Scan Line BPM

Scan Line BPM refers to the speed of the scan line moving up and down the screen. The standard Scan Line BPM is set to 120, where the scan line touches the bottom of the screen 1 second after reaching the top of the screen.

In Cytus

In Cytus, unlike the BPM of the song, the Scan Line BPM of a song is unchanged throughout the whole song. However, the Scan Line BPM of the same song could be different in Easy and Hard difficulties.

Here is a list of relationships between the Scan Line BPM and the actual BPM of the song:

Scan Line BPM = Actual BPM
This is the most common relationship. These songs have a Scan Line BPM equal to its actual BPM. Examples are Future World and L. In total, 145 songs have this relation.
Scan Line BPM = 1/2 of Actual BPM
Some songs have a slower scan line BPM. It is often quite tricky to get a high TP on these songs as packed notes are commonly found in them. Examples are Area184 and Megaera. Some songs have their scan line using half of the BPM on Easy while the Hard difficulty remains with its original BPM. Examples are D R G and Let's Go On An Adventure. 8 songs have this relation on both difficulties, with 24 songs having this relation on only one difficulty.
Scan Line BPM = 2x of Actual BPM
There are currently only 5 songs with a scan line BPM equal to two times the actual BPM. Note that all of them either have a speedy scan line, or a very slow original BPM.
Scan Line BPM = 2/3 of Actual BPM
This relationship is only (and commonly) found in songs with a triple beat (3/4 or 3/8). Examples are Brionac and Vivere La Vita.
Scan Line BPM = 4/5 of Actual BPM
Currently there are only two songs that has its Scan Line BPM equal to 4/5 of its Actual BPM, which are Fight With Your Devil and Recollections. It is due to the song's quintuple beat (5/4).
Scan Line BPM having no relationship with the Actual BPM
There could be two reasons leading to this situation: The song has a constantly changing BPM. Since the Scan Line BPM is always kept unchanged, it is impossible for the Scan Line BPM to follow the actual BPM. An example is Morpho. Gate of Expectancy is partially a special case, though the Scan line BPM doesn’t change, but the pulsing gradients on the sides of the screen do. The scan line is set fixed as a constant unrelated to the actual BPM. Examples are Sacred and The Riddle Story. Songs with this relation fit with the total 24 songs that have uneven or unrelated scan BPMs.

In Cytus II

With the release of Cytus II and the introduction of the Chaos difficulty, the scan line can now speed up and slow down. Right before the speed changes, the scan line will turn red (if it speeds up) or green (if it slows down) and the words "Speed Up" or "Speed Down" will appear at the bottom of the screen.

Some charts speed up or slow down without warning. Two examples are CHAOS and Pure Powerstomper.

Here is a list of possible Scan Line BPM relationships.

