TOOLS FOR INFORMATION PROCESSES:
MIXING SOFTWARE FOR AUDIO
Audio data is organised as either a series of sound samples or as descriptions of various attributes of each individual note. In this section we restrict our discussion to software applications used to process sampled audio data. In Chapter 2, we discussed sounds samples as representing the instantaneous amplitude of a sound wave recorded at precise time intervals; this is how sampled audio data is organised. It is structured as a sequence of separate samples where each sample represents the amplitude of the sound wave at a particular point in time. For example, every second of CD quality sound contains 44100 samples for both the left and right channels, each of these samples is 16 bits long, in effect an integer in the range 0 to 65535. Software applications that process audio data are able to analyse and alter these sound samples;
that is they change the integers used to represent each amplitude sample. As is the case with other software applications, most mixing software is able to organise the data into various formats in preparation for storage and subsequent retrieval, however during processing mixing applications operate on the raw sound samples.
Mixing software is used to alter a sound sample and also to combine sound samples from multiple sources. The processes available within mixing applications operate by automating the alteration of multiple sound samples. Sequences of individual sound samples, in almost all cases, need to change progressively; this ensures that a smooth rise or fall in amplitude is maintained. It would be tiresome to manually edit individual sound samples directly; maintaining appropriate differences between each sample would be near impossible. Most mixing software applications display the sound data graphically as a wave. Remember the amplitude, or height of the wave
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Fig 4.7
MS-Word provides various options for editing nodes on Bezier curves.
GROUP TASK Discussion
How does the organisation of vector image data assist draw applications to perform the processes described above? Discuss.
GROUP TASK Activity
Open Microsoft Word, or some draw software application. Try to reproduce the Bezier curve shown in Fig 4.7. Make a list of the attributes of the Bezier curve and briefly describe their effect.
determines the volume or level of the sound whereas the frequency or number of waves per second determines the pitch of the sound. Fig 4.8 below shows two screen shots from ‘Cool Edit’, a mixing software application written by Syntrillium Software Corporation. The left hand screen shows an entire wave, indicating that the level or volume increases at the start of the sound and steadily decreases as the sound finishes.
The right hand screen has been zoomed to display just 0.001 seconds of this sound so that the individual sound samples can be seen. The sound displayed on these screens is in stereo (two channels); the top wave represents the sound played through the left speaker and the bottom represents the sound played through the right speaker.
The level of a sound is a measure of the relative differences in amplitude. To maintain the fidelity or detail of digital audio it should be collected (recorded) using the widest possible range of amplitudes. For example, if 16 bit samples are used then the loudest sounds in the recording should ideally have a value of 65535. If the level is set low such that the loudest sample is represented as only 32000, then all the sound samples recorded will be compressed to be within a range from 0 to 32000. In effect much of the detail of the original sound will be lost. Often mixing software is used to adjust the average levels of different tracks within a music compilation; this process is called normalisation. Normalised recordings allow listeners to set the volume on their amplifiers once in the knowledge that each track on the compilation will on average play at the same volume.
Some processes in mixing applications alter the sound without analysis of the existing sample e.g. trimming silence, fading in or out, and combining two sounds, whilst others first analyse the sound to determine the nature of the alterations to be made e.g.
filtering out noise. Let us consider these processes applied to a single channel and discuss how the raw sound samples are altered.
Trimming is a process similar to cutting, where parts of the sound are removed; in fact the familiar cut, copy and paste functions are also present in most mixing software.
Commonly when sound samples are collected they contain initial periods of near silence and they also conclude with a period of near silence. Such samples will appear on the display as areas with low or zero amplitude. In most applications the user highlights the required section of the wave pattern and then initiates the trim function.
The trim function removes all the sound samples that do not lie within the selected range; hence the trim function reduces the total number of raw sound samples.
Fig 4.8
The screen at left shows an entire 4.814 second stereo wave form. The right hand screen shows just 0.001 seconds of this wave so the individual sound samples are visible.
Screenshots courtesy of Cool Edit by Syntrillium Software Corporation.
Fades progressively alter the level or amplitude of a sound; a fade-in occurs when the sound level progressively increases and a fade-out occurs when the sound level progressively decreases. Fades to do not alter the frequency of the wave so the pitch remains constant, just the volume changes. In
Fig 4.9, the top wave, which has a constant frequency, has first had fade-in applied and then fade-out applied. Notice that the wavelength and hence the frequency of each wave is the same. In most mixing software it is possible to adjust the nature of the fade using various envelopes – an envelope describes the change in a wave’s shape over time. The graphs in Fig 4.9 show the effect of a simple straight-line envelope being applied.
When an envelope is applied each new sound sample is calculated as a percentage of the corresponding old sample amplitude. In the fade-in example in Fig 4.9 the envelope is a straight line so the percentages used increase constantly from 0 to 100. Envelopes that are not straight lines will vary the percentages used to mirror the shape of the envelope.
Combining two sounds so they play simultaneously is commonly used to add additional instruments or vocals to an existing audio track. Such functions are implemented in mixing software using a special case of the familiar paste function, however instead of inserting the new sound samples into the existing data, the new sound samples are combined or mixed with the existing samples. For example, in
‘Cool Edit’ the edit menu contains a ‘Mix Paste…’ function. When this menu item is selected various criteria, including the file to be pasted are specified prior to the mixing process commencing. So how are the raw sound samples altered when one sound is combined with another? Let us consider a simple example where a 200Hz and a 400Hz sound sample are combined (see Fig 4.10). Each sound sample represents a particular amplitude at a particular point in time. When two samples are
Fig 4.9
Fades progressively alter the amplitude of a wave without affecting its frequency.
