Intro to digital audio Flashcards
How does digital audio differ to analogue?
Represents sound as a sequence of discrete numbers, rather than a continuous variation of pressure / voltage
Why is digital audio easier to transmit, store and copy reliably?
Has a fixed temporal (and thus frequency) resolution and fixed amplitude resolution. Whereas analogue has a theoretically infinite temporal and amplitude resolution.
When was the first computer based digital recording system made?
1957 by Max Matthews
When was the first CD player made?
1982 - Sony CDP-101
When was the ADAT made?
1991 - used video tape to store 8 tracks of audio.
What do more bits mean?
More bits mean more values representable. Adding a bit doubles the range of possible values.
What is increasing bit depth used for?
- To record audio samples with greater precision / resolution.
- Because each bit doubles the range of amplitudes, it adds 6dB to the signal-to-noise ratio (SNR): e.g. 16-bit = 16 x 6 = 96dB
What are individual bits transmitted as?
- fixed high and low voltage levels (e.g. S/PDIF +/- 0.3V)
- on/off flashes of light (optical)
What is the sampling theorem?
- Provided certain limitations are observed, any arbitrary wave form can be recorded by taking individual samples of amplitude at fixed intervals.
- The wave form is played back by outputting the same samples at the same fixed intervals.
What is the sampling frequency?
The number of samples taken over the course of one second:
- measured in Hz
- known as the sample rate
What are different sample rates used for?
Different purposes based on different needs (quality, bandwidth).
What is the sample rate of telephony?
8kHz
What is the sample rate of broadcast (eg. radio)?
32kHz
What is the sample rate of a CD?
44.1kHz
What is the sample rate of audio for video?
48kHz
What is the sample rate of a professional recording?
96kHz
When does aliasing occur?
When the input frequency is greater than half the sampling frequency. This point is called the Nyquist limit.
Describe the frequencies above and below the Nyquist limit (fN).
Frequency components above fN are mapped to (and heard as) frequencies below fN. Thus, all frequencies in the input signal must be less than half the sampling rate
What happens to frequencies above the Nyquist limit?
Frequencies above the Nyquist limit are mirrored around it: At a 48kHz sample rate: an ordinarily inaudible frequency of 10kHz above Nyquist (34kHz) will be audible as 10kHz below Nyquist (14kHz).
How are frequencies above the Nyquist removed?
This is done with a low-pass filter known as an analogue antialiasing filter.
What is oversampling used for?
To avoid the use of analogue brick-wall filters (which exhibit phase problems). Instead, an analogue gentle-slope filter and a digital brick-wall filter are combined
What are the advantages and disadvantages of a brick wall filter when oversampling?
- Good frequency response
- Poor phase response
What are the advantages and disadvantages of a gentle filter when oversampling?
- Poor frequency response (higher frequencies attenuated)
- Good phase response
How do you avoid the gentle slope filter from affecting audible frequencies?
Oversample the audio at a much higher rate than required (e.g. from 4x to 128x).
How can a digital filter help with phase shifting?
Digital filters can have linear phase response, so the subsequent brick wall filter does not cause a phase shift.
What happens to the oversampled audio?
The oversampled audio is digitally down-sampled to the required sample rate.
What are band limited synthesisers used for?
In digital synthesisers, band-limited versions are used to ensure the harmonics do not exceed the Nyquist frequency.
What happens to the amplitudes after sampling?
After sampling, the amplitudes are stored as 16-bit (CD) or 24-bit (DVD) integer values.
When is the best SNR (signal to noise ratio) achieved?
The best SNR is achieved when signal spans the full range of levels.
- CD: 16-bit (96dB), DVD: 24-bit (144dB).
Weaker signals use fewer levels (bits) and so have a worse SNR.
What does dithering do?
Adds a small amount of random noise to de-emphasise the steps
- Digital ‘blurring’
to hide the lines
What does dithering improve?
Improves playback sound perceptually
Ensures noise is broadband, which is harder to notice ( not correlated with the audio)
What else will distort audio?
Uneven / irregular clock pulses will also distort the audio.
- Leads to phase issues, spectral distortion, poor stereo imaging.
- Fundamentally a hardware limitation (not easily solved in software)
