Audio & Midi Protocols

Question 1

Vinyl

Answer 1

Earlier models were accompanied with an additional ‘scratchy noise’. When the needle passes over a groove, it changes it slightly (overtime erasing it).

Question 2

Cassette

Answer 2

Recording tape is coated with a metallic compound (Iron oxide) capable of storing a magnetic pattern with varying strength and direction to represent the original sounds waveform. Modern tape formats use polyester as the tape base, with a suitable magnetic coating.

Question 3

Cassette factoid

Answer 3

Harmonic and dynamic distortion are part and parcel of tape recordings, but these qualities are so ingrained in our audio culture that technically more accurate digital recordings often seem ‘flat’ by comparison.

Question 4

Digital

Answer 4

Digital recording converts the analogue wave into a stream of
numbers and records the numbers instead of the wave. To play back the music, the stream of numbers is converted back by a digital-to-analogue converter, amplified and then fed to the speakers to produce the sound.

Question 5

CD

Answer 5

the sampling rate is 44,100 samples per second and the number of gradations is 65,535. At this level, the output of the DAC so closely matches the original waveform that the sound is essentially “perfect” to most human ears.

Question 6

Limitations of an Analogue System

Answer 6

Copies are never as good as the original. Analogue Recording always adds noise.

Question 7

Digital Representation of Sound

Answer 7

ADC captures samples of audio at a specified rate and size, which are referred to as the sampling rate and the bit depth.

Question 8

how many samples at a sample rate of 50khz would be taken to represent 1 minute of stereo audio?

Answer 8

2(50,000 x 60) = 6,000,000 (Note it’s important to times by 2 as stereo)
Key numbers: 50khz = 50,000. 1 minute = 60 seconds. 2 as stereo (so 2 channels)

Question 9

Exemplifying an Aliased Frequency

Answer 9

Imagine a sample rate of 48khz. The highest frequency that could occur before aliasing would be 24khz (half the sample rate). If a frequency of 30khz went through the ADC, as this is over 24khz, the frequency would be under sampled, resulting in a aliased frequency of 18khz. 48000 (sampling frequency) - 30000 (Pitch frequency) = 18000 (Aliased Frequency).

Question 10

Anti-aliasing Filters

Answer 10

Occurs between the ADC and lowpasses input signals at a frequency higher than half the sampling frequency to make sure
as little as possible input signals occur. Another filter is placed after the DAC. Its main function is to change the samples stored digitally into smooth, continuous representation of the original.

Question 11

Bit depth

Answer 11

The number of bits in a word determines how precise the values are. Working with a higher bit depth is like measuring with a ruler that has finer increments: you get a more precise measurement.

Question 12

Bit Dynamic Range

Answer 12

A 16-bit digital audio has a maximum dynamic range of 96dB while a 24-bit depth will give us a maximum of 144dB.

Question 13

Dither

Answer 13

is a low-level noise that is intentionally added to a digital audio file. Dither noise masks what is called quantization error. It makes it random and therefore harder for ears to discern.

Question 14

Audio data compression

Answer 14

In which the amount of data in a recorded waveform is reduced for transmission. This is used in CD and MP3 encoding, internet radio, and the like.

Question 15

Audio level compression

Answer 15

In which the dynamic range of an audio waveform is reduced. This is used in guitar effects racks, recording studios, etc.

Question 16

Audio data compression calculation

Answer 16

2(44.1 x 16) = 1411.2kbps
44.1 = Sample rate
16 = bits
2 = stereo

Question 17

Adaptive Differential Pulse Code Modulation

Answer 17

Exploits redundancy on a sample by sample basis and was designed around the concept that the differences between each sample are often smaller than the amplitudes of the samples themselves. Fewer bits are therefore required in order to encode.

Question 18

Lossless Compression

Answer 18

Preserves all of the original data whilst decreasing size by looking for patterns / probabilities. Includes file types .zip and FLAC (audio)

Question 19

Lossy Compression

Answer 19

Compression followed by expansion results in something similar but not identical to the original file.

Question 20

MP3

Answer 20

MP3 works by encoding a wave file and then stripping it of the elements we won’t notice if they are missing (i.e. sounds above 16KHz).

Question 21

PLL (phase-locked loop)

Answer 21

works by comparing and adjusting the phase of an oscillator’s clock (produced by a voltage controlled crystal oscillator) to the phase of an external signal.

Question 22

USB (Universal Serial Bus)

Answer 22

The benefit of USB is it can be Hot connected or disconnected. USB is an all in one solution because it can be used to link many different external devices.

Question 23

Firewire

Answer 23

can (was) be used to link high-speed external devices. Offered substantial improvements (particularly for multichannel audio) compared to earlier USB, and was the protocol of choice for audio interface connections

Question 24

Thunderbolt

Answer 24

Enough bandwidth to daisy-chain multiple high-speed devices without using a hub or switch. Capable of delivering 10 watts and more to peripherals like external solid-state drives for improved performance on the go. Thunderbolt requires active cables, (which is why they’re so expensive).