Autosave: 2024-12-27 14:21:56

neuron/3cd71ed3-2c64-460c-92b8-94889befa1fe/Audio_file_formats.md

@@ -0,0 +1,98 @@
+---
+tags: [sound, binary]
+created: Friday, December 27, 2024
+---
+
+# Audio file formats
+
+## CD
+
+- CDs store audio in uncompressed PCM (Pulse Code Modulation) format
+- They have a sampling rate of 44.1kHz
+- In two channels for stereo
+
+## WAV: Waveform Audio File Format
+
+- CD-quality encoding with no compression
+- Bit-for-bit identical to the CD source
+- Historically developed for Windows machines but can play on all operating
+  systems
+
+## FLAC: Free Lossless Audio Codec
+
+- Basically the same as WAV but in a (losslessly) compressed format
+- The difference between a novel in a text file (WAV) and as a zipped file
+
+## MP3: MPEG-1 Audio Layer MP3
+
+- Lossy format.
+- When a WAV file (or other lossless format) is converted to MP3 a Fast Fourier
+  Transform analysis is performed to determine the frequency of certain sounds.
+- This is used by the encoder to decide which parts of the sound are
+  imperceptible and thus which can be discarded to reduce the file size. This is
+  done through the application of psycho-acoustic models.
+- The remaining data is then compressed
+- Examples of the data reduction applied:
+  - Removing frequencies that humans cannot hear
+  - Removing quieter sounds that are masked by louder sounds
+  - Combining similar frequencies
+  - Reducing stero information where it is less noticable
+- A conversion to MP3 from, for example, a CD source is always a one-way process
+  and is not reversible. Once information is discarded in the compression
+  process it cannot be retrieved. This is obviously in contrast to lossless
+  methods like FLAC where the original CD audio can always be reconstructed.
+- It follows from the above that if you repeatedly encode a CD source to MP3, it
+  will deteriorate in quality since more data is being removed each time.
+
+## OGG: Ogg Vorbis
+
+- An open-source alternative to MP3
+- Typically achieves better quality than MP3 at the same bit rate, especially at
+  very high/ low frequencies
+- Also better stereo handling at low frequencies
+- Uses a more modern psycho-acoustic model
+
+## Variable and constant bit rates
+
+For lossy formats like MP3, the amount of data that is being encoded from the
+uncompressed source is expressed via the unit of "bit rate": **how many
+thousands of bits are being used to represent each second of audio**.
+
+There are two methods of encoding that impact on the bit rate.
+
+With _variable bit rate_ encoding, the encoder dynamically adjusts the bit rate
+depending on what is happening in the music. During complex passages (e.g. a
+full orchestra) it uses a higher bit rate to capture the detail. During simpler
+passages (like a single instrument) it uses lower bit rates since less data is
+needed and during silence it can drop to very low bit rates.
+
+_Constant bit rate encoding_ uses the same bit rate throughout, it is
+consequently less efficient.
+
+When talking about the quality of MP3s there are generally two bit rates cited:
+
+- ~128kbps: acceptable but significantly reduced
+- 320kbps: the highest quality you can get whilst still using a lossy method
+  like MP3
+
+With VBR, these are sometimes expressed as an average.
+
+Subjectively, A 128kbps MP3 might sound "underwater" or "swishy", while a
+320kbps version would preserve much more detail.
+
+Still, the bitrate of a CD is 1411kbps! A 128kbs MP3 is therefore only capturing
+about 9% of CD quality and a 320kbps MP3 is capturing about 23% of CD quality.
+
+## Streaming services
+
+Spotify uses Ogg Vorbis throughout but uses different bit rates for its diferent
+tiers. The free tier has a range from 24-160kbs at VBR with the option of 320kbs
+on the premium tier.
+
+Other services offer FLAC or FLAC-equivalent quality at their most expensive
+tiers (Apple Music, Amazon Music, Tidal).
+
+Og Vorbis is particularly well-suited to streaming. It can seamlessly switch bit
+rates during the stream which is beneficial with changeable network conditions.
+Plus data is organised into independent packets so if a packet is lost there is
+no perceptible difference.

