Sound is stored the same way as other digital files. However, a soundwave is a continuous signal, that isn’t directly digital.
An ADC (Analogue Digital Converter) is required to convert a soundwave to a digital sound file. It requires thousands of samples to be taken per second.
A sample is a snapshot of what sound was detected at a specific point in time. If samples are played together at a high enough rate, sound that is close to the original can be played.
Factors Impacting Audio Quality
- Sampling Rate: Number of samples per second
- Bit depth: Number of bits used to represent each clip
- Bit rate: Number of bit used per second of audio
Sampling Rate
- Number of samples taken per second
- The higher the rate, the greater the detail of the sound
- Unit of sampling is represented in Hz
- Each sample represents the amplitude of the wave at a certain point in time.
- The most common sampling rate for music is 44,100 samples per second which is 44,100Hz (44.1kHz)
- A VOIP typically has a sampling rate of 8kHz, which is enough for a human voice to be heard quickly, but the quality is reduced to a certain extent.
Frequency
- The number of oscillations per second is called frequency
- It is measured in Hz and controls the pitch of the sound
Nyquist’s Theorem
- The theorem states that:
- For accurate sampling, the sampling rate must be at least twice the frequency of the highest frequency in the original sound signal.
- When sampling rate is less than the frequency suggested by the theorem, the recording will not be accurate to the original sound.
Bit Depth
- Bit depth is the number of bits available for each sample
- The higher the bit depth, the higher the quality of the audio
- A CD has a bit depth of 16 bits, and a DVD has a bit depth of 24 bits
- An n bit system can have 2^n different values.
PCM
- High-quality audio files are created as pulse-code modulation (PCM)
- WAV or AIFF are uncompressed examples
- The pulses of the induced electrical signal in the microphone
Bit Rates
- The amount of data sampled per second (kbps)
- Bit rate = sampling rate x bit depth x channels
- Typically, stereo is the minimum channel count (2 channels)
Audio Quality
- A reasonable music audio must have a minimum bit rate of 128bkps.
- The greater the bit rate, the higher the quality.
- This is why the audio quality of a CD is higher than that of Spotify.
- Tidal is a music streaming solution that provides the highest streaming quality over the internet.
Analogue to Digital conversion
- An analogue signal is a continuous signal which represents physical measurements.
- A signal from a microphone is an example of an analogue signal.
- However, a digital signal is a discrete signal that uses discontinuous data.
File size check
- 24 bit depth
- 2 channel - L & R
- Sample rate 16kHz
- 2.5 minutes long sample rate * bit depth * channels
24 x 16000 x 2 = 768kbps 768000 x 150= 115200000 bits / 8 = 14.4 MB
(Musical Instrument Device Interface) MIDI
- A communication protocol that enables electronic musical instruments to interact with each other using information and control signals.
- Asynchronous serial 8-bit transmission
- MIDI file instructs device on how to produce a musical note using several commands.
- A command is identified by its specific sequence of bits.
- You can think of it as the musical equivalent of an
SVG
file.
Types of bytes
-
Status bytes
- Indicates instruments about the type of action to be performed
- The action of 16 channels are represented in this byte
- Notes are ON or OFF
- Key pressure—how hard a key is pressed
-
Data bytes
- Information to status bytes
- Pitch byte: denotes note to be played
- Velocity note: denotes the loudness of the note.
-
Using a MIDI file, music identical to the original can be played every time.
-
A sequence program is required so that commands can be recognised.
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Smaller in size than MP3
-
MIDI does not contain any audio track
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Used in web pages, ringtones etc
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Used for creating electronic orchestras.
- Bit depth of 8 bits, what value range can be represented? 2^n = 2^8 = 256
- Calculate the bit rate of a 32-bit system with a sampling rate of 44,100 samples per second and 2 channels. 32 * 44100 * 2 = 2822400 bits
2822.4kbps
- What is the file size of a 4-minute song with bit rate of question 2/ 2,822,400 * 240 = 677,376,600 bits = 84.67MB
- Analogue sound is converted to digital using a DAC to sample.
- Sound files from the internet are typically lower quality than a CD sound file, because online services tend to use lower bit rates and higher compression to reduce hosting costs and increase speed.
- If the bit depth is increased, then there is a greater range of sound possible. The file size will also be increased.
- VoIP uses a sampling rate of 8kHZ because it is enough for you to understand a human voice. Anything higher would be inefficient.
- Factors affecting audio quality: sample rate, bit depth & bit rate.