Advanced audio techniques for slide projection programs

In my article, "Advanced Two-Projector Program Techniques" (PSA Journal, February 2000, pgs. 16-18), I discussed tape recording equipment and the preparation of sound tracks for sound-synchronized slide projector programs.

Tape recording, however, has a number of weaknesses. If you inadvertently fail to include a music or narration segment, or make a mistake in a segment and don't catch it right away, it is necessary to make insertions later on, which is much more work. The traditional solution is to copy the tape to another one, making corrections as needed. This is a royal pain since it means starting and stopping both tapes, finding the proper insertion points, and using a third tape for the replacements. Moreover, the clicks that result when starting and stopping a tape further degrade its quality.

However, for those readers who have access to a home computer, these sound tracks can be improved and edited with a great deal more ease and precision than that afforded by cassette-to-cassette tape recorders.

Equipment

A cable is needed to connect your computer's sound card to your audio equipment. The cable (available from Radio Shack, Cat. No. 42-2483) has two RCA plugs on one end (this connects to the "line-out" of your audio equipment, typically the amplifier) and a 1/8" stereo plug on the other (this connects to the "line-in" of your sound card or onboard audio. (Note: Many computers have their audio built right into the motherboard; the connectors, however, are available at the back of the computer just as they are with sound cards.)

Transferring the sound from a tape to the computer takes place in real-time, i.e., if the tape contains 30 minutes of sound, it will take 30 minutes to digitize and record it (once you push the "record" button, you can, of course, walk away and have lunch or whatever!). For each minute of stereo sound recorded at a sample rate of 44,100 Hz and a 16-bit sample size, you must have roughly 10MB of free disk space; e.g., a 30-minute recording will result in a 300 MB sound file. Therefore, a hard drive with sufficient free space is required. However, I find that recording at a sample rate of 22,050 Hz and an 8-bit sample size results in a quality level that is entirely satisfactory for slide show audiences. At this level a 30-minute recording results in a file size of only 75 MB. Note that the actual file sizes you will experience may be 5 to 10 percent larger because edited sound files invariably contain additional information that you supply, such as commentary and marking points.

You also will most likely need some RCA-type cables to complete the connections between the computer and the audio equipment, the kind and number of which will depend upon your particular setup. To avoid having to connect and disconnect these cables, a pair of audio switch selectors - one for computer input and one for output - is useful (Radio Shack, 42-2112).

There are two methods for getting the sound file out of your computer:

1. Transferring to Tape: Using another cable of the same kind referenced previously, the 1/8" stereo end is connected to the auxiliary "line-out" of your sound card (the standard "line-out" is usually connected to computer's amplified speakers), and the RCA plugs to the "line-in" of your audio equipment. This is a digital-to-analog conversion. One problem that may arise is that some sound cards, and even more frequently on-board audio, do not have more than one "line-out" connector. In this case you must split the signal with a 1/8" stereo headphone Y-adapter (Radio Shack, Cat. No. 42-2463), one leg of the Y going to your speakers and the other going to the "line-in" of your audio equipment.

2. Transferring to CD: If you have a CDR and there is a CD player included in your audio equipment, the sound file can be "burned" to a CD. This is a lot faster than transferring the file to tape where again the transfer occurs in real time. Since it is a digital-to-digital conversion, there is no loss in quality. Also, if you don't have to re-do the editing and re-bum it is cheaper since CD-R blanks cost less than a tape. (Note: If you have a CD-RW, don't copy the file to a RW disk, as these usually cannot be played in an ordinary CD player. However, CD-RWs can also copy files to a CD-R disk, so this poses no problem for those who have such equipment.)

Software

Software is needed to digitize and record the sound from a tape and to edit the resulting sound file. The sound-editing program I use is Sound Forge XP 4.5 (www.sonicfoundry.com). It is easy to use and inexpensive (I got mine from www.buy.com for $30 plus shipping). Once the analog sound on the tape is converted into a digital form and recorded, it is saved as a digital WAV file. Then, using the sound editor, the WAV file is edited.

