The Deadly Analog to DV Audio Level

Tutorial: Page (1) of 1 - 02/17/03

The Deadly Analog to DV Audio Level By Douglas Spotted Eagle

Without going into a history lesson, dB levels were established in the early days of telephone and radio. In fact, the deci-?bel? was named after Alexander Graham Bell, inventor of many things including the telephone. The early years determined that 0dB would equal one milliwatt.

Converting Analog to Digital
Unfortunately, this measurement was distorted by a number of factors, such as fluctuating changes in voltage. Also, the meters back then would bounce so fast due to the dynamic range of speech or music, that the meters really did nothing but look interesting. So, a damping device was created that kept the meters at an averaging point, making the meters usable, and more readable. Specific voltage combined with a determined length of time created the average, and led to what we now call the Volume Unit (VU) meter.

Once a standard was established, it became easier for manufacturers to calibrate their equipment so that everyone?s media had the same level. Except that aging magnetic media, machines with lesser or greater voltage, and a number of other variables created still yet more inconsistencies. So, it was determined that a test tone should be applied to

Converting analog to digital

the head of all recorded media in order to calibrate the equipment of the broadcast or duplicator to the equipment of the recording studio. 1K became the defacto standard, with video color bars generally added for video projects. This was supposed to assure consistency of audio level between program material, announcements, and commercial advertisements (most of us can recall the commercials that used to come on being a lot louder than the television series we might have been watching in years past).

Mix audio at ?5dBfs (Fig.1)

The digital environment is substantially less forgiving than its analog counterpart. Analog levels that exceed 0dB often have a warmer, more ?saturated? sound. Digital audio that exceeds 0dB sounds akin to glass crunching in the ear. In the digital realm, most software is calibrated to an equal standard. Therefore, when a recording is finished in one application, it can be opened for processing or layback in another, with consistent levels, as there is no hardware flow in the chain that can alter the level.

So, where?s the problem?
The problem lies in matching the currently analog world of broadcast, i.e., Beta SP machines and their counterparts, to the digital realm. Many broadcast engineers or duplication houses will use the test tone from a DV tape to calibrate their meters, thereby setting the loudness for the peaks of a project. Except that most broadcast machines use VU meters and digital gear generally use peak metering. Now, the industry has determined that ?20dBfs (decibel-full scale) is the standard setting for nominal values for broadcasting audio. Fine and well. Understand that if you are using most NLE systems, a -20dB signal won?t even draw a line in your audio window, and you can?t see what?s going on. As a musician, I want to ALWAYS know what?s going on. As a result, I tend to recognize that this might be the industry standard, but we set mix levels somewhat hotter in the studio at approximately -6dB from 0dBfs, and worry about output volume at the end of the project, just prior to printing to a master tape destined to make a transition from the digital to analog realm. Remember, if the media is staying in the digital realm, it never needs to be worried about, so long as it NEVER crosses 0dBfs.

Izotope Compressor (Fig. 2)

Given that peak dynamic ranges of processed digital signals average to peak around 15db above 0VU, I set my test tone at ?20dB, and leave it at that. This gives me some extra headroom in the event of a large dynamic spike. To further safeguard my audio, in the final process/print to tape, I insert a compressor, (Fig. 2) with a soft limit at ?3dB. This assures that my signal will never cross 0dBfs. At this point, engineers educated in the traditional way of thinking believe that they are throwing away signal to noise ratio.

Not so?
It?s important to remember that the s/n ratio is approximately 6dB different in analog to digital comparisons. In other words, an analog tape will sound 6 dB louder than its digital counterpart when calibrated to the same peak level. So there is nothing actually lost, and preserving signal integrity is clearly more critical as compared to s/n ratio. The argument comes in when studios, equipped with DAT machines or other mechanical digital devices look at their meters. The meters may be calibrated to -20 dBFS, -18 dBFS, and -14 dBFS translating to 0 VU. However, uncompressed, or live media easily can cover the narrower dynamic ranges, and cross the dreaded line of no return and mashed bits.

The question that continually is asked however, is ?where do I take my reference level from? Average or peak measurements?? In the digital world, it?s not so much where you take the level from, as it is critical to know what the level IS. By making sure that signals have less than 12dB of dynamic range through the use of a compressor, and by making sure that PEAK levels are less than 0dBfs, then there should be no issue at print. This is why I use plugins like the Izotope and WAVES Ultramaximizer. Sonic Foundry has a tool known as ?WaveHammer? that comes with Sound Forge as well.

To summarize in safety, set tones for final output at ?20dBfs. Use a compressor/dynamic EQ to control dynamic range of live material or excessively varied peaks. Be certain the duping house is aware of the unique relationship between 0 dB analog and 0dBfs. The resulting benefit is that broadcast standards will be met without exceeding digital 0dBfs. If the audio has been handled properly throughout the processing stages, signal to noise and quality will also be maintained. And you?ll look like a hero for delivering media the way it was meant to be delivered: professionally. For further, in-depth information, check out the ATSC website, the SMPTE site, Mix Magazine, or many of the other great forums on the web.

Happy editing!
Douglas Spotted Eagle
Sundance Media Group

Download Test tones for free

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More tutorials and reviews by Douglas Spotted Eagle may be found here on the DMN Forums or on the VASST website, home of the shared veg files and other training media. "Spot" is the author of more than a dozen books on the subject of DV, audio for video, DVD authoring, and Sony Vegas. He is the co-author of "HDV-What You NEED to Know."

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