Audio Troubleshooting Guide

A step-by-step guide to diagnosing and rectifying some common technical problems encountered when preparing a recording system

Overview

This guide covers some of the common problems encountered when setting up a digitisation workspace and preparing for initial recording. It will suggest some checks which can be carried out, and quick fixes, and will provide a simple guide to "best practice", to help minimise unwanted artifacts.

In addition to the problem-solving suggestions listed, all kinds of problems (too many to list!) can occasionally be caused by malfunctioning equipment. If all the potential causes we suggest here have been eliminated, or you suspect a faulty unit, then you should have your equipment inspected by a qualified service technician. Never attempt repairs to any audio device yourself - they are complex instruments, which can deliver potentially harmful electric shocks if opened up; even when disconnected from the mains.

Step-by-step diagnosis

What is your problem ?

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Loss of signal / No sound

Complete absence of sound output from your recording chain. Several potential causes, so methodically work through your chain of equipment, unit by unit, checking off each of these points (where applicable):

Causes:

1. Power loss to one or more units in signal chain
2. Break in the audio signal chain
3. Zero input gain/output level on one or more device(s)
4. Lack of phantom power to microphone

Solutions:

1. Check power to all units (mains splitters/cables/adaptors, power switches and fuses if power indicators do not light)
2. Securely re-seat (i.e. unplug and then plug back in) all audio connections, starting from the source.
3. See Gain Management to troubleshoot and optimise levels in the signal chain
4. Check whether any of your microphones require +48V 'phantom power', and if so, that your microphone preamplifier - into which the microphone's cable is plugged - can deliver it, and is set to do so. If in doubt, consult the units' user manuals.

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Distortion

The distinctive 'fuzz' or overdriven sound of a signal exceeding the limits (or 'headroom') of the recording chain.how about an mp3 of non-distorted and distorted speech? -Steve Hull 22/01/2009 11:42

Causes:

1. Excessive gain (usually a 'fuzzy' overdriven sound) at some point in the recording chain

Solutions:

1. Use input meters on each device (and/or software application) to verify where signal is overloading input, and attenuate (turn down) accordingly
2. Monitor all input and output levels, and attempt to set them so that peak levels hit around 75-80%

• visually check gain and level controls
• watch the input and output signal meters
• where possible monitor output with headphones

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Background Noise

Background noise can take many forms, but the most common are hiss (a high, unpitched 'fizzing' noise), hum (a steady background tone). and crackle (intermittent bursts of noise)

1. Hiss

Causes:

1. Insufficient Gain; amplifying a very quiet signal by a large factor will also similarly amplify any background noise present in the system, so any background hiss or hum will be more noticeable.
2. Equipment self-noise

Solutions:

1. Check input meters on each device in the chain for a healty (50%+) signal level
2. Insufficient gain will exacerbate equipment self-noise, so check gains (as above) fiirst. If any unit is still excessively noisy, have it checked by a qualified engineer.

2. Hum

Causes:

Most hum (as distinct from hiss) is caused by the 50-60Hz alternating current (AC) used for mains electrical supply in most countries. This frequency can be picked up through the electronic circuitry of audio devices, if not properly shielded, or if connected in certain configurations (especially those which cause an 'earth loop').

Solutions:

Tracking down and completely eliminating background hum can be very difficult, but these are some steps which can help reduce its presence

1. Earth-lift. Some interconnection devices (usually D.I. or 'direct injection' boxes) have an 'earth-lift' switch which disconnects the shield of the connected audio cable, and can have beneficial results in defeating earth hum
2. Separate audio cables from power (and other) cables. Keep cables tidy so that audio cables are at least 50cm from power cables, and do not run in parallel. Where they must cross, try to ensure they cross at right-angles.The same rule applies to separating analogue audio cables from video, phone, digital audio and network cables.
3. Warning : You may be advised to disconnect the earth connections for the mains cable of your equipment to avoid hum. This is extremely unwise and unsafe, so DON'T DO IT ! The earth is there for a very good reason - to help prevent electric shock.