neuron/3cd71ed3-2c64-460c-92b8-94889befa1fe/Binary_encoding_of_sound.md

@@ -0,0 +1,9 @@
+---
+tags: []
+created: Friday, December 27, 2024 
+---
+# Binary_encoding_of_sound
+
+
+
+

neuron/3cd71ed3-2c64-460c-92b8-94889befa1fe/a4601796_trapdoor_functions.md

@@ -1,5 +1,5 @@
 ---
-tags: [cryptography]
+tags: [cryptography, encryption]
 created: Friday, December 20, 2024
 ---
 

neuron/3cd71ed3-2c64-460c-92b8-94889befa1fe/bbdcb54f_public_key_cryptography.md

@@ -1,5 +1,5 @@
 ---
-tags: [encryption]
+tags: [encryption, cryptography]
 created: Friday, December 20, 2024
 ---
 

neuron/3cd71ed3-2c64-460c-92b8-94889befa1fe/index.md

@@ -13,12 +13,14 @@ computer science.
 
 ![not-by-ai-badge](static/not-by-ai-alternative--light.svg)
  
-**Build ID:** dc239556-fd21-4147-b56e-3d8b474984ad
+**Build ID:** 3cd71ed3-2c64-460c-92b8-94889befa1fe
 
-**Published:** Fri 20 Dec 2024 13:12:09
+**Published:** Fri 27 Dec 2024 14:21:54
 
 ### Recent edits 
 
+- [[Audio_file_formats]] 
+- [[Binary_encoding_of_sound]] 
 - [[a4601796_trapdoor_functions]] 
 - [[bbdcb54f_public_key_cryptography]] 
 - [[Dynamic_and_static_websites]] 
@@ -29,11 +31,9 @@ computer science.
 - [[HTTPS]] 
 - [[Bauman_quote]] 
 - [[fbbfbc32-political-accommodation-inveigelment-surveillance-capitalism]] 
-- [[a247fa9b_surveillance_capitalism_not_necessary]] 
-- [[AWS_SNS]] 
 
 
-### All notes (509) 
+### All notes (511) 
 
 - [[0716531c_rewilding_the_internet]] 
 - [[241fe1a3_the_Web_versus_modem_BBSs]] 
@@ -74,6 +74,7 @@ computer science.
 - [[Assembly]] 
 - [[Associative_Property_of_Addition_and_Multiplication]] 
 - [[Atomic_and_molecular_propositions]] 
+- [[Audio_file_formats]] 
 - [[Awk]] 
 - [[Axioms_of_set_theory]] 
 - [[Bash_colour_output]] 
@@ -91,6 +92,7 @@ computer science.
 - [[Binary_arithmetic]] 
 - [[Binary_colour_encoding]] 
 - [[Binary_encoding]] 
+- [[Binary_encoding_of_sound]] 
 - [[Binary_encoding_of_text]] 
 - [[Binary_number_system]] 
 - [[Binary_units_of_measurement]] 

