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-Audio monitoring & metering.
-Thiết bị xử lý tín hiệu (processing).
-Kỹ xảo (effect).
Audio Monitoring & Metering
Audio Metering means using a visual display to monitor audio levels. This helps maintain audio signals at their optimum level and minimise degradation. There are two common types of meter which are used to measure audio levels:
- VU Meter (Volume Unit)
- PPM Meter (Peak Program)
Both types of meter are available in various forms including stand-alone units, components in larger systems, and software applications. Whatever the type of meter, two characteristics are important:
- The scale which defines which units are being measured.
- The ballistics of the meter which determine how fast it responds to sound and returns to a lower level.
A VU (volume unit) meter is an audio metering device. It is designed to visually measure the "loudness" of an audio signal.
The VU meter was developed in the late 1930s to help standardise transmissions over telephone lines. It went on to become a standard metering tool throughout the audio industry.
VU meters measure average sound levels and are designed to represent the way human ears perceive volume.
The rise time of a VU meter (the time it takes to register the level of a sound) and the fall time (the time it takes to return to a lower reading) are both 300 milliseconds.
The optimum audio level for a VU meter is generally around 0VU, often referred to as "0dB". Technically speaking, 0VU is equal to +4 dBm, or 1.228 volts RMS across a 600 ohm load.
VU meters work well with continuous sounds but poorly with fast transient sounds.
Peak Program Meter (PPM)
A Peak Program Monitor (PPM), sometimes referred to as a Peak Reading Meter (PRM), is an audio metering device. It's general function is similar to a VU meter but there are some important differences.
The rise time of a PPM (the time it takes to register the level of a sound) is much faster than a VU meter, typically 10 milliseconds compared to 300 milliseconds. This makes transient peaks easier to measure.
The fall time of a PPM (the time it takes the meter to return to a lower reading) is much slower.
PPM meters are very good for reading fast, transient sounds. This is especially useful in situations where pops and distortion are a problem.
Audio compression is a method of reducing the dynamic range of a signal. All signal levels above the specified threshold are reduced by the specified ratio.
The example below shows how a signal level is reduced by 2:1 (the output level above the threshold is halved) and 10:1 (severe compression).
How to Use a Compressor
Audio compression is a method of reducing the dynamic range of a signal.
You will need:
- A compressor with manual controls.
- An audio source to be compressed (eg. microphone, musical instrument, output of sound desk, etc).
- A destination device with which to feed the compressed output (eg. tape deck, sound desk, amplifier, etc).
- Connect the source to the compressor's input, and the compressor's output to the destination device.
- Adjust the compressor's input and output gains to appropriate levels.
- Set the threshold level to the point at which you wish compression to take effect. Signals below this level will not be affected. Signal levels above the threshold will be reduced according to the compression ratio.
- Set the compression ratio. Ratios of 5:1 or less will produce fairly smooth compression; ratios of 10:1 or more will produce more severe cutting off.
- Set the attack time. This is the delay between detection of a signal above the threshold, and the commencement of compression (ie. the time it takes to "attack" the signal).
- Set the decay time. This is the time taken to release the signal from compression.
- Adjust any other settings on the compressor. If you don't know what they are, try to put them on automatic, or disable them.
Set the compressor to a threshold of 0db, and a compression ratio of 3:1. In this case, all signals below 0db will be unaffected, and all signals above 0db will be reduced by 3db to 1 (ie. for every 1db input over 0db, 1/3db will be output).