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Digitising sound answers
Activity 1
- Why does the use of the audio file need to be determined before digitisation should take place?
The use will determine the type of audio file as well as the required quality. If compression is set too high then the quality would not be high enough for a radio broadcast but may be quite suitable for a web podcast. If an uncompressed format is used then the quality would be suitable for any application but the file size would be very large and not suitable for web use.
- Explain the purpose of an analog to digital converter and what is the difference between this and a digital to analog converter.
An analogue to digital converter will convert sound from a cassette or microphone into a digital format.
- Research methods used to digitise sound.
Activity 2
- What is the difference between sampling and sample rate.
Sampling is the process by which the sound is digitised. The sample rate is the number of slices of a sound wave used. That sound that is not used is data that is disregarded. The shape of the sound wave is extrapolated from the sound slices used. As such sample rate is a reflection of the quality of the sound.
- Research the following terms and explain them with the use of a drawing:
- amplitude
- wavelength
- frequency
The frequency of this wave is a measure of the number of waves measured over a second. This is measured in Hertz. If this wave took one second to pass a point then the frequency would be 1 Hertz.
Activity 3
- Use the Windows sound recorder to record sounds and make a list of the various settings such as sample rate. Make a recording and then use sound recorder to convert to various formats with a variety of different settings. Examine the difference in file size and quality.
The screen dumps below show you how to save sound with various sample rates and file types using Windows "Sound Recorder" found under Programs> Accessories> Entertainment
Once you have the program running you will find that is you click 'Save As' you will have a choice of options.
What you should notice is that different file formats will have varying degrees of quality and compression. Mostly you will probably not notice the difference in quality between the file formats. However you will notice the difference as you begin to play with the quality of the sound which is to be found under attributes. As you decrease the quality the file size will also decrease.
- Examine the table above and then explain and justify the different criteria that needs to be examined when choosing a sound format. For example what file type would you record in if you were:
- a professional musician with voices
A professional musician would need a file type with excellent quality as well as the "equipment used for playback, and trade-offs between file sizes (smaller files mean more music) and listening quality (more compression normally means lower audio fidelity)." A lossless format such as PCM (.WAV or .AIF) may suit him or her but MP3s with a high sample rate can give excellent quality and are also used by radio stations. However an uncompressed format would then give flexibility with rerecording at a later stage.
- a professional instrumentalist
A midi file may be appropriate. Remember midi files will play individual notes and can be made to sound like any instruments with special effects. "The MIDI (Musical Instrument Digital Interface) is a format for communicating music information between electronic sound devices like synthesizers and PC sound cards. MIDI format is flexible and can be used for basic to professional music." As such if the musical instrument is electronic this maybe a real option.
REMEMBER MIDI files "cannot record sounds" and "MIDI files do not contain sampled sound". This means that if the musician wants to record a particular sound then they probably want to sample the sound. Midi files will not sample. As such we come back to the same choices as in the previous example. However if they want to do mixing with the sound file at a later stage and it is not for immediate playback in a CD player then many of the uncompressed formats are appropriate.
- a web page designer
A major issue for web designers is download speed. If users of a website are left waiting too long then web masters know they will go elsewhere. A compressed format is critical. One of the common formats is important so that it is easily played by as many people as possible. MP3 is ideal. Ogg Vorbis may be a problem if browsers and their associated media players will not play it. Real player or QuickTime are also common formats. An increasingly common one is mp4a which has the extension of .aac. These files are commonly used by ipods.
- a professional musician with voices
- about to record to CD for playback in a standard CD player
The critical issue here is that it must be playable in a CD player. As stated This mainly restricts it to .cda (compact disc audio), .wma (Windows Media) or mp3. Many of the other formats will not be recognised by CD or DVD players.
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