Recording compact disc Digital Audio

Recording audio compact discs, also called Red Bookdisks" or " Red BookCD-DA" disks, is a popular subject these days. With the advent of the new generation of low cost CD-R drives, more and more people are making their own audio disks. Firstly, I'd like to point out that you should take care not to infringe on the copyrights of others when creating audio disks. Frequently, users want to make custom disks with tracks from their favorite albums. That's okay as long as you own the CD and the copy is for your own personal use - like to play in your car or take to work to play. But it is illegal to copy copyrighted disks to sell or give away. The information presented here is intended for legitimate applications.

So, why would you want to produce audio disks with a CD-R drive? If you are a recording studio, have a home studio, are a musician or a producer, the reason is clear. Recording studios, by and large, still give their clients a cassette tape as a reference. As we all know, tapes are poor quality compared with compact discs.

In this case, the client won't know how the music will sound on CD until disks are pressed. If a change is needed, say in EQ, then the disk must be pressed again. This is very costly in terms of time and money. If the studio could give the client a one-off CD as a reference, changes can be made quickly and at minimal expense.

If you are a musician and you want to send your project to a label, or even to a radio station for some free air time, a compact disc lends some credibility. How many cassettes do you think a major record label receives for consideration? Now how many compact discs do you think they receive? See the advantage? They are more likely to listen to the CD.

Since most CD-R drives are computer-based, I will cover the ways your computer can be used to burn an audio CD.

Sampling - Direct to Hard Disk Recording

The most basic approach is using a sound board with a compact disc player or tape player and a compact disc recorder (CD-R). Most contemporary sound boards can sample at 44.lKhz. at 16 bits. What does that mean? The sample rate is simply how many samples the sound card takes in 1 second of the signal you send into it's inputs. 44.lKhz. means that the sound board takes 44,100 samples per second. 16 bits is the resolution of the sample, or how deep it is.

In both cases, sample rate and resolution, higher is better. The higher the sample rate and resolution, the better the quality of the sound. Use 44.lKhz/16-bit when recording audio for compact discs. Connect the CD player or tape machine to the line input of the sound card. Follow the directions for the sound board software to start recording and send the audio you want to record into the sound card's inputs. This will record the audio to the hard disk, usually in the .WAV format for Windows (AIF or Sound Designer II for Macintosh). Check to be sure that your CD-R software supports the file format used by your sound card.

Generally, you will need a hard disk with an access time of 15ms or less to do this. Also, keep in mind that at 44.lKhz/16-bit you will need 10Mb of disk space per minute of audio. The average song is 3.5 minutes - that's 35Mb of disk space! When you are finished recording, you can check your recording by playing back the sound file now present on your hard drive. When you are ready to make a CD, follow the CD-R software directions to cut the disk. This approach is okay for many people, but it is really not the best. Most PC sound cards have, at best, mediocre analog-to-digital (A/D) converters. On top of that, all the RF (radio frequency) energy that bounces around inside a PC can cause all sorts of unwanted artifacts in the sound (pops, clicks, etc.). In my experience, the sound cards that produce the best sound quality are made by Roland and Turtle Beach.

The second approach yields the best sound quality and is suitable for professional use and commercial replication. Digidesign of Menlo Park, CA has been producing hard disk recording systems for the music industry for years. They have several levels of products that fit all levels of users. The concept of operation is the same as described above. However, the quality of the sound is superb. For the PC, the best product is Session 8. For Macintosh, the best system is Pro Tools. In addition, Digidesign offers accessory programs such as Masterlist CD for cutting audio disks with CD-R drives. If you want to sync sound to video, Digidesign offers PostView. This is the best solution, in terms of sound quality and integration. These system are surprisingly affordable!

Transferring Audio in the Digital Domain

What if you already have a CD with your audio on it and you want to transfer it digitally to your hard disk for editing? Or what if you have several sound effects disks and you need to take a track off each one for a special compilation? There is a way to accomplish this. Whether you have a PC or Macintosh, get a copy of any CD recording software that supports audio recording-- which includes most software packages today such as CD-Maker, Gear(Gear for Unix, Gear Windows, Gear Audio, etc.) or Nero Burning ROM. With the software packages, your computer and a compatible CD-ROM drive, you can "bounce" tracks from a CD to your hard disk. This bounce is a digital bounce; audio data is transferred digitally from the CD to the hard disk. Once the audio is on your hard disk in the appropriate format, you can play back the sound files to check them. When you are ready to cut a CD, follow the directions of your CD-R software to produce your CD.

Another digital approach I will cover is when you have DAT tape and you want to get the audio on your computer in a .WAV format (PC), or AIFF/Sound Designer II (Mac). Since the DAT (digital audio tape) is already digital, you don't need (or want) to send the sound through another set of A/D converters. The DAT must be a regular audio DAT (recorded on a consumer audio or professional audio DAT deck) recorded at 44.lKhz sample rate. If the tape is recorded at 48Khz, you will need a sample rate converter. For the Macintosh, Digidesign has a low end audio card with digital input/output. This card is called Audiomedia III. For the PC, Digital Audio Labs makes a card with digital I/0 called the Digital Only Card. With either product, connect the digital outputs of the DAT deck to the digital inputs of the Digidesign or Digital Audio Labs sound card. There are no level adjustments with digital I/0, just follow the directions for the sound card software to transfer the audio digitally to the PC in the correct file format (.WAV, AIFF, or Sound Designer II). The Digidesign cards can do sample rate conversion (48Khz to 44.1Khz). There is no generation loss because the transfer was done digitally.

The last digital strategy I will mention concerns the Alesis ADAT digital multi-track recorder, Digidesign Pro Tools or Session 8, and the Digidesign ADAT interface. For those of you who are unfamiliar with the ADAT, it is an 8-track recording system designed around the S-VHS cassette tape. The ADAT can record/playback 8 tracks of audio at a time. It accepts analog or digital inputs. Using the Digidesign ADAT interface, it is possible to simultaneously transfer 8 tracks of audio to Pro Tools or Session 8. If you only have a 4-channel Pro Tools system, you can only transfer 4 tracks at a time. Look into expanding your system and check out Pro Tools III! Connect the optical connector on your ADAT to the Digidesign ADAT interface and follow the directions of the ADAT interface software to transfer the audio to Pro Tools or Session 8. Once the audio is in, it can be edited and bounced down to a regular stereo .WAV file for Session 8, or Sound Designer II for Pro Tools. Use Digidesign MasterList or other CD-R software to cut the disk.

*Original article by Bob Demoulin
May 1999