CD-DA Format

After spending some time with my mother on the phone last night involving audio CD formats and why or why iTunes won’t import them, I’ve decided to do a brief write-up on the now ancient CD audio format.

I first knew this format as “Red Book” audio format. It’s a spec dually held by Philips and Sony and was released in 1980. It calls for the following:

  • 2 channels of LPCM (or PCM) audio, 16-bit signed values samples at 44100 Hz.
  • A maximum capacity of 74 minutes of audio (later expanded to 80 minutes)
  • Maximum number of tracks is 99
  • The audio data is in a 2,352 byte block

Wikipedia tells me the format is apparently still licensable for $5000 from Philips which is kind of shocking to me considering how old the format is. Why isn’t this open-sourced yet? I can only assume the logo that Philips lets you license is also part of that 5k. Anyway, at its simplest CD-DA or Compact Disc Digital Audio is stereo audio at 44.1 Khz. When you pop it in a traditional CD player the device reads the information at 1x (about 150 Kib/second) , decodes the LPCM audio and streams it to your speakers.

CD-DA Logo

The CD-DA logo, imprinted upon many a CD.

I recall playing a brand-new CD-ROM based game that I had as a kid with our new Sound Blaster 8-bit audio card. This game loaded data off of the CD-ROM at 2x (making this horrible seeking noise), and then played Red Book audio from the audio portion of the image, thus freeing the CPU to continue rendering the game loop and not having to worry about decoding that audio on its own. This was also my first exposure to IRQ’s (Interrupt Requests) and how you could tell the processor that you wanted to deal with a specific hardware entity, in this case the decoding engine on the sound card. I though it was awesome.

Back in the mid-90’s I was starting to share media with friends across the Internet. With broadband having almost zero penetration at the time I was stuck using a 56k modem and a phone line to get out to the world. Some of you will recall that few people were ever able to get up to the so-called 56,000 bits of data that our modem’s were supposed to bleep merrily down the phone line so often I was reduced to an even slower mode–44k most of the time. On the web I found guys who were trading effectively CD quality audio via encoded files called MP3’s. Developed by the Fraunhofer Institute, MP3 went on to become a standard format for easily encoding and transporting audio–it sounds decent enough, files went from bulky 60 meg .WAV files to 3.5 meg MP3’s. It was great and it transformed the industry. CD burners became cheap and fast enough to put in home computers, media became widely available using a dye sandwiched in between two layers of plastic. Almost overnight we started using these devices more and more and seeking new and higher resolution audio formats for our media.

One of those was the Super Audio CD (SACD).  SACD was also developed by Sony and Philips and offered almost 8 GB of storage on a single disc and a higher audio fidelity for those who had new SACD playback devices. The format never took off and really the only remnant I have of it is a John Williams soundtrack CD from 1999.

Another extension known as CD-Text buried Artist, title and other relevant information in the 5 kb storage area know as a lead-in on a CD-DA disc. This does not adhere to the Red Book standard, but most newer playback devices (car stereos for example) can read this data and output it in some way to the user.  Apparently its rare to find the old Philips CD-DA logo on discs any more since rarely do the adhere to the standard. Not having bought a CD in years I don’t know about that but it stands to reason.

I think at a later date I’ll delve into some of the stranger formats I’ve used included hybridized discs.