zk/Audio_file_formats.md

@@ -0,0 +1,98 @@
+---
+tags: [sound, binary]
+created: Friday, December 27, 2024
+---
+
+# Audio file formats
+
+## CD
+
+- CDs store audio in uncompressed PCM (Pulse Code Modulation) format
+- They have a sampling rate of 44.1kHz
+- In two channels for stereo
+
+## WAV: Waveform Audio File Format
+
+- CD-quality encoding with no compression
+- Bit-for-bit identical to the CD source
+- Historically developed for Windows machines but can play on all operating
+  systems
+
+## FLAC: Free Lossless Audio Codec
+
+- Basically the same as WAV but in a (losslessly) compressed format
+- The difference between a novel in a text file (WAV) and as a zipped file
+
+## MP3: MPEG-1 Audio Layer MP3
+
+- Lossy format.
+- When a WAV file (or other lossless format) is converted to MP3 a Fast Fourier
+  Transform analysis is performed to determine the frequency of certain sounds.
+- This is used by the encoder to decide which parts of the sound are
+  imperceptible and thus which can be discarded to reduce the file size. This is
+  done through the application of psycho-acoustic models.
+- The remaining data is then compressed
+- Examples of the data reduction applied:
+  - Removing frequencies that humans cannot hear
+  - Removing quieter sounds that are masked by louder sounds
+  - Combining similar frequencies
+  - Reducing stero information where it is less noticable
+- A conversion to MP3 from, for example, a CD source is always a one-way process
+  and is not reversible. Once information is discarded in the compression
+  process it cannot be retrieved. This is obviously in contrast to lossless
+  methods like FLAC where the original CD audio can always be reconstructed.
+- It follows from the above that if you repeatedly encode a CD source to MP3, it
+  will deteriorate in quality since more data is being removed each time.
+
+## OGG: Ogg Vorbis
+
+- An open-source alternative to MP3
+- Typically achieves better quality than MP3 at the same bit rate, especially at
+  very high/ low frequencies
+- Also better stereo handling at low frequencies
+- Uses a more modern psycho-acoustic model
+
+## Variable and constant bit rates
+
+For lossy formats like MP3, the amount of data that is being encoded from the
+uncompressed source is expressed via the unit of "bit rate": **how many
+thousands of bits are being used to represent each second of audio**.
+
+There are two methods of encoding that impact on the bit rate.
+
+With _variable bit rate_ encoding, the encoder dynamically adjusts the bit rate
+depending on what is happening in the music. During complex passages (e.g. a
+full orchestra) it uses a higher bit rate to capture the detail. During simpler
+passages (like a single instrument) it uses lower bit rates since less data is
+needed and during silence it can drop to very low bit rates.
+
+_Constant bit rate encoding_ uses the same bit rate throughout, it is
+consequently less efficient.
+
+When talking about the quality of MP3s there are generally two bit rates cited:
+
+- ~128kbps: acceptable but significantly reduced
+- 320kbps: the highest quality you can get whilst still using a lossy method
+  like MP3
+
+With VBR, these are sometimes expressed as an average.
+
+Subjectively, A 128kbps MP3 might sound "underwater" or "swishy", while a
+320kbps version would preserve much more detail.
+
+Still, the bitrate of a CD is 1411kbps! A 128kbs MP3 is therefore only capturing
+about 9% of CD quality and a 320kbps MP3 is capturing about 23% of CD quality.
+
+## Streaming services
+
+Spotify uses Ogg Vorbis throughout but uses different bit rates for its diferent
+tiers. The free tier has a range from 24-160kbs at VBR with the option of 320kbs
+on the premium tier.
+
+Other services offer FLAC or FLAC-equivalent quality at their most expensive
+tiers (Apple Music, Amazon Music, Tidal).
+
+Og Vorbis is particularly well-suited to streaming. It can seamlessly switch bit
+rates during the stream which is beneficial with changeable network conditions.
+Plus data is organised into independent packets so if a packet is lost there is
+no perceptible difference.

zk/Binary_encoding_of_sound.md

@@ -0,0 +1,9 @@
+---
+tags: []
+created: Friday, December 27, 2024 
+---
+# Binary_encoding_of_sound
+
+
+
+

zk/a4601796_trapdoor_functions.md

@@ -1,5 +1,5 @@
 ---
-tags: [cryptography]
+tags: [cryptography, encryption]
 created: Friday, December 20, 2024
 ---
 

zk/bbdcb54f_public_key_cryptography.md

@@ -1,5 +1,5 @@
 ---
-tags: [encryption]
+tags: [encryption, cryptography]
 created: Friday, December 20, 2024
 ---