Audio Electronics III - Week 3 Blog

I found some additional information on building the Hand of Light.

Here's the schematic of the project (PCB & power transformer):

Here's the PCB board layout:

While in class, I went online to search for information on compressors and optical compression. Here are my notes:

What is Optical Compression?

• Types of compressors 
• Tube
• FET (field effect transistor)
• VCA (voltage-controlled amplifier)
• Optical photocell arrangement (light source and light detector)
• Digital processor

Optical Compressors

• Basic principle of optical compressor - changes electricity into light, and then back to electricity 
• During the process there are certain kinds of nonlinearities that occur—producing a very desirable effect when applied to audio signals in a controlled way (logarithmic/exponential deviations)
• Like most common compressors, the incoming audio is split into 2 parts: 
• the part that passes through the gain and is ultimately output
• the part that is used in the detector circuit to control that gain stage (this is what is unique in optical compressors)
• A light source and sensor are employed to control the gain reduction
• “the real magic in an optical compressor as an audio device lies in the photocell”
• the photocell has a gradual and predictable release curve, which is very gentle and musical
• the first 50% of its release occurs very rapidly (tens of milliseconds)
• complete release can take several seconds
• this type of release is not found in typical solid state or tube design
• the cell also exhibits a phenomenon known as memory
• the release time of the cell is dependent on the duration and intensity of the light to which it has been exposed
• (when the input signal has been quite loud, and the compressor has been working harder, the release time will be longer)

Photocell/Photoresistor

• Also known as a light-dependent resistor (LDR)
• Light-controlled variable resistor
• The resistance of a photoresistor decreases with increasing incident light intensity (photoconductivity)
• Made of high resistance semiconductor
• In the dark, a photocell can have a resistance as high as a few megaohms
• In the light, a photocell can have a resistance as low as a few hundred ohms

Optical VS Electronic Compressor

• Inside compressors - voltage controlled amplifier (automatically controls the volume of the signal)
• Optical - less distortion, quieter, slower/smoother control sweep (how the compressor smashes sound over time)

Sources/Links

http://www.sweetwater.com/insync/what-so-good-optical-compressors/

http://en.wikipedia.org/wiki/Photoresistor