Prepare for the Interview
The What.CD membership interview will test your knowledge of What.CD's acceptable transcoding policies, BitTorrent, various audio formats and codecs, the Golden Rules of What.CD etc.. Already feeling overwhelmed? Don't worry! This guide will provide you with all the information needed in order to pass the interview.
Let's get started!
Lossless Compression
Lossless compression is a compression methodology in which the result of the compression can be faithfully restored, i.e. bit-by-bit identical with the uncompressed data. Similar to ZIP or RAR, lossless compression for audio is especially tuned and designed for the characteristics of waveform data, thus achieving compression far greater than that of generic compression utilities.
As lossless compression preserves all information of the original waveform file, audio compressed with lossless compression is unavoidably larger than audio compressed with lossy compression. However, this disadvantage is more than offset by lossless compression's ability to be transcoded to other lossless formats without quality degradation.
Compressed Lossless formats always have variable bitrates.
The only lossless format allowed on What.CD is FLAC:
However you should be familiar with other lossless formats such as ALAC, WAV (PCM) and APE.
Lossy Compression
Lossy compression is a compression methodology that significantly reduces audio file size by discarding information imperceptible to humans. The amount of audio information discarded is dependent upon the target bitrate selected at the time of encoding.
While lossy compression is perfect for audio devices that have limited file storage, it should be noted that the waveform reconstructed from lossy compressed information will never match the original waveform.
Lossy formats can have average bitrates, constant bitrates, and variable bitrates.
The only Lossy Formats on What.CD are:
Container Formats
Although WAV and AIFF are formats not allowed on What.CD, it is important to be aware of these formats. As "container formats", WAV and AIFF can hold uncompressed and compressed audio data, however for interview purposes, these formats will most likely be used as examples of uncompressed lossless audio data (PCM).
Bitrates
A bitrate is the data rate (i.e. how many bits get transferred in a certain amount of time) and is usually expressed in bits per second. The three types of bitrates used when encoding audio are average bitrate (ABR), constant bitrate (CBR), variable bitrate (VBR).
For example, the minimum constant bitrate (CBR) allowed on What.CD is 192 kbps. This means that the file contains an average of 192 kilobits for each second of audio (~1.4 MB per minute). This contrasts with CD audio, which is encoded as 1411.2 kilobits for each second of audio.
LAME
Before we look at the various types of transcodes, let's take a look at the encoder we will be using to transcode between lossless and lossy formats.
LAME is the What.CD recommended encoder. Developed by the open-source community since 1998, LAME has become the highest quality MP3 encoder for several reasons:
- Highly optimized settings
- Fast encoding
- ABR, CBR, and quality-optimized VBR encoding methods
- Gapless playback with LAME-header compliant decoders
- Supported by Exact Audio Copy
- Highly tunable
For information on recommended encoder settings, go here.
Transcodes
Transcoding means converting a file from one encoding method (i.e. file format) to another. Transcoding can be performed from lossless to lossless, lossless to lossy, lossy to lossy, and lossy to lossless.
The only transcodes allowed on What.CD are lossless to lossless and lossless to lossy. Using spectral analysis, we will take a closer look at three of the types of transcodes.
Lossless to Lossless
When performing a lossless to lossless transcode, the quality of the audio file will not decrease. Thus, you may transcode from one lossless format to another as often as you like (e.g. to take advantage of better compression or better error-correction or better hardware support).
Take for example, this spectral analysis of a FLAC file (click image to enlarge).
http://whatinterviewprep.webs.com/flacvv2.png
Notice how the range of frequencies extends beyond 20 kHz (the end of most human's audible range of frequencies). When transcoding between lossless formats, all of the audio information will be retained and in the case of lossless compression, the file size will be reduced.
Lossless to Lossy
Keeping lossless archives gives you the opportunity to transcode music in the future to other lossy formats as encoder technology improves. However, it may be convenient to transcode to lossy formats for use with portable audio players (iPod, Zune, etc.) to maximize storage capacity while sacrificing some audio quality.
When the above FLAC file is transcoded to a MP3 320 kbps (CBR) using the LAME encoder, one can notice a cut-off around 20 kHz. Furthermore, the MP3 320 kbps (CBR) has a smaller file size, while the quality lost will be imperceptible to most humans.
http://whatinterviewprep.webs.com/320lamelg3.png
Of course, even better means of compression exist. Let's take a look at two more MP3s encoded using the V0 and V2 LAME presets.
