Digital Sound – Music and Audio, Digital Sampling
What is Sound? Sound is caused by a change in air pressure. When something vibrates the air pressure is changed, the picture, fig.1.shows how the beating of a drum causes changes in air pressure, the harder the drum is hit the more compressed the air becomes and so the louder the sound wave is, in this case represented by a sine wave the more compressed the air waves the higher the peak and vice versa the less the air is compressed the deeper the trough. It should be noted that sound is the movement of air molecules or for example the molecules in water and that Sound waves can travel through a variety of mediums including water, metal, glass etc. but not in a vacuum. As there are no air molecules in a vacuum there can be no sound.
Fig.1. Sound Waves. Rockwell International http://www.rockwool.com/acoustics/faq Accessed 19/03/2010
1.0 Definition of what sampling is and how to sample/record
When we talk about sampling we are usually talking about Digital sampling these days, a basic description is that a specifically designed application listens to the sound and takes a slice of the sound wave, think of it as a snapshot of the sound wave. The length of the snapshot is measured in bits and the number of snapshots taken is called the sampling rate and is measured in seconds. The more snapshots, that is the higher the sampling rate the better the quality of the sound (Fidelity) as it more closely resembles the original sound waveform.
Fig.2. Picture represents the difference in quality of the sampled sound depending on the bit-depth used. What is audio. http://musikality.net Accessed 22/03/2010
The sampling rate varies but typically a CD is sampled at 44,100 times per second at 16 bits. These samples are then stored by either on the CD itself or say on a computers hard drive for possible re-sampling and/or manipulation by effects processors.
An example of Analogue sampling can be found in the effects pedals used by guitarists, some of which remain in use today examples of these being Reverb and Phaser units where a proportion of the incoming signal is affected or indeed the whole signal is shifted out of phase or added to the original signal to produce the reverb effect. Other examples include echo units which do similar things to the signal although it does not change the actual sound but just samples the incoming sound and adds a copy to the outgoing signal but with a delay that can be adjusted to give a varying echo.
Sampled sounds can be used in conjunction with a range of effects systems to produce unique sounds for the Radio, TV and Film industries. Sampled sounds can be assigned to keys on an electronic keyboard – either a different sound for each key or a single sampled sound which is then processed by the keyboard to give a complete range of sounds.
2.0 Analogue Vs Digital
Fig.3. Analogue Vs Digital Sound. http://www.webbasedprogramming.com Accessed 19/03/2010
Analogue is a continuous medium and Digital is made up of steps. To sample or record sound it must first be converted from sound waves into an electrical signal. In Fig.3. There are two diagrams; the top one is of a Analogue signal which has been converted into an electrical signal for example by using a microphone which converts the movement created by sound waves impacting on a material such as thin plastic inside the microphone which in turn creates an electrical signal either by varying the capacitance in a condenser microphone or moves either the coils or a magnetic core inside a coil which then also generates an electrical signal. This continuous signal can now be recorded by using this electrical signal to for example move a needle, scratching an electrical representation of the sound onto the surface of a vinyl disk or onto magnetic tape using a recording head which is a form of electro magnet which marks the surface of tape with a signal.
Fig.4. Cross Section of Dynamic Microphone. http://www.mediacollege.com Accessed 19/03/2010
A digital signal starts off as an Analogue signal that is a sound wave converted to an electrical signal and this electrical signal is then converted into a digital signal by sampling it at a determined rate. That is the number of times it is sampled in a second. If we look again at Fig.3. The bottom diagram shows a representation of what a digital signal looks like compared with the analogue signal above it. You can see how the waveform has been converted into steps with each step approximating the peak or trough of the original signal. This approximation also supplies us with a clue as to why many sound purists prefer analogue over digital, as the analogue signal is continuous or step less while digital is an approximation of the original sound and as such does not contain the richness of the original analogue sound. Of course this can be alleviated to some extent by sampling at a higher rate and so more steps will be produced therefore more closely following the original signals waveform.
