don't often listen to audiobooks. Here's some clarity. When playing audiobooks, iTunes can display both chapter and parts information. You'll see the parts an audiobook contains by clicking on its icon in iTunes' main window. These are the separate audio files that make up the book. But if you double-click an audiobook and start playing it, look in the menu bar and you see the Chapters menu, which lists the book's chapters--the audio that equates to the chapters in the printed form of the book.

iOS 7's Music app displays parts but not chapters. You access a list of parts by tapping on the List button in the top-right corner of the Now Playing screen. Earlier versions of the Music app gave you access to chapters, but Apple removed that feature (unfortunately) with iOS 7 on the iPhone (and it was removed long ago on the iPad). I found it useful to know how long a chapter was, which is something you can't easily find out now. The only thing you can do is navigate from chapter to chapter by tapping the Forward and Back buttons. You'll see the name of each chapter after the author's name, but nothing more. For example, you won't see the length of each chapter, which you did see in the past.

The reader didn't say if they had bought this audiobook from the iTunes Store or from Audible. If it's the latter, using Audible's own app lets you view a chapters menu. It also lets you save bookmarks and change the speed in more increments than the iOS Music app. Audible's books can be cheaper than the same books from Apple--you can get a book-a-month plan for $15 a month, and other plans can cost even less. If you're a big audiobook listener, Audible is the better deal.

Have questions of your own for the iTunes Guy? Send them along for his consideration.