3. Crackle

Causes

1. Loose connection
2. Dry joint (literally - old solder crumbling and flaking away from an internal connection point, leading to intermittent or lost connection)

Solutions:

1. Check and reseat connectors throughout the chain
2. Have internal connections inspected by a qualified service engineer

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Reverberation

Excessive echo or reverberation on your recording is all but impossible to remove post-production, but here are a few ideas to minimise it at the recording stage:

• Soft furnishings - sofas, beanbags, cushions etc - help soak up reverberation
• Heavy curtains, rugs or duvets can be hung across reflective surfaces - walls, windows etc
• Experiment with different microphone positions within the room and see which has the best reverberant characteristics for the source
• Some specially designed screens are available, either freestanding or attached to the microphone stand, to help cut acoustic reflections from behind the mic
• By moving closer to the microphone, you will increase the ratio of direct, or 'dry' signal in comparison to reverberated sound

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Feedback

Causes:

Feedback is caused by the signal from a microphone being amplified through a speaker, which is then literally 'fed back' into the microphone, causing a reverberant or 'ringing' effect, or in the worst case an endless loop and a constant tone (the familiar whine or squeal which most of us recognise as 'feedback')

Solutions:

1. Most recording situations do not require re-amplification of the recording microphone(s) through monitor speakers, which will contribute an element of feedback, even if this does not reach the constant feedback 'tone'.
2. Where necessary, monitoring should be done through closed-back, over-the-ear headphones
3. However, in a situation where an audience is being addressed while recording takes place (a lecture for example), re-amplification through a sound reinforcement system or public address may be necessary. If experiencing feedback problem in this type of enviroment, you should refer to our guide to Digital Equalisation for a full explanation of how to use graphic equalisation or automatic feedback destroyers within your public address system to neutralise feedback.

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Computer Audio routing

Another common cause of problems occurs once the signal has passed safely through the analogue recording chain, has been correctly managed and adequately monitored and metered, plugged into the computer soundcard and then... nothing.

Routing an audio signal within a computer can be a complex procedure, with important stages and control panels often not clearly marked or obvious to the user. Although it is impossible for us to cover here all possible combinations of factors which could lead to failure of the signal to negotiate the internal workings of your Digital Audio Workstation, there are some key areas where you should start looking.

Audio Control Panels

This path will access the Windows Control Panel for Audio Devices:

Start -> Control Panel -> Sounds and Audio Devices

The tabs at the top of this control panel give access to the various parameters available for your audio interface (soundcard) and any other audio devices.

Soundcard/Audio interface Drivers

Check that Drivers for audio devices are installed an up-to date:

Start -> Control Panel -> Sounds and Audio Devices -> Hardware

Double-clicking on a device will open a pane in which you can inspect, and if necessary update, its driver(s)

Input/Output assignment

Start -> Control Panel -> Sounds and Audio Devices -> Audio

Start -> Control Panel -> Sounds and Audio Devices -> Voice

These paths access the control panels where you can choose the audio input/output device you want to use for different types of recording and audio application. Generally you will choose the same device for all these tasks, but it is wise to check that the correct device is selected if your audio recorder program is not receiving a signal.

Selecting and enabling inputs/outputs

Every audio application will have a 'Preferences' or 'Options' panel which will allow you to select the audio device it will use by default. Additionally, many other parameters may be available which will affect the quality and format of recording (sample rates, bit rates etc etc). A more in-depth guide to this complex area is in development, but in the mean time please contact our helpdesk for assistance with specific enquiries.

As an example, Audacity's Preference pane is located at

Edit -> Preferences

and gives access to a huge number of user-definable settings. A more advanced application will offer far more. Most critically, ensure that the correct audio device is selected for recording and playback:

Edit -> Preferences -> Audio I/O

 

Published in: Creating

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