Digital signals are made up from 1’s and 0’s that is Binary code, so each one of those steps in Fig.3. is made up from a series of these 1’s and 0’s. This Binary code is the language of computers and so this allows the digital sound or data to be stored and manipulated by a computer. Digital sound can be recorded onto a computers hard disk, flash drive (USB Key), CD and DVD disk. As the digital sound is made up of 1’s and 0’s it is also possible to reduce the size of the data (or Digital sound file) by removing excess 1’s and 0’s for example the Mp3 format. The computer will remember to add these back in when the sound needs to be reproduced as will other sound reproducing equipment for example an iPod as it will recognise that the file is in an Mp3 format and so it will know to add those 1’s and 0’s back in during playback.
Fig.5. Copy degradation experienced when copying from a copy of a copy etc. Car on the left is a copy and you can see the pixilation of the image compared with the image of the car on the right. One of the key differences between Analogue and Digital recording is unlike Analogue recordings it does not matter how many times you record a digital sound file it remains at the same quality. Whereas an Analogue sound file loses it’s quality (Fidelity) the more times that it is copied
that is a master file when copied becomes a 2nd generation copy a copy from this becomes a 3rd generation and so on. This occurs in the same way that when the copy of a document is not quite as good as the original and a copy taken from this copy is even less perfect. This is best viewed by images in Fig.5. which is of two images of the same car, the one on the right is the original and the image on the left is a copy of a copy of a copy as you can see the final copy is very unclear and this represents the degradation in sound quality (loss of Fidelity) which you can expect when copying analogue sound files. For this reason for example, a music producer will have a master copy and it is from this that copies are made so that the copies are at worst 2nd generation copies.
3.0 Programmes and Equipment used to sampling & re-sampling
Fig.6. Audacity 1.3.11-beta Sound sampling and editing software application Audacity is a free open source application that can sample and re-sample digital sound and to facilitate the editing of sound files and even offers several effects including Echo, pitch and tempo changes etc. Coming back to its main features Audacity can let you in a non-destructive way edit digital sound tracks, cutting and pasting sections, adding new tracks with other digital sound files to enable mixing etc.
It is also possible to re-sample sound files by selecting the track to resample and then from the Track menu select resample and this will pop up another menu box where you are able to select a different sampling rate from 8000 to 96,000. Another option is to be able to export a sound file in different formats including .WAV (uncompressed 16 bit) and Mp3 (Compressed).
Audacity can do more than just re-sample sound it is also possible to carry out non-destructive editing which means unlike editing of old where say a tape was cut and new sections of tape added or removed in a process known as splicing. In non-destructive editing a section of the sound waveform can be cut and pasted from and into tracks. Alternatively tracks from another music file can be imported and ether inserted into the original track or added as another track and therefore effectively mixed with the original tracks.
A more professional application called Pro Tools does very similar things to Audacity but takes it several stages further also to having it’s own Analogue to Digital convertor, superior in quality to a Mac or PC’s own internal hardware called an M-Box. Pro Tools is an industry standard sound editing application used by TV, Film and music production companies to create and edit professional quality sound and music.
Fig.7. Pro Tools screenshot, courtesy of http://www.lennonbus.org. Accessed 22/03/10/2010
There are many other sound editing and re-sampling programs many of which are free to use or open source which include:-
- Adobe Audition
- Microsoft Media Player
- Sony Soundforge 9.0
- Rockwell International http://www.rockwool.com/acoustics/faq Accessed 19/03/2010
- What is audio. http://musikality.net Accessed 22/03/2010
- Analogue Vs Digital Sound. http://www.webbasedprogramming.com Accessed 19/03/2010
- Cross Section of Dynamic Microphone. http://www.mediacollege.com Accessed 19/03/2010
- Pro Tools screenshot, courtesy of http://www.lennonbus.org Accessed 22/03/10/2010