A file encoded using the V0 preset will have a average bitrate of ~245 kbps, however since V0 uses a variable bitrate, the bitrate will fluctuate throughout the song depending on the complexity of the waveform.
http://whatinterviewprep.webs.com/v0...extremels8.png
A file encoded using the V2 preset will have a average bitrate of ~190 kbps, however since V2 uses a variable bitrate, the bitrate will fluctuate throughout the song depending on the complexity of the waveform.
http://whatinterviewprep.webs.com/v2...tandardxj0.png
Lossy to Lossy
Every time you encode with a lossy encoder, the quality will decrease. There's no way to gain quality back even if you transcode your MP3 128 kbps into a MP3 320kbps. The sound quality of the result will always be worse than the (lossy) source file along with having a larger file size. Even transcoding from a 320kbps CBR MP3 to a 192kbps CBR MP3 will result in worse quality than if you just transcoded directly from a lossless format to 192kbps CBR MP3 in the first place. Transcoding between lossy formats is not allowed on What.CD.
Lossy to Lossless
You already know what happens when you transcode a lower quality bitrate file to a higher quality file bitrate; the result is a file with a larger file size and no added quality. The same is true of a lossy to lossless transcode. When one downloads a lossless file, he/she expects a bit-by-bit replication of the original source. Therefore, the uploading of lossy to lossless transcodes is not allowed on What.CD.
BitTorrent
While the quality of music files is of the utmost importance, the quality of our swarms is equally important. Familiarize yourself with the following BitTorrent-related topics:
BitTorrent (General Information)
http://www.dessent.net/btfaq/
Distributed Hash Table (DHT)
Distributed hash tables (DHTs) are a class of decentralized distributed systems that provide a lookup service similar to a hash table: (name, value) pairs are stored in the DHT, and any participating node can efficiently retrieve the value associated with a given name. If you have enabled DHT, you could be uploading to people who don't even have access to the site, so you won't be receiving the proper credit for your uploads & downloads. You will be sharing data out of the tracker and those ghost peers will be using your bandwidth to download from you.
Make sure it is disabled in your torrent client.
Port Forwarding (Becoming Connectable)
Port forwarding, sometimes referred to as port mapping, is the act of forwarding a network port from one network node to another. This technique can allow an external user to reach a port on a private IP address (inside a LAN) from the outside via a NAT-enabled router. Port forwarding allows remote computers (e.g. public machines on the Internet) to connect to a specific computer within a private LAN.
If your ports are not properly forwarded, please visit http://www.portforward.com.
What.CD's Golden Rules
- Do not defy the expressed wishes of the staff.
- Access to this website is a privilege, not a right, and it can be taken away from you for any reason.
- Staff decisions are final. Arguing about our decisions is hazardous for your account.
- One account per person per lifetime. Anyone creating additional accounts will be banned.
- Avatars must be safe for work, non-pornographic or semi-pornographic, and not even slightly offensive. Use common sense.
- Do not post our .torrent files on other sites. Every .torrent file has your personal passkey embedded in it. The tracker will automatically disable your account if you share your torrent files with others. You will not get your account back. This doesn't prohibit you sharing the content on other sites, just the .torrent file.
- This is a torrent site which promotes sharing amongst the community. If you are not willing to give back to the community what you take from it, this site is not for you. In other words, we expect you to have an acceptable share ratio. If you download a torrent, please, seed the copy you have until there are sufficient people seeding as well before you stop.
- Do not browse the site using proxies or TOR. The site will automatically alert us.
- Trading, selling or offering invites in public is strictly prohibited, and will get you permanently banned.
- Trading, selling, sharing, or giving away your account is prohibited as well. PM a mod to disable you if you no longer want it.
- You're completely responsible for the people you invite. If your invitees are caught cheating or trading/selling invites, not only will they be banned, so will you. Be careful who you invite. Invites are a precious commodity.
- Attempting to find or exploit a bug in the site code is the worst possible offense you can commit. We have automatic systems in place for monitoring these activities, and committing them will result in the banning of you, your inviter, and your inviter's entire invite tree.
- We're a community. Working together is what makes this place what it is. There are well over a thousand new torrents uploaded every day and sadly the staff aren't psychic. If you come across something which violates a rule, report it! Ignoring it is only damaging the community.
