Tim Carroll /
08.20.2003 12:00 AM
Giving Dynamic Range Control to Consumers
Last time we wrapped up our investigation of audio distribution. This time we are going to sidetrack just a little bit and take a look at a different approach to controlling dynamic range. Normally, dynamic range control (i.e. processing) is done before program material is sent to consumers(be it CD, DVD, television or radio(but there are a couple of ways this control can be placed in the hands of consumers.

METADATA

You might wonder why in the world it would be desirable to enable consumers to change the dynamic range of the audio they receive. The first example that comes to mind is the delivery of films on DVD. When you watch a film in a modern theater you hear the soundtrack reproduced with a dynamic range that matches or comes close to that of the mix stage where it was recorded. In a room with two hundred of your closest friends, this can be an exciting way to watch a film. Once this film is transferred to DVD, which has the capability of carrying the full dynamic range of film recordings, reproducing it in your living room is a different story. Yes, it can be just as exciting, but what if you want to watch it at night after the kids have gone to bed or if you don't want to disturb the neighbors again? While you can turn the volume down to where the dialogue is just audible, the effects have another 20dB of headroom. This can be astonishingly loud even though the dialogue is soft and you believe that you are safe. Of course, the loud effects will wait until you leave the room to grab some refreshments, then the real excitement is finding the remote control before the kids wake up.

The answer to this situation is to control the dynamic range, making the loudest and softest sounds closer together. This allows the volume to be parked at a level with no fear that the effects will bark out or that the dialogue will be too low to understand. Traditionally, this work has been done at the transmitter or during the mastering of a CD, and once it is done it cannot be easily undone. With DVDs and digital television, Dolby Digital metadata provides a different approach. In past articles, we have discussed the dynamic range control (DRC) function of Dolby Digital, but to briefly recap, the control information is generated in the encoder and via metadata, is passed on to the decoder. The default mode is to apply the DRC information if it exists. This is a good thing, because it means that you have to know how to change it and probably won't do so by accident.

CONSUMER DYNAMIC RANGE CONTROL

One of the benefits of the Internet boom was a relatively quick increase in computer processing power. This allowed for not only faster Internet access, but also some fantastic multimedia applications such as streaming audio and video. MP3 audio data rate reduction and MPEG video compression helped all of this become a reality, and the door was open for other applications to take advantage of the increased processor performance. Octiv, Inc., from Berkeley, Calif. is one such company. Started by Keith McMillen, Leif Claesson, Amy Huson, and a few other ex-Orban employees, they developed an entire multiband audio dynamic range control system that runs on a PC under Windows or Linux. The processing system, called Octimax, was initially designed as a pre-processor for Internet audio delivery but has found many applications off the desktop as well.

Octimax is a full-featured audio processing system containing a wideband automatic gain control (AGC), five-band compression and limiting, and look ahead peak limiting (also called negative attack time limiting) and can also perform multiband source noise reduction. It has all of the features professionals are used to finding in a typical broadcast audio processor, but is completely software-based.

(click thumbnail)
Fig. 1: Octiv's Optimax processing structure. Note that the "Negative Attack Time Dynamics Processors" are also known as look-ahead limiters. Drawing courtesy of Octiv.

Due to the complexity (See Fig.1) of a true multiband dynamic range control system, it has historically been relegated to digital signal processing (DSP) farms where multiple complex processors do the math required to control the audio in a good sounding manner. Octimax changed that, at least for audio on a PC. How does it sound? I find its quality equal to and in some cases better than hardware-based systems with the similar features.

The next logical step was to try to move the Octimax technology to a stand-alone device for consumer use. Luckily, quite a bit has changed in the DSP world since the first so-called general purpose devices were released. While these early devices provided quite a bit of processing power, they did many things well, but were not great at any one specific task. As consumer audio products started gaining momentum, the DSP manufacturers started developing devices targeted at specific operations such as Dolby Digital decoding for DVD players and television set top boxes, or MP3 decoding in portable devices.

These new devices also allowed Octiv to develop a low-cost single chip implementation of its Octimax technology. In cooperation with Terk (of powered antenna fame), they have produced a device called the "TV Volume Regulator model VR-1." This is somewhat of a misnomer as it can be used to regulate the volume of any two-channel audio source such as DVD, CD, and even cassettes. The idea is to place the VR-1 in between any source and the destination. An example might be to connect the left and right audio outputs of a cable set top box or a VCR to the VR-1, then connect the outputs of the VR-1 to the audio inputs on a television set. The result will be remarkably consistent loudness regardless of which channel you select. If you wish to watch a program with all of its transmitted dynamic range, simply flip the VR-1 into bypass. I purchased one and have it connected between my DirecTV receiver stereo output and the television set. This is how I usually watch television, but when I watch a DVD or rent an action adventure movie (and it's not too late at night), I will turn on my Dolby Digital receiver and turn off the television volume so that I get the full impact.

This is the same rationale behind DRC in Dolby Digital-different listening environments and listening times demand different dynamic ranges. The amount of control is now not only user-selectable, it also is now applicable to other sources that may not have Dolby Digital and thus not have DRC. It will also help with two channel or downmixed Dolby Digital sources that may not have DRC (or enough DRC) generated during the encoding process.

There are many additional possibilities for consumer side dynamic range control-from portable devices that may contain music selections from many sources, each possibly with a different loudness, to car audio where highly dynamic content tends to either fall into the road or wind noise when quiet and rattle the windshield (and the driver) when loud, high quality multiband dynamic range control can be a very useful solution. I am contemplating purchasing another VR-1, fitting it with some batteries and bringing it with me the next time I go for a jog with my MP3/WMA/AAC player. Could be very interesting- especially the jogging part.

One more application is also possible. Imagine driving in your car and tuning into a digital music broadcast via satellite, terrestrial digital (i.e. the new IBOC system), or even perhaps from your local television station on a little extra bandwidth they lease to a music service. Let's imagine that you are listening to Beethoven's Ninth Symphony and its dynamic range has been adjusted so that it fits comfortably in your car. You pull into the driveway, go into your home and tune in the same broadcast to continue listening, except now you decide you would like to hear the music with all of its dynamic range, so you switch it to "wide" mode. Everyone still listening in his or her cars will hear restricted dynamic range and anyone who wishes can hear the music with full dynamic range. It is now absolutely possible. Will it happen? I'm not sure, but I can certainly dream!

In honor of TV Technology's 20th anniversary, next time we will take a look at how audio technology in television has evolved over the past 20 years. Until then, please keep the excellent questions and suggestions coming. Thanks for your time!


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