I think i understand the bottom number on a time signature now (Update)

Question

I know bottom number will tell me what note gets the beat but my question was always "Why does it matter what notes get's the beat if it don't change anything in practicality?" but know i figured out the difference will be that the music will be quicker, a video demonstrating weird time signatures in dream theater songs shows that the higher the bottom number is the fastes you counted too, because when its for on the bottom you'll count one beat and one its eighth note you will count half the duration of the beat so will be quicker, this is the video:

, i just want to know if i am right,thank you guys, sorry for possible english mistakes.

Answer 1

So in the question "What's the difference between 4/4 and 2/2," the answer is "How the conductor would move their arms." That's partly a joke (obviously irrelevant for music that doesn't have a conductor), but it gets at the heart of the matter. 4/4 has four "beats"; you feel (and maybe communicate, in the way you play) four significant pulses. If you take a measure of four quarter notes and give it the time signature 2/2, then you feel and communicate only 2. The notes might still pass by at the same "speed"—say, your 4/4 would be 120 bpm per quarter note, and your 2/2 would be 60 per half note: either way, the four quarter notes are the same duration, but the "feel" of the music would be slower and broader for the 2/2.

In other words, in these two examples, the notes would pass by at the same rate, but one would "feel" and communicate four pulses, and the other would feel two larger pulses:

X: 1
K:C
L:1/4
M:4/4
Q:1/4 = 120
A A A A

X: 1
K:C
L:1/4
M:2/2
Q:1/2 = 60
A A A A

Now, you might ask "So why not call it 2/4, with 60bpm per quarter note, and write a measure of four eighth notes?"

X: 1
K:C
L:1/8
M:2/4
Q:1/4 = 60
AA AA

Theoretically, this would sound and feel the same as a 2/2 measure of quarter notes. But if you also wanted other notes that were twice as short, or four times as short, then you're left writing a lot of 16th or even 32nd notes. Sometimes composers might choose a certain time signature to make the music easier to read. Sometimes they might even use it to "trick" the performer into psychologically communicating a faster or slower tempo: if you do see a lot of 16th notes, you might think "Gee, this is fast," but if you notate the same thing as 2/2 and see half or quarter notes you might communicate more relaxation.

EDIT: I wrote all this without watching the video. I'll add: the video contains a measure of 7/8 and also of 7/4. Yes, in this context the eighth notes are equal to each other so you "count slower" in the 7/4 measure. This can't be assumed when you compare one piece to another, though; there's no reason that one piece in 5/8 has any faster or slower of a beat than one in 5/4. The video is something of an extreme example, stringing together "unusual" time signatures. What you might encounter more often would be, for instance, 4/4 and 7/8 juxtaposed, where you might think of the 7/8 as "a bar of 4/4 but with one eighth note missing."

X: 1
K:C
L:1/8
M:4/4
Q:1/4 = 120
AA AA AA AA |\
M:7/8
AA AA AA A |\
M:4/4
AA AA AA AA |\

In this case, the change in time signature doesn't communicate anything about the pulse necessarily; it's just that there's no other time signature for a bar with seven eight notes in it, even if you mean to have four "beats" with one of them shorter than the other three.