If the music you are planning to use is on a tape, then transfer is in real time, i.e., it will take 30 minutes to transfer 30 minutes of music from the tape to your computer. However, if it is on an audio CD, it can be transferred to your sound file directly in a matter of a minute or two (a process known as "ripping"). For this you will need another piece of software and the one I use is a "freeware" program (nope - doesn't cost anything !) called "Audio Crusher," downloadable at www.sysdesk.de. It is easy to use, fast, and the price is right. PSA Journal November 2001

Editing Sound

Unlike tape, where one must spend considerable time forwarding or reversing the tape to get to a target point, moving around in a sound file is both precise and fast. On the sound file you can, among other things, do the following:

1. Remove the noises made when starting and stopping the tape;

2. Add or replace narration or music segments with ease;

3. Adjust the volume of individual segments;

4. Fade sound in or out;

5. Add, delete, or modify segments of silence on the tape.

The procedure for removing starting and stopping noises is simple. When you play a sound file on your computer using a sound-editor, you not only hear the sound through the computer's speakers, but you see the waveform on the monitor. The unwanted sounds are shown as "blips" on the waveform. The playback is stopped at these points and the segment containing the blips is selected with your mouse and "muted," in effect replacing them with silence.

Adding or replacing narration or music segments is just as easy. After recording the additions or replacements on tape they are digitized to a different file. The segments are then inserted from this secondary file into the main file precisely where you want them. Another advantage is that although it is not always possible or easy to tape-record replacement music or narration to match the sound volume of the rest of the tape, in a digitized sound file a sound editor can adjust the volume and/or tone of any part of the file with just a few keystrokes. Also, although you can fade sound in or out using a tape deck, a sound editor can do this automatically with a single mouse click or, alternatively, you can specify the exact shape of the fade-in or fade-out amplitude.

The ability to add or delete segments of silence in a sound track comes in handy. For example, when adding music to the end of an existing music selection, you must first press the record button on the tape and then the play button on the music source device. Since this takes some time there is always a small segment of silence between the selections. The segment of silence can easily be removed in a sound editor, making the two musical selections sound as if they came from the same sound track.

Although there is not sufficient space in this article to examine sound editing in detail, the accompanying figure shows what a wave file looks like and some of the advantages of audio editing. The top row shows the narration for each slide and the bottom row shows the continuous music accompanying the narration. First, note that the intervals between the narrations are set precisely (to a thousandth of a second if you want!). The first four intervals are set to 8 seconds, the fifth to 16 seconds since there is a single non-narrated slide between them, and the sixth at 24 seconds since there are two non-narrated slides between them. Next, observe that the maximum volume of the music has been adjusted so that there are no sudden, loud changes that would be distracting or irritating to the audience. Many music selections that were unsuitable previously, now become useful.

Thirdly, note that the sound level of the s music has been lowered during the narrations so it does not interfere with the audience's ability to hear the narrator. This is a smooth reduction that starts about 1 second before the narration starts and fades the music gently to a low level, and then fades it back up after the narration is completed. It is accomplished by a single keystroke, using the Sound Forge "Graphic Fade" command. Finally, note that there is an annotation called "Mark" in the middle of the fifth narration. This is where I have changed the music. The change is made during a narration when it is low enough so that the audience doesn't notice the transition. Annotations can be as simple as this one word or as complex as a sentence. Either way they do not show up on the sound track when it is played and a single keystroke takes you instantly to any one of them.

In addition to perfecting your sound tracks, be they narration or music, computer-based sound editing is kind of fun. You can take your time editing, it is easy, and if you make a mistake you can re-do it and try again. It requires a modest investment in cables and software, but the end result is professional-quality sound in your presentations. As the saying goes, "The devil is in the details," and there is the inevitable tinkering with computer settings before everything works just right. I would be happy, therefore, to respond to any PSA member who would like additional information or suggestions. Contact me at aklee@fuse.net.

COPYRIGHT 2001 Photographic Society of America, Inc.
COPYRIGHT 2001 Gale Group