Square One: Audio Poltergeists

Feb 1, 2009 12:00 PM, By Brian Smithers

           
 

HOW TO EXERCISE AN ASSORTMENT OF SONIC SPOOKS

Time to Worry

FIG. 2: SoundHack is a venerable freeware program that offers many useful, if sometimes obscure, functions for operating on audio files.

Several types of timing problems can be hazardous to digital audio. The most obvious timing problem is a file with the wrong sampling rate. There are various ways this can happen, most of which boil down to somebody or something telling the DAW one thing and the master clock another. In this way, a file gets recorded at an actual speed of, say, 48 kHz when its file header identifies it as a 44.1 kHz file. Import it into another session — or play the original session back with the master clock set correctly — and it will sound almost a whole step lower than it should. The easiest fix is to edit the file header to reflect the proper sampling rate, using software such as Tom Erbe's SoundHack (soundhack.com; see Fig. 2). In some DAWs, you can accomplish the same thing by importing the file into a 48 kHz session without sampling-rate conversion, then importing it back into the 44.1 kHz session and letting the DAW convert the sampling rate.

Jitter is the inherent irregularity of a clock signal, and it causes converters to sample earlier or later than they should. Jitter in A/D conversion captures that distortion to the file, whereas jitter in D/A conversion has no permanent effect on the file or sessions being played back. Use high-quality converters, and use a stable word-clock source when synchronizing multiple digital devices. If you have no master clock and are simply daisy-chaining the word-clock signal, use your A/D converter as the master clock when recording and your D/A converter as the master when mixing or printing a mix.

Jitter between digital devices is irrelevant as long as all devices follow the same clock. If there is any confusion about the clock chain, however, samples will be dropped or doubled as clocks drift apart. Use the correct cables, terminate the end of the chain, and learn your gear's temperamental side. When rational analysis fails, don't be afraid to move past what should work and experiment with different chain orders.

Ex Files

Applying lossy data compression, such as MP3, inescapably reduces the audio quality of a sound file, and converting the file to an uncompressed format such as WAV does not undo the damage. A less obvious artifact of data reduction is a tendency for a waveform to drift out of sync compared with the original. Trying to remarry a soundtrack to a video when each has been compressed can be frustrating. Careful editing and judicious use of time compression and expansion can save the day. It's better to use the uncompressed originals if they are available.

Sampling-rate conversion is sometimes a necessary evil, but it should be handled with care. Always use your DAW's highest-quality conversion algorithm. When possible, don't convert between multiples of 48 kHz and multiples of 44.1 kHz, as the required math is complex. (For an explanation of the math involved in converting between 44.1 and 48 kHz, see en.wikipedia.org/wiki/Sample_rate_conversion.) I may say “24/96” for simplicity, but I actually record CD projects at 88.2 kHz. If you can, leave the sampling-rate conversion to the mastering engineer.

There is, however, no sonic difference between the various PCM formats. You can convert from WAV to AIFF to SDII and back to WAV and never alter the sound.

Keeping Murphy's Law at bay means using the left side of your brain to manage your right-brain activities. Plan ahead for your creative sessions so you can face the music with as few distractions as the law allows.

Brian Smithers is department chair of workstations at Full Sail University and the author of Mixing in Pro Tools: Skill Pack (Cengage Learning, 2006).