Sound files The need for compression and basic techniques for compression.
Sampled Sound and Nyquist-theorem Sampling resolution, sampling rate.
Sound Synthesis Describe MIDI and its advantages for storing sound digitally.
Streaming audio Explain what it is and why it is used.
Analogue and Digital Data
Differentiate between analogue and digital data and analogue and digital signals.
Analogue and Digital Signals
Analogue to Digital Converter (ADC)
Describe the principles of operation of an analogue to digital converter.
The world outside of digital computers is analogue, light and sound reach us as continuous waves. Computers require these continuous analogue forms to be sampled and converted into numeric format. A key device in the conversion of analogue data to digital format is the ADC or analogue-digital converter. To see or hear digital data a DAC or digital-analogue converter is used (humans cannot read digital data as sounds or pictures!)
Digital data are discrete, they are precise numbers that change in steps rather than a smooth curve.
Analogue sound must be sensed and converted into electrical current that represents the original. This is done by a transducer e.g. a microphone. The values of the electrical signal can be sampled and converted into numbers; when stored in binary these numbers can be stored, manipulated and communicated through networks.
Digitised data is processed and saved in binary format. Digitising generally involves sampling, that is recording the values of analogue data and converting it to digital (numeric) form. Sound, for example, is sampled.
The analogue digital process is as follows: Samples are taken at fixed intervals. The samples are represented as narrow pulses of a height proportional to the original signal (pulse amplitude modulation or PAM). The PAM samples are quantised, turned into numbers, which are rounded to the nearest whole number. The PCM encoder produces a sequence of fixed-height pulses representing binary 0 and 1.
The Nyquist-Shannon sampling theorem states that a source can be captured accurately if it is sampled at twice the highest frequency. For example, if the highest frequency is 20KHz then the sampling rate needs to be 2 x 20,000 samples/second = 40KHz (double this in stereo). The actual sampling rate for CD audio is 44,100Hz ("The exact sampling rate of 44.1 kHz was inherited from a method of converting digital audio into an analog video signal for storage on U-matic video tape, which was the most affordable way to transfer data from the recording studio to the CD manufacturer at the time the CD specification was being developed." - Wikipedia. DVD audio and Blu-Ray audio is sampled at 96,000Hz.
Sampling can be done at lower rates to produce less data but with some loss of quality depending on the rate chosen.
A DAC reverses the process of encoding, converting numbers in binary back to analogue sounds.
The quality of recorded sound is influenced by the sampling rate and the number of levels of sound that the PCM encoder can distinguish e.g. 16 bit recording gives 65,536 levels. This is bit depth. Studios may record in 24 bit sound but it is doubtful whether the human ear can distinguish between 20 and 24 bit sound.
For audio files there are both lossy (such as MP3) and lossless methods of compression (such as Monkey's Audio and FLAC).
When images and sounds are compressed they take up less storage space so they take up less space on a storage device and can be transmitted more quickly.
Sounds can be synthesised from wave forms: generate a wave form and convert it to analogue so it can be heard. Instruments can be synthesised by recording them digitally and then playing them back at specific pitches.
MIDI devices store the instrument note and duration information about sounds in binary form.
A broadband speed of 2.5 Mbit/s or more is recommended for streaming movies.
| Digital Basics | Digital Signals | Digital Audio |
| ADC | DAC | Sound Cards |
| Compression | Quantized Music |
Explain how analogue sounds can be converted to digital format. What factors control the quality of the sound recording?
Explain how sounds and music recorded digitally can be converted to a form that is audible to humans.
Discuss the significance of audio compression in the development of modern audio devices.
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