200Hz
400Hz
Mixed result
Fig 4.10
Mixing simply adds the raw amplitudes of each sample. If any results are greater than the range the sample size allows, then all the final samples are scaled proportionally.
A
B
A+B
combined each pair of amplitudes occurring at the same time are added. For example, in Fig 4.10 amplitude A and amplitude B occur at the same time, hence the resulting amplitude is A+B. If the sum A+B is greater than the range of values that can be represented (i.e. greater than 65535 for 16-bit samples) then all the resulting values are scaled proportionally. This scaling affects just the amplitude not the frequency of the wave at each point, hence the average level of the resulting sound will change but the pitch will be correct.
Let us now consider the processes involved to filter out background noise from a sound. Background noise is any unwanted sounds present throughout an audio clip; it commonly includes noise from the environment or generated by the recording equipment. To remove background noise from a clip involves first analysing a section of the clip that should be silent, that is a
series of sound samples is examined that represent the noise that is to be removed. In Fig 4.11 suitable groups of sound samples for noise analysis are shown at A and B. These noise samples are analysed to determine the frequencies and levels (amplitudes) present. For example, the analysis may find that the noise contains a frequency of 100Hz that is 2% of the maximum level. Finally the original wave is analysed to find and remove occurrences of these frequencies that occur at the level determined in the noise sample. The processing occurring
during noise reduction is complex, as the raw data is not organised into different frequencies and their corresponding levels; these properties must be determined from the raw sound samples. However once these properties have been determined the final alteration of each raw data item is a simple subtraction process. Fig 4.11 shows an original waveform before noise reduction and then the resulting waveform after noise reduction; notice that the sections corresponding to A and B on the original have essentially become straight lines indicating silence.
A B
Fig 4.11
A wave form before and after background noise reduction.
GROUP TASK Discussion
How does the organisation of audio data into sound samples assist mixing applications to perform the processes described above? Discuss.
GROUP TASK Activity
Record two sounds; say your voice saying hello and goodbye. Use a mixing software application, and your recorded sounds, to perform examples of each of the processes discussed above.
GROUP TASK Research
There are various formats used to organise audio data in preparation for playback (display). Make a list of as many different audio formats as you can and specify the advantages and disadvantages of each format.
SET 4A
1. Organising involves:
(A) deciding what to organise.
(B) structuring and representing data.
(C) altering the data.
(D) permanently storing data.
2. Which of the following is true for all examples of the organising process?
(A) It formats data in preparation for display.
(B) It prepares data for use by other information processes.
(C) It determines the storage format for files.
(D) It alters the data so it is can be understood by humans.
3. Analog to digital conversion:
(A) is purely an organising process.
(B) is primarily an organising process.
(C) is a collecting information process.
(D) is a processing information process.
4. In terms of data organisation, the essential difference between a bitmap and vector image is:
(A) Bitmaps are composed of individual pixels; vectors images describe each shape mathematically.
(B) Vector images are composed of individual pixels; bitmaps describe each shape mathematically.
(C) Bitmaps require greater storage than vector images.
(D) Vector images can be scaled without loss of quality; this is not true of bitmaps.
5. The nodes on an object within a draw application are used to:
(A) alter the fill colour, line colour and thickness of the object’s outline.
(B) move the object relative to other objects within the image.
(C) determine the storage format used when the image is saved.
(D) determine and alter the shape and size of the object.
6. A 24-bit RGB colour is represented:
(A) using 24 bits for each of the red, green and blue components.
(B) as a sequence of pixels, where each pixel value is compressed.
(C) using a colour table that includes at least 256 different combinations.
(D) using 8 bits for each of the red, green and blue components.
7. A line is drawn on top of an existing image, if this line is later selected and moved behind the image then the image must be a:
(A) bitmap image.
(B) vector image.
(C) sequence of pixels.
(D) Window’s Metafile.
8. A wave displayed in an audio mixing application is really:
(A) a sequence of amplitude values joined to form a smooth curve.
(B) a sequence of frequency values joined to form a smooth curve.
(C) a representation of the raw analog data.
(D) a combination of volume samples taken at various periods in time.
9. Combining or mixing two sounds involves:
(A) simple substraction of each corresponding sound sample.
(B) simple addition of each corresponding sound sample.
(C) multiplying each corresponding sound sample.
(D) playing both sounds at the same time whilst collecting new sound samples.
10. Evenly reducing the volume of a sequence of sound samples involves:
(A) altering each sound sample to the same amplitude.
(B) decreasing the frequency of each wave form.
(C) multiplying all sound samples by a number between 0 and 1.
(D) multiplying all sound samples by a number greater than 1.
11. Describe an example of the organising process occurring before, after or during each of the other six information processes.
12. In terms of the organising process, what does structuring and representing mean? Provide examples as part of your answer.
13. Describe the organisation of both bitmap and vector images and describe the nature of images suitable for each method of organisation.
14. Describe the organisation of CD quality stereo sampled audio data.
15. “The organisation of bitmap images and sampled audio data is a compromise between quality and storage size.” Do you agree? Justify your answer.