Sound FX Metadata Tech Specs: Demystified

Metadata is a bit mysterious. It’s complex. It’s invisible. It secretly gloms onto the side of sound fx files. It is revealed only when viewed through specialized “metadata apps.”

For such a small, hidden element of sound fx libraries, metadata has enormous importance for sound pros.

Earlier articles here introduced metadata. A recent post shared what good metadata looks like.

But what is metadata itself? How does it work? What’s the difference between the different flavours of metadata, such as ID3, BWAV, iXML, and others? Is one type better than others?

As with a lot of things in pro audio, most information is more complicated than it has to be. It’s common to see discussions about metadata filled with wordy, confusing terms.

So, this post will be an overview meant to briefly explore the tech aspects of sound fx metadata. Why bother?

Well, metadata software are complex apps. Each supports metadata differently. Knowing the broad strokes will help you choose the best app for yourself and your sound library. It will help you add proper metadata to your files to give your fans precisely the metadata they need.

Please note: I am very detailed. This article will take you 10 minutes to read. Click the button below to email the article to yourself to read later.

What is Metadata?

The Concept of Sound FX Metadata

What is sound fx metadata?

Metadata is text information that describes a sound clip. It uses words and letters (as opposed to sound) to explain a sound effect using a number of different fields or columns, such as Description, Category, Location, Microphone, and others. Sound fx metadata may also include images, too. Album covers or photographs of field recording locations are examples.

The goal of sound fx metadata is to describe the sonic aspects of field recording as fully as possible using text. This text creates valuable search terms or keywords that help sound pros find the clip later, often from amongst thousands of other similar clips.

Depending on the app, the information in these fields is sometimes called tags.

Optional Metadata

One cool thing about metadata is that it isn’t pushy or controlling. We’re all familiar how some products lock you into using a feature forever (e.g., the way printers demand you only use their approved toner cartridges). With metadata, however, if you don’t want to use it, you don’t have to.

In fact, you may see and listen to a WAV file in your MacOS Finder without ever knowing there is metadata attached to the sound. You’ll be able to enjoy it regardless.

Invisible Metadata

Metadata is cleverly hidden. It’s invisible in the sense that you can’t hear it or see it. Metadata’s cascade of bonus text is revealed when the sound files are viewed through metadata apps like Basehead, Soundminer, Soundly, and others. View a list of metadata apps.

The downside to this is that in most cases everyone will need a metadata app to see the information. So, if you write metadata in Basehead, Soundly, or Soundminer, there’s no guarantee the text will be seen by Windows or MacOS, or in iTunes. Most of the time you and your fans will need a specialized app to detect it and view it.

Storing Metadata

Where is this metadata stored? Is it part of the audio? Can you hear it?

No. The metadata will not interfere with the audio whatsoever. Why not?

Well, the metadata is not saved within the audio portion of a sound file. Instead, it is saved to a sound file’s header. This is a tiny bit of data that is located before the audio begins. This header guides the sound, but isn’t part of the audio, much in the same way a book’s table of contents describes a novel, but isn’t part of the story.

Within this header are smaller portions called chunks. You may hear people mention metadata chunks. Each chunk stores info for a different flavour of metadata. Now, there are many types of metadata. Examples are BWAV BEXT, ID3, iXML, and others. Each of these formats will have their own chunk hanging out beside the others in the sound file header.

Together these chunks surround the sound effect like an invisible wrapper around a candy.

In other words, the metadata becomes attached to the sound effect, but doesn’t change it inherently.

Writing Metadata

How is metadata written to a sound file?

That’s the tricky bit. Normally, this cannot be done from Windows or MacOS itself. Instead, that’s a metadata app’s task. The app will write all new text changes to the file. It will save them to the chunk we learned about earlier, in the sound file’s header. The app may write metadata as soon as text is entered in the app. Other apps require the text to be manually saved with a key command or a menu selection.

Whichever the case, this commits the text to the file itself. So, if the file is copied, or sent to someone else, the metadata will travel with it. This is a major benefit of sound fx metadata: it allows the bonus text to be shared with others. This is particularly helpful to sound library creators. It provides all the rich information to sound library fans to help them find sound fx quickly and accurately.

Writing metadata to a sound file is often called embedding, burning, wrapping, or injecting metadata.

Types of Metadata

And this is where the similarity of metadata ends. Why?

