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Phần 5: Chất lượng âm thanh (Sound quanlity).
How to Prevent Distortion
Unwanted distortion is caused by a signal which is "too strong". If an audio signal level is too high for a particular component to cope with, then parts of the signal will be lost. This results in the rasping distorted sound.
To illustrate this point, the pictures below represent a few seconds of music which has been recorded by a digital audio program. The maximum possible dynamic range (the range from quietest to loudest parts) of the signal is shown as 0 to +/-100 units.
In the first example, the amplitude (strength / height) of the signal falls comfortably within the +/-100 unit range. This is a well-recorded signal.
In the second example, the signal is amplified by 250%. In this case, the recording components can no longer accommodate the dynamic range, and the strongest portions of the signal are cut off. This is where distortion occurs.
These examples can be used as an analogy for any audio signal. Imagine that the windows above represent a pathway through a component in a sound system, and the waves represent the signal travelling along the pathway. Once the component's maximum dynamic range is breached, you have distortion.
Distortion can occur at almost any point in the audio pathway, from the microphone to the speaker. The first priority is to find out exactly where the problem is.
Ideally, you would want to measure the signal level at as many points as possible, using a VU (Volume Unit) meter or similar device. Generally speaking, you should keep the level below about 0dBu at every point in the pathway.
If you can't measure the signal level, you'll have to do some deducing. Follow the entire audio pathway, beginning at the source (the source could be a microphone, tape deck, musical instrument, etc). Here are some things to look for:
- Is the distortion coming from a microphone? This could be caused by a very loud noise being too close to the mic. Try moving the mic further away from the noise source.
- Are you seeing any "peak" or "clip" lights on any of your equipment? These are warnings that a signal level is too high.
- Are any volume or gain controls in your system turned up suspiciously high? Are there any obvious points where you could drop the level?
- Are your speakers being driven too hard? If you have an amplifier which is pushing the speakers beyond their design limits, then be careful - you may well find that the distortion becomes permanent.
- If the distortion is coming from occasional peaking, consider adding a compressor.
- Could the distortion be caused by faulty equipment?
- Is the problem really distorion? There are some other unpleasant noises which could be confused with distorion; for example, the graunching sounds made by a dodgy cable connection or dirty volume knob.
How to Eliminate Feedback
Audio feedback is the ringing noise (often described as squealing, screeching, etc) sometimes present in sound systems. It is caused by a "looped signal", that is, a signal which travels in a continuous loop.
In technical terms, feedback occurs when the gain in the signal loop reaches "unity" (0dB gain).
One of the most common feedback situations is shown in the diagram below - a microphone feeds a signal into a sound system, which then amplifies and outputs the signal from a speaker, which is picked up again by the microphone.
Of course, there are many situations which result in feedback. For example, the microphone could be replaced by the pickups of an electric guitar. (In fact many guitarists employ controlled feedback to artistic advantage. This is what's happening when you see a guitarist hold his/her guitar up close to a speaker.)
To eliminate feedback, you must interrupt the feedback loop.
Here are a few suggestions for controlling feedback:
- Change the position of the microphone and/or speaker so that the speaker output isn't feeding directly into the mic. Keep speakers further forward (i.e. closer to the audience) than microphones.
- Use a more directional microphone.
- Speak (or sing) close to the microphone.
- Turn the microphone off when not in use.
- Equalise the signal, lowering the frequencies which are causing the feedback.
- Use a noise gate (automatically shuts off a signal when it gets below a certain threshold) or filter.
- Lower the speaker output, so the mic doesn't pick it up.
- Avoid aiming speakers directly at reflective surfaces such as walls.
- Use direct injection feeds instead of microphones for musical instruments.
- Use headset or in-ear monitors instead of speaker monitors.
You could also try a digital feedback eliminator. There are various models available with varying levels of effectiveness. The better ones are reported to produce reasonable results.
Feedback can occur at any frequency. The frequencies which cause most trouble will depend on the situation but factors include the room's resonant frequencies, frequency response of microphones, characteristics of musical instruments (e.g. resonant frequencies of an acoustic guitar), etc.
Feedback can be "almost there", or intermittent. For example, you might turn down the level of a microphone to stop the continuous feedback, but when someone talks into it you might still notice a faint ringing or unpleasant tone to the voice. In this case, the feedback is still a problem and further action must be taken.