What is an analog music source?
A medium in which music is played through mechanical means e.g. grooves on a vinyl / magnetic tape
How can you listen to the contents of an analog music source on a computer?
Analog material must be converted into binary data through A-D conversion - such as ripping a vinyl through serato / rekordbox / preamp & soundcard, or connecting a tape deck to a sound card.
What is a digital music source?
A source that can already be interpreted by a computer - such as an audio file (mp3, flac, wav, aif, etc..) / CD / DVD, or a streaming method through DSPs
List the 3 main audio file quality types
Uncompressed lossless, Compressed lossless, Lossy
List 5 points about uncompressed lossless audio formats
- Complete retention of quality from original binary source (analog or digital) - highest fidelity
- All data is treated equally in the reproduction of audio - so silence is given as many bits per second as a busy part of a track
- Main formats include WAV and AIFF - which use Pulse Code Modulation (PCM) as the storage algorithm
- Take up the most memory of all formats - this is why you do not stream uncompressed lossless video files online / download
- Can be compressed into compressed lossless without any perceived loss of quality OR compressed into lossy with a potential perceived loss of quality
List 5 points about compressed lossless audio formats
- Complete retention of quality from original binary source (to our ear) - highest fidelity
- Unperceiveable data such as silence is given almost no bits in order to save memory and efficiently store the audio
- Main formats include FLAC and ALAC, with FLAC being the most popular
- Happy medium between lossy and uncompressed lossless
- Can be uncompressed into PCM (WAV, etc..) or compressed further into lossy
List 4 points about lossy audio formats
- Some degree of quality loss, as some audio data is negated or lost in order to save memory space (e.g. frequencies above 20kHz)
- Main formats include mp3, aac, wma.
- Most efficient for storage, providing sufficient quality audio for the overwhelming majority of the population with only a fraction of the memory space
- Cannot be converted into lossless, because not all of the original source material is present (e.g. high frequencies) and cannot be recreated properly
Define ‘Transparent’
Refers to the threshold at which a lossy file is undistinguishable to a lossless file to the average listener - roughly 192kbps at a constant bitrate
List the accepted lossless and lossy formats on Orpheus
LOSSLESS: flac only, most efficient and conversion possible
LOSSY: mp3, aac (digital download, iTunes respectively) and ac3, dts (DVD material)
mp3 > aac every time unless there are exclusives in aac format from iTunes
What is the most popular mp3 encoder and why?
LAME - open source, reliable, high quality
When converting an audio file into mp3 with LAME, there are three options regarding how the bitrate is manifested in the output. List them
- CBR (Constant Bitrate)
- VBR (Variable Bitrate)
- ABR (Average Bitrate)
Explain CBR
- CBR involves setting a target bitrate at the start of conversion, in which LAME will output an mp3 with that set bitrate across the entire track.
- This means that regardless of sound complexity, each second of music is treated and compressed in the same nature
- Predictable file size and most uniform listening experience
Explain VBR and list its sub-formats
- VBR involves setting a target bitrate at the start of conversion. LAME will then convert the audio into an mp3 with that target in mind, however some negative deviance from the quality will occur depending on sound complexity
- Each second of music is treated differently depending on complexity - therefore silence is given significantly less bits per second than complex audio
- Outcome is a file that has a general bitrate that is close to the target, but there will always be dips in quality.
- V0 - V9. V0 = 245kbps, V2 = 190kbps (lowest acceptable on Orpheus)
Explain ABR
- ABR involves setting a target bitrate at the start of conversion. LAME will then convert the audio into mp3 which has a varying bitrate throughout the material, but the cumulative average will always equal the target
- Not the best method of conversion, as the variability of quality will vary depending on the nature of the source
Explain transcoding
Transcoding is the action of converting an audio format into another audio format
Bad transcodes involve working against the optimal conversion methods, and do not make sense. These involve:
- Lossy –> Lossless
- Same br lossy –> Same br lossy
- High br lossy –> Low br lossy
- Transcodes can be detected via spectral imaging and are undesirable
How do you navigate Orpheus?
BitTorrent
Explain how BitTorrent works, including seeding, leeching, and upload credit
- BitTorrent allows users to share data with each other through .torrent files - these files are not the target files specifically, instead they refer to the target files and the source material is downloaded through different peers (users who also have the .torrent in their client) each giving small parts of data automatically
- When you are downloading a torrent, you are leeching - once you have downloaded and continue to stream data through the .torrent, you are seeding – meaning you are giving chunks of the torrent data to other peers who are downloading the same file
- You receive upload credit for having people leech off you, so it is beneficial to leave torrents seeding. This is a fair system where everyone works together to share files
Explain the ratio system on Orpehus
- Each user has a ratio figure. This is a weighting of how much they have uploaded compared to downloaded. A ratio of 1 implies they give as much as they take, therefore it is healthy.
- Naturally, it can be hard to maintain a ratio like this therefore each user has a required ratio to satisfy in order to keep downloading. This figure is flexible and achievable as long as you don’t take the piss
- The required ratio is calculated by taking into account how much data you have downloaded, as well as how many torrents you are seeding
Explain how the required ratio system works depending on the amount of data a user has downloaded
- Users have a required ratio of 0 for the first 5GB of downloads, if they do not seed anything subsequently. If they continue to seed each torrent for more than 72h per week, then they can continue to have this ratio for up to 20GB of downloads.
- It is generally good practice to seed everything as this helps maintain a good ratio, but it also reduces your required ratio up to a certain point (100GB+)
What happens if you dip below the required ratio?
Ratio watch –> 2 week period where you need to improve the ratio to above the standard. If this is not achieved then leeching privileges will be removed.
You can pull it out the bag in ratio watch by uploading new torrents, trumping files, filling requests, and seeding
If you download more than 10GB in ratio watch, you will be shut off from leeching. If you do not adhere to the upload quality rules, you will also be shut off
Explain freeleech and neutral leech torrents
- Freeleech: the data downloaded from these torrents is not counted in your ratio calculations, but your seeding data is. This is a good way to improve your ratio / maintain a good ratio
- Neutral leech: neither data downloaded or data uploaded is counted in ratio calculations - basically a void torrent
What is port forwarding?
- Port forwarding is the process of allowing users on public internet to connect directly to your computer
- This involves specifying a certain port and telling your modem to allow users to connect to your computer specifically
Why is port forwarding preferable?
Port forwarding ensures you can connect to the entirety of the Orpheus network, and ensures that leechers can actually leech data that you are seeding.
What are DHT’s and what are they useful for? Why do they need to be disabled for Orpheus?
DHT’s (Distributed Hash Tables) are a system for finding peers to connect to that do not have a centralised network - meaning it is easier to find them for downloads.
They are not allowed for Orpheus, as having them enabled for your computer allows non-users to connect and download torrents.