Well, there are a handful of metadata formats and approaches. Generally, they break down like this:

Open vs. Closed Metadata Formats

• Open metadata formats. This is a format that any app creator can edit, save to, and view. A simple example is the ID3 format used by MP3s.

• Closed metadata formats. This type of metadata is created by one special metadata app. It may also be called a “proprietary format.” The text information will be encrypted. That means no other app can access it. Only the original app can create this proprietary metadata, edit it, and save it.

  Proprietary formats are often sophisticated and powerful. The theory is that they limit access to provide a reliable metadata standard and to protect their investment creating their metadata app. Soundminer’s native Metawrapper metadata format is an example.

Fixed vs. Open Metadata Specifications

When metadata first was applied to sound files, hardware manufacturers and software apps couldn’t agree on what fields to include, or what should be stored in them. The result was a complete mess of incompatible fields and data. Metadata applied by one app couldn’t be shared with another.

The solution? Create metadata standards that everyone must follow.

• Fixed metadata specification. This metadata style has rigid field names. They cannot be altered. What’s more, they may only store one type of information in them, say, either text or numbers, or a date. For example, BWAV has fields such as:

  • Description (text, 256 characters).
  • Originator (text, 32 characters).
  • OriginatorDate (date, YYYY-MM-DD)

   

  There are other fields. However, the point is that if you want to save data to a BWAV file, it must be added to an existing field in the way it expects. This is done to create a reliable standard that will work across a variety of apps or hardware. So, it’s not possible for a metadata app to create a new BWAV field called “Field Recordist” and write “Sound Wizard” to the field. If an app wants to use the BWAV spec, it must play by the rules and use only the existing fields as described by its specification or spec.

• Extensible metadata specification. Some metadata formats allow apps to tack on extra fields. This allows the app to use all the rigid, fixed, existing fields and add on whatever bonus fields they desire.

  For example, iXML can use fields such as Project, Scene, and Tape. However, it also allows an app to build upon this. So, it can include those fields, and then add a field called “Field Recordist” for our Sound Wizard.

  This allows the format to be extended, which is why it is referred to as an extensible format.

  This allows a metadata app to create flexible field types and names and expand upon established metadata formats. However, since the new fields are beyond the rigid, original field names, there’s no guarantee any other app will see them. However, it’s a good option to extend the power of a metadata format when the information will be used only within one metadata app.

Formats

Here are some common metadata formats you may discover when using the most popular audio metadata apps.

Note: this list is not exhaustive. Do you know of additions to this list? Please contact me and I’ll keep it updated.

• BWAV. The Broadcast WAV standard is the most common metadata format for sound pros. It writes metadata to a BEXT chunk.

• iXML. The iXML metadata format was designed by a group of manufacturers to create a new standard of metadata to work with BWAV files.

• XMP. The Extensible Metadata Platform was developed by Adobe to add metadata to all audio and video file types. It builds upon the iXML format.

• Soundminer Metawrapper. This format writes to a proprietary metadata flavour that includes fields specifically designed for post-production sound pros such as Microphone, Category, Subcategory, and so on.

• ID3v2. The ID3 metadata consumer format is a typically associated with MP3s.

  ID3 may also tag along with BWAV files (Basehead, Soundminer) giving two metadata formats for the price of one. In a nice bonus, when paired with AIFF files, ID3v2 tags embedded in AIFF files will display metadata in iTunes (unlike WAV files).

• MP4. The MP4/M4A audio and video format evolved from Apple’s QuickTime. It has its own metadata version. As a bonus, XMP metadata can tag along with it as well.

• Vorbis. The Vorbis comment metadata format was designed to be used with Vorbis, FLAC, Speex, and Opus audio file formats in a manner similar to the ID3 spec.

• LIST-INFO. This format was the original metadata specification for WAV files, created in 1991. It is still supported by apps today.

Chart of Metadata Formats

How Metadata Formats Help You

How can this information help you?

Perhaps your fans are video editors. You will want to choose a metadata format that they can use in their video editing software. And, since not each metadata app writes to every format, the list will help you choose an app that will write to the format you need.

In most cases you will be safe with BWAV, since it is widely adopted. iXML is more flexible because it is allows additional fields to be tacked onto the original specification.

Metadata tech specs are complicated. Don’t let them overwhelm you. Instead, use the info here to wrangle the tech specs to deliver the best metadata to your fans in the style they crave.

Read More

• A good summary of metadata formats with metadata apps and software options.
• A PDF report on the BWAV metadata format.
• A 2011 paper that examines which editing apps accept what metadata formats.

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