Scan Line BPM does not change
13 songs do not change their scan line's BPM at all, making them more akin to original Cytus charts. Examples include Gravity and Alterna Pt.1 -Cosmogony-.
Scan Line BPM slows down to 1/2x of Actual BPM
This is by far the most common change that will happen, as it appears in 109 songs. This is usually used during the calmer sections of the music (however that does not mean the section is always easier, like in CHAOS) Other examples include IOLITE-SUNSTONE and Nostalgia Sonatina. Some especially fast songs like Turnstile Jumper will have its fastest scan line speed at 1/2 the actual BPM.
Scan Line BPM slows down to 1/4x of Actual BPM
In 21 songs, the scan line can further slow down to 1/4 of the song's BPM. Except for Return, all of the songs that do this also have the scan line at 1/2 the original BPM. This usually results in notes being extremely close together, making it especially hard to be accurate. Examples include Fireflies (Funk Fiction remix) and Blow my mind.  hunted is a special case, as its scan line slows down to 1/8 times the actual BPM, giving it the slowest scan line speed of a regular song at 26.5 BPM. This is done in the middle of a long pause in the song.
Scan Line BPM speeds up to 2x of Actual BPM
With this relationship, the scan line is going double the original tempo and is usually used to ramp up the intensity of a song. The rhythms here are very spread apart as notes are hard to aim with the scan line being so fast. This relationship is found in 29 songs, most notably Keep it up, Devilic Sphere, Luolimasi and EMber.
Scan Line BPM speeds up to 4x of Actual BPM
Of these songs with 2x BPM scan lines, 9 songs feature a scan line with quadruple BPM: Chrome VOX, Grimoire of Crimson, Baptism of Fire (CliqTrack remix), Celestial Sounds (KIVΛ Remix), Brain Power, Phantom Razor, Praystation (HiTECH NINJA Remix), Perspectives, and Bastard of Hardcore. In almost all of these cases, the player is in the middle of hold notes and the speed is simply for visual effect. This is because playing any normal rhythms at these speeds would require near inhuman levels of aim and reaction time. Brain Power is a special case, as its scan line speeds up to 8 times the actual BPM, giving it the fastest scan line speed of 1384 BPM. Not only does this make the scan line extremely difficult to follow, but it also makes the notes right after hard to react to as appearance speed scales with BPM.
Scan Line BPM changes speed with some other relationship to the Actual BPM
There are several reasons for this type of relationship: The song switches to 3/4 time, so the scan line adjusts to 2/3 or 4/3 the original BPM. Examples: Light of Buenos Aires and Still . The song switches from 3/4 time to 4/4 time. This means the scan line adjusts to 3/4 of the main scan line BPM. Example: Sairai The song switches to 5/4 time, so the scan line adjusts to 2/5 or 4/5 the original BPM. Examples: Gekkouka and 風の声. The song switches to 7/8 time, so the scan line adjusts to 4/7 or 8/7 the original BPM. Examples: Fighting and FUJIN Rumble. The song switches to another odd time signature, like 11/4 (Take me to the Future, which switches to 4/11 the main BPM) or 10/4 (Alexandrite, switcing to 1/5 the original BPM). The song's BPM changes constantly, so the scan line adjusts accordingly to the new BPM. Examples: Whisper in my Head and Pure Powerstomper. 小悪魔×3の大脫走！？ is a special case as the scan line doesn’t adjust its BPM at all when the time signature changes. Ramen is God's GLITCH difficulty also changes the scan line BPM to 8/5 and 4/3 the original BPM but the time does not change. The scan line stops completely, before resuming from the same position. Examples: V. and II. They are also the only case where the scan line speed changes on Easy or Hard. Usually, this is done with as little change as possible. Note: Ivy's Hard charts can actually change BPM despite being 1/2 of the new BPM. In total, 16 songs currently change to an odd BPM.
Teleporting Scan Lines
Glitch charts have teleporting scan lines. These instantly displace scan lines, but does not actually change the BPM. Unlike BPM changes, there is no indication that a scan line will teleport except the incoming notes. The most frequent type of teleporting scan lines is the one way scan line. At the end of the scan line, it teleports back to the other side instead of rebounding. This is often used as a way to add challenge without actually changing the position of the scan line. One way scan lines often happen in parts of the song that repeat themselves. Examples include Gravity and Chrome VOX. Alternatively, it can be used simply for visual effect in a part of the song with no notes. Example: Deadly Slot Game. Another example of teleporting scan lines is combining it with a stop, where the scan line alternates between the top and the bottom. An example of this is XODUS (on Chaos difficulty) V. is unique for combining the teleportation with a stop when the scan line is at the middle. It teleports to the end of a hold note, but stops there until the note actually ends, serving only to disorient players. Notably, this is the only time a teleporting scan line teleports somewhere in the middle of the screen. This, along with XODUS are the only instances where the teleporting scan line is not a one-way scan line and the only occurrences of teleporting scanlines in CHAOS instead of GLITCH difficulty. Chocolate Missile has audio glitching before the first chorus, and so the scanline difficulty "glitches" as well on GLITCH 13.

For a complete list of the Scan Line BPM of all songs in all difficulties, see Songs by BPM.