Analogue and Digital Recording

Analogue and Digital Recording

• An analogue waveform is a representation of the changes in air pressure which are a result of the vibrations created when we make a sound

• Microphones convert the changes in air pressure to an electrical signal - this is an analogue signal, because its amplitude can have any value between the minimum and maximum

• A digital signal can only have values of 0 and 1, or on and off

• It uses these values to store a representation of the signal’s amplitude as numerical data.

Analogue to Digital Conversion

• The process of analogue to digital conversion involves us taking digital samples of the momentary amplitude of a waveform

• A continuous analogue signal is converted into a series of binary numbers

Sample Rate

• The number of times the amplitude of the analogue signal is ‘read’ during the conversion / digital sampling process

• Measured in hertz, where 1 Hz is equal to 1 sample of the waveform’s amplitude taken every second

• Sample rates of 44.1kHz (CD quality) and 48kHz (DVD quality) are common

• At CD quality the amplitude of the waveform is being read 44,100 times every second

• Other higher sample rates exist, but are less common. Due to fact that the limitation of human hearing means you would be unlikely to hear any difference, even with very high-quality playback equipment

• The sample rate determines the frequency response of the piece of equipment - the higher the sample rate, the greater the highest frequency captured

Nyquist’s Theorem

• States that the sample rate of a digital system should be a minimum of twice the highest frequency captured

• A sampl e rate of 44.1kHz, more than adequately captures the top end of the human hearing range at 20kHz.

Aliasing

• When the sample rate is not high enough, aliasing can occur

• Frequencies above the Nyquist frequency are caputred and incorrectly recreated within audible range below the Nyquist frequency

• These inaccuracies manifest as unwanted artefacts in the reproduced signal

Bit Depth

• The level of detail the amplitude is measured to for each sample

• It provides a snapshot of the amplitude of the waveform at the instant when the sample is taken

• The higher the bit depth, the more detail with which you can measure the amplitude of the analogue waveform and the greater the dynamic range and therfore the signal-to-noise ration.

• CD quality is 16 bit but 24 bits are also commonly seen in digital audio.

• A bit depth of 24 is usefel as when working on a project, the noise on multiple tracks soon adds up.

Dithering

• The introduction of small amounts of unobtrusive randomly generated noise into the conversion process.

• It randomises the effect of quantisation error as part of the analogue to digital conversion process.

Discussing frequency responses

• A frequency response graph shows the frequencies that are captured by a microphone, and how much louder or quieter they are compared to the actual sound

• It shows whether frequencies are captured louder or quieter than they should be by the amount of difference in dB either side of 0 on the Y axis.

Noise and interference in analogue and digital systems

• Digital signals are less susceptible to interference than analogue signals

• If an analogue signal picks up noise, this forms part of the waveform

• However, a digital siganl is still identifiable as 0 or 1, and doesn’t degreade in quality if it picks up noise.