If you've ever delivered a finished album to a CD pressing plant, you've probably handed over a "DDP" and never once looked inside it. This is entirely normal, and it works right up until the day it doesn't. Here's what a DDP actually is, why it's worth checking before you send it, and a free online tool I built to make that easy.

What is a DDP image?

DDP stands for Disc Description Protocol, a format developed by Doug Carson & Associates in the 1980s to solve a very specific problem: a pressing plant needs one complete, unambiguous description of everything that goes on a disc, because at the moment a glass master is being cut there is nobody left to ask. It remains the standard master format the CD manufacturing industry uses to go from a finished mix to a pressed disc. A DDP fileset typically contains:

• DDPID and DDPMS: descriptor files that identify the set and map out its contents.
• PQDESCR: the PQ subcode data, meaning track start times, pre-gaps, ISRC codes per track, and the disc's UPC/EAN barcode.
• The audio image itself (a .DAT file): the full programme as one continuous 16-bit, 44.1kHz stereo stream.
• CDTEXT.BIN (optional): track titles, performers, composers and other CD-Text metadata.

So a DDP is really the entire disc: audio plus all the subcode information a plant needs to cut a glass master exactly as the mastering engineer intended. Think of it as the audio equivalent of a print-ready PDF, everything baked in, down to the smallest detail.

Why it's worth checking before you send it

A DDP is usually the last stop before manufacturing, which makes it an exceptionally bad place to discover a mistake, since anything that slips through gets reproduced, faithfully and at industrial scale, onto every disc in the run. A few things that can go wrong and are easy to miss without opening the image:

• A track's pre-gap is set to the wrong length, or missing entirely.
• An ISRC code wasn't embedded, or ended up on the wrong track.
• The UPC/EAN barcode doesn't match what's on the artwork.
• CD-Text titles or performer names have typos, wrong characters, or are missing altogether.
• The PQ sheet and the actual audio don't line up, so track 4 starts a second early, or the album fades out before the disc actually ends.

None of this is visible just by looking at the audio files. You need something that reads the DDP the way a pressing plant does.

A free, in-browser DDP player

That's why I built the Knurl DDP Player, a free tool that opens a DDP fileset, plays it back, and shows you everything that's encoded in it.

It's a web app, so there's nothing to install. Drop your DDP folder (or a .zip of it) into the page in any modern browser, and it:

• Plays the disc continuously, the way it will sound off the pressed CD, with a waveform and a track-by-track player.
• Shows every ISRC code, the UPC/EAN, and the DDP version.
• Reads CD-Text: titles, performers, composers, arrangers, even genre codes, when present.
• Lets you export a PQ sheet as a clean, columnar reference document.

It's also entirely client-side. The DDP is parsed and decoded right there in your browser using the Web Audio API, and nothing is ever uploaded to a server. That matters because a DDP is usually unreleased material, and the last thing you want, on the final step before manufacturing, is to hand a master-in-progress to a stranger's server just to double-check the track titles.

The more established desktop DDP players deserve a fair word here: they are good tools, made by people who care about the same details, and this isn't an attempt to replace them. They do, however, require an install and, in some cases, a licence. The Knurl DDP Player is free, runs on anything with a browser (Mac, Windows, Linux, even a tablet), and needs zero setup. If you just need to sanity-check a DDP someone sent you, or confirm your own export before it goes to the plant, it's the fastest way to do it.

Try it

Head to knurlmastering.com/en/ddp/, drop in a DDP folder, and check your PQ, ISRC, UPC/EAN and CD-Text in a few seconds. It's free, it's private, and there's nothing to install. And if everything in your DDP turns out to be perfect, you'll have spent half a minute confirming it, which is the cheapest peace of mind available anywhere in the manufacturing chain.