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Lampifier Technology Overview 

Pronunciation: Lampifier (lamp'ə-fī'ər) - rhymes with amplifier

In the 1930’s during the depths of the Great Depression, Americans fell in love with “the movies”, especially during the hot summers. Movie houses gave inexpensive entertainment to an out-of-work nation feeling down on its luck. And movie houses had a new luxury called air conditioning. But the background noise and commotion from so many movie goers made it difficult to hear the dialog. Audio amplifiers and speakers of the time just couldn't compete with all that noise.

So the movie house owners turned to a another new invention called an audio dynamic range compressor. In those early years, photo cells were expensive and unreliable so audio engineers designed compressors using light bulbs as the active elements. The compressors made the audio track louder without amplifier clipping. The light bulbs of that era were primitive by today’s standards but they did the job. However, the bulbs had to be fed enough power to keep them lit continuously in order achieve satisfactory audio quality. And that made them burn out quickly.

When the US went into World War II, the military commanders needed compressed audio for radio communications. But they could not tolerate the low reliability and high battery consumption of continuously lit light bulbs. So a lot of research and development went into photocells. Photocell-based audio compressors quick outpaced the earlier lamp-only compressors and until now, the lamp-only compressors have been forgotten.

Now the Lampifier audio processor brings back the lamp-only compressor. But with modern light-bulb technology, the Lampifier “audio bulb” light bulb is more reliable, uses less power, and is better optimized for audio quality than the 1930’s predecessors. It also cost less and is more environmentally friendly than photocells, which contain toxic cadmium sulfide.

Intelligently Controlled Dynamics

The Lampifier® processor intelligently controls dynamics. First, the high intensity passages are tamed to avoid unpleasantly overpowering everyone and/or overloading your sound system. Second, singers and public speakers have added strength in the more softly delivered passages, including falsetto as well as the more difficult-to-project lower vocal range. Typically, the processor allows speakers a wider range of dynamic input—from a "whisper to a shout" - without getting lost in the mix one moment or blasting above it the next. Unlike ordinary compressor/limiters, however, the Lampifier processor does not simply reduce the sound level above a given threshold. Instead it retains and delivers the intended dynamics within a reduced range utilizing a unique, self-adjusting "natural" compression ratio. (There's more about that below.) Therefore, compression is applied intelligently at all levels, creating a smoother and punchier sound with an increased clarity of articulation in passages—loud or soft.

Feedback Rejection

The Lampifier processor reduces feedback in two ways. First, a noise gate turns off the mic automatically at a predetermined level. This prevents low level feedback from ever getting started. Of course if your sound system is loud enough, high level feedback can still occur. (Any microphone will create a feedback loop if the speaker output volume is high enough. For example, if you place the mic directly into the cone of its speaker output!) Typically, the Lampifier processor immediately clamps down on such feedback, reducing it from an ear-splitting, equipment-damaging level to simply an annoyance. Most of the time feedback is eliminated altogether, but if it does occur it is controlled and its volume is greatly reduced.

User Programmable - The Ultimate in Flexibility

Thanks to the natural compression characteristics of the Lampifier processor, a Lampifier microphone is distinctly beneficial, and quite forgiving in most applications. The pre-programmed setting for each specific application will therefore typically work well. However, it can easily be fine-tuned as needed. See the Customize Your Mic page for detailed instructions. The compression slider controls how hot the signal hits the Lampifier® compressor, so the lower the setting, the more compression is applied at all levels.

The processor also has a programmable noise gate. If set too low, feedback may occur more easily, particularly in at high volume. If set too high, the gate may block wanted source sound. The programming seeks to place this level as high as possible, without cutting out any wanted material. For example, a strong singer could use a relatively high gate setting. But if your style requires soft, breathy "trailoffs" that need to be fully captured, a lower gate threshold setting is more appropriate.

In addition there are two other programmable settings. First, the gate can close fast or slow. Slow is the more common setting, and will work best for most vocals and musical instruments. Fast gating can be used where feedback is a problem, or for capturing a crunch-tone electric guitar — where the sound is essentially either "on" or "off" and when off, you want to mute immediately.

Second, the output level is programmable. It can be either "low" or "high". Typically, the low output level is used when the compression threshold is -12db, -16dB, -20dB, or -24dB. These are the higher compression programs which can boost the output signal from the microphone. Programming a low output level compensates for (decreases) the output signal. The other compression thresholds have lower output so a high output level is typically programmed for those. The low output level program setting is for compensating for a loud signal source (or higher compression programs) so that the microphone output signal does not cause distortion at the input of your mixing console. The high output level program setting is for compensating for a quiet signal source (or lower compression programs) so that the microphone output signal is not too weak at the input of your mixing console.

Theory of Operation

Common uses of Lampifier microphone are in sound recording, live sound reinforcement, broadcasting, and communications. Benefits include an increase in the perceived audio volume, better control over acoustic feedback, and a reduction in accidental sound system overload. The compressor-circuit of the Lampifier processor has two preamplifiers, a load resistor R, and an audio bulb. The audio bulb is a special kind of miniature light bulb (or lamp) that is optimized for sound enhancement, extremely long life (typically 25+ years), and durability instead of light output. In operation, preamp 1 amplifies an audio input signal which is applied to the input terminal to provide a drive signal to the audio bulb. The audio bulb has a filament with a positive temperature coefficient - usually a wire made of tungsten. When the audio signal increases, the drive signal heats the filament and increases its electrical resistance. The resistance reduces the level of the drive signal applied to the second preamp (and the output terminal). When the audio signal decreases, the filament cools and its resistance decreases. This action increases the level of the drive signal applied to the second preamp and the output terminal.

   

The curves and table above (click to enlarge) are created from laboratory measurement data of the input/output characteristic of an operational circuit having a tungsten filament and a load resistor having values (R) of practically zero, 1.5 x T0, and 5 x T0, where T0 is the filament resistance at room temperature. The curves show that the peak compression ratio of about 5:1 occurs when the input signal is -5.0 dB and R is practically zero. The compression ratio gradually drops to 1:1 for lower input signals and it gradually drops to 2:1 for higher input signals. When R = 1.5 x T0 the peak compression ratio is 3.5:1 and when R = 5 x T0 the peak compression ratio is 2.5:1.

In most applications, the filament is rarely operated to the point of incandescence (an input signal of 0 dB). Instead, the input signal level is attenuated so that only the very loudest parts of the audio program are above -5.0 dB input. Not only does this dramatically increases the service life of the lamp (because the audio bulb rarely lights up), it also improves the sound quality. The charts above show that the Lampifier circuit reduces its compression ratio above -5.0 dB input. When this occurs, more of the intended dynamics of the original audio program are retained. In contrast, ordinary audio level compressors do not reduce their compression ratios. Instead, they merely "flatten" the audio program's dynamics by compressing the very loudest parts the most.

Natural Audio Compression

Because the compressor circuit has no digital signal processor or VCA amplifier, the audio signal's noise, distortion, group delay, and frequency response are practically unchanged. There are only two components that actively process the audio signal - the lamp filament and the load resistor. Control parameters such as threshold, compression ratio, attack and release time are governed by the input level of the audio program, the of audio bulb selection, the load resistor, and other circuit parameters. A small audio bulb is preferred because its small filament heats and cools quickly providing short attack and release time. The compression ratio and compression threshold can be changed by simply adjusting audio input level. The peak compression ratio is governed by the load resistor. As shown by the graph above, the lower the resistor value, the higher the peak compression ratio.

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