- We respect the wishes of other sites here, as we wish for them to do the same. Please refrain from posting links or full names of sites that want not to be mentioned.
Note: These are only some of What.CD's rules. Following a successful interview, it is extremely important for you to be aware of What.CD's rules concerning uploading, requests, etc. Members can find What.CD's rules here.
Ratio Watch
Amount Downloaded
Minimum Ratio 0% Seeded
Minimum Ratio 100% Seeded
Extra Info
0-5 GB
0.00
0.00
Initial grace period for all users
5-10 GB
0.15
0.00
N/A to members less than 2 weeks old
10-20 GB
0.20
0.00
Ratio watch starts for members < 2 weeks old
20-30 GB
0.30
0.05
30-40 GB
0.40
0.10
40-50 GB
0.50
0.20
50-60 GB
0.60
0.30
60-80 GB
0.60
0.40
80-100 GB
0.60
0.50
100+ GB
0.60
0.60
If your ratio does not meet the standards required for the amount you've downloaded, the percentage of torrents you're seeding and the length of time you've been on the site, you will be put on ratio watch. You will have two weeks to get your ratio to the required level for the amount you've downloaded - failure to do so will result in your account being automatically disabled.
If you download over 10GB while you're on ratio watch, you will be instantly disabled.
Everyone gets to download their first 5GB before ratio watch begins. If you've been a member for less than 2 weeks, ratio watch does not begin until the 10GB barrier.
What.CD keeps track of your Actual Ratio, and your "Required Ratio". The more of your snatched torrents you have seeded for at least 72 hours in the past week, the lower your Required Ratio will be. To stay out of ratio watch, you can either improve your Actual Ratio by uploading, or reduce your Required Ratio by seeding more of your downloaded torrents.
If you pass one of the barriers while on ratio watch, you are immediately subject to the new requirements. For example, if you've been a member of the site for two weeks and you've downloaded 9.5 gigs with a ratio of 0.10, and you're not seeding anything, then you go ahead and download another 0.6 gigs while on your first day of ratio watch, you will have 13 days to get your ratio up to your new target of 0.20.
You are only allowed to go on ratio watch twice. The third time your ratio dips below the required level, your account is instantly disabled.
The ratio watch system is completely automatic, and cannot be altered by staff, however if you have been disabled because of ratio watch, you are welcome to come to talk to a staff member on IRC and ask for a second chance.
Client Whitelist
Client rules are how we maintain the integrity of our swarms. This allows us to filter out disruptive and dishonest clients that may hurt the performance of either the tracker or individual peers.
Allowed Client Additional Notes btgdaemon 0.9.x
btgdaemon 1.0.x
btpd 0.13
btpd 0.15
Deluge 1.2.1 / libtorrent (Rasterbar) 0.14.9
Deluge 1.3.x
Enhanced CTorrent (dnh3.2)
KTorrent 2.1.x
KTorrent 2.2.x
KTorrent 3.0.x
KTorrent 3.1.x
KTorrent 3.2.x
KTorrent 3.3.x
KTorrent 3.5.x
libtorrent (Rakshasa) 0.10.4 (rtorrent)
libtorrent (Rakshasa) 0.11.x (rtorrent)
libtorrent (Rakshasa) 0.12.x (rtorrent)
libtorrent (Rakshasa) 0.14.6.0 (rtorrent)
Transmission 1.5.4 For OSX 10.4 Transmission 1.6.x
Transmission 1.7.x Excluding version 1.7.4 Transmission 1.8.x
Transmission 1.9.2
uTorrent 1.6.1 Last release before BitTorrent Inc. began influencing the codebase. uTorrent 1.7.6
uTorrent 1.7.7
uTorrent 1.8.x
uTorrent 2.0.x
uTorrent 2.1.x
uTorrent Mac 0.9.x
The modification of clients to bypass our client requirements (spoofing) is explicitly forbidden. People caught doing this will be instantly and permanently banned. This is your only warning.
The use of clients or proxies which have been modified to report incorrect stats to the tracker (cheating) is not allowed, and will result in a permanent ban. Additionally, your information will be passed on to representatives of other trackers, where you are liable to be banned as well.
The testing of unstable clients by developers is not allowed unless approved by a staff member.
