Realtime Audio Processing

Whether it is a live performance and you are at the mixer, a radio broadcast, or you're the DJ at an event or nightclub, you will most likely be involved in audio processing in real time. Once performed by electronic equipment such as mixers, equalizers, and filters, the new age of digital audio processing via computer has revolutionized these industries. This is a very expansive subject to which this article will try to break down three popular but very different applications.

As an example, the goals of a radio broadcast technician, the guy running the sound board at a live concert, and the DJ at a simple mixer are not necessarily the same. In some instances they may be very opposite. However, all instances will involve some type of audio processing and the application either by means of a physical device such as a mixer and equalizer, or by digital processing being done via a computer will be used. In some instances hardware and software are combined.

Relations to:

• Compression
• Normalization
• Limiter
• AGC
• Equalization

For Applications:

• Home Theater
• DJ
• FM Broadcast

Compression and Limiting: Audio compression is not the same as audio data compression, as in this is not a discussion on how to make audio data files take up less disk space. Audio compressors use the peaks or the immediate RMS levels to help prevent sound from exceeding a certain threshold. The audio limiter does the job of stopping all sound at a given level, usually at 0 dBFS or some predefined limit.

Normalization: In contrast to compression, the normalization process doesn't actually manipulate audio dynamics within a piece, but rather elevates the more silent parts nearer to the louder ones. The default of most music mixing, recording or mastering applications would be 0 dBFS. The concept of Normalization can also be split into Loudness Normalization and Peak Normalization. We are more concerned with Loudness Normalization since we are not involved in the production process, only the enjoyment of a piece.

Home Theater: Normalization

Watching programming, most likely a modern motion picture, you might notice that speech dialog is quiet, often difficult to hear whereas action scenes including music and sound effects that are intolerably loud. You might not give a damn that the producer of the film wishes to express himself this way as you seek remedy for your headache thanks to this new trend in "full spectrum sound".

The solution is "normalization." This can be accomplished with either software (if you are viewing video media from your computer) or with a physical device between the media player and the sound system.

See: Normalization Options for Home Theater

FM Broadcast: Limiter and Compressor

Your LPFM station or home stereo transmitter just doesn't have the apparent loudness and "boom" of those big broadcast stations, and when you turn up the audio gain it just starts to distort. You have now been exposed to the FM Broadcast Loudness War, a product of FM Broadcasters using audio compression techniques to make the music they play on their FM stations as loud as possible or stand out over other broadcast stations. Your little FM transmitter on your iPod just can't compete. You need to investigate and learn about AGC (Automatic Gain Control) circuitry which should be incorporation into a good FM transmitter as well as some kind of audio compression / audio limiter solution which could be a physical device or software. To work within the legal FM Broadcast Radio Frequency Deviation it is possible to have your transmitted music sound "louder" by compressing the audio and applying a limiter to prevent distortion or clipping.

See: Audio Compression Options for FM Broadcast

DJ: Audio Spectrum Manipulation

This is included for the purposes of disambiguation when discussing things like audio compression versus equalization. The use of an audio equalizer or sometimes referred to as a graphic equalizer is a means to manipulate the sound spectrum of music, as an example, giving a piece more bass or boosting midtone for more pronounced guitar sound. What you might do to manipulate a live performance or DJ activity is likely going to be the opposite of what you would try to do as a broadcaster. In the broadcast world the audio needs to remain as flat as possible after compression while a live performance would be adjusted to suit a particular venue and audience taking into account any number of variables.