Re: Loudness and TV-sound? ("Richard F. Lyon" )

Subject: Re: Loudness and TV-sound?
From:    "Richard F. Lyon"  <DickLyon(at)ACM.ORG>
Date:    Thu, 14 Aug 1997 10:39:18 -0700

>As a humanist doing research on sound in television, I wonder If someone on >the Auditory list might be able to help me. I recently read an article in >New Scientist written by Barry Fox, taking up an issue I am interested in: >Loudness in TV-commercials ><> > >After describing the common technique of compressing sound to make it sound >loud, Fox writes: "Another technique, pioneered in the 1960s by the Tamla >Motown studios in Detroit, uses both filters and compressors to separate >the audible spectrum into narrow bands and pack as much energy into each >one as possible. This equalises the level of loudness for all the backing >instruments. The voice of one singer is then mixed to peak at a carefully >chosen frequency so it stands out." > >Are anyone on the list familiar with the technique Fox describes? And would >you be able to say something more about it, or guide me to literature or >references? Daniel Levitin did a good job of describing this technique in terms of a "frequency selective compressor," but let me add a little. This kind of processing is also referred to in the literature as a "multiband compressor" and has been succesfully applied to hearing aids over the last three decades (recently commercially by Resound Corp.). Why use it for television sound? Because of the nature of loudness perception and the TV sound signal limitations. Power produces loudness most effectively when it is spread more or less equally over all the critical bands that the ear resolves. TV sound has a peak-power constraint (an FM peak deviation constraint, really, but effectively a power constraint for a given volume control playback setting). Typical sound signals concentrate the power in a few bands at a time. The multiband compressor reduces the power in those high-intensity bands so that the power can be used in the lower-intensity bands to increase the overall loudness. If it's done right, it affects the quality and intelligibility of the sound very little, which is why it's a good technique for hearing aids, too. Compressors have long been used to equalize intensities across time. It is less well known that they can also work across frequencies to get even better loudness for your power. Dick

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