From what I understand of time signatures, the top number indicates the number of beats, except for compound or odd beats. Like 4/4 has 4 crotchet beats, while 6/8 has two groups of 3 quaver beats, so two beats in total(?). Now, it seems to me that listening to upbeats and downbeats is the only way tell the time signature of a song by ear. From what I can tell, in 4/4 time, it goes one & two & three & four &, and so on, so the beginning of every other beat group would get a downbeat. But in 6/8 time it seems to go one two three one two three, where the beginning of each beat group gets a downbeat. So does the beginning of each beat get one downbeat, or does every other beat get a downbeat? Or is it inconsistent?
As a general rule, 'down beat' refers to the first and strongest beat in the bar. In a very simple bar of 4/4 the first beat (down beat) is the strongest, the third beat would be strong too but not as strong as the first beat, and then beats 2 and 4 are more like off beats (a bit lighter than the other two). In a 6/8 bar, beats 1 and 4 are the strong beats, therefore 2,3,5 and 6 being lighter. However, remember that it's all quite subtle. The down beats shouldn't be over emphasized or the piece would sound clumpy.
Pulse is more to do with the speed of a piece. But it can also refer to the toe-tapping emphasis you feel when listening to a rhythmic piece.
But in 6/8 time it seems to go one two three one two three, where the beginning of each beat group gets a downbeat. So does the beginning of each beat get one downbeat, or does every other beat get a downbeat? Or is it inconsistent?
This will depend on the tempo:
In popular use, beat can refer to a variety of related concepts, including pulse, tempo, meter, specific rhythms, and groove.
6/8 can be felt (heard, conducted and played) quite differently:
In a very slow tempo as a Largo each beat can be felt as a downbeat. A guitarist might play each beat as an arpeggiated downbeat 1 2 3 4 5 6
Adagio: A conductor could vary his figures accentuating 1 2 3 4 5 6
Referring to rhythm and style a minuet, English Waltz can be played 1 3 4 6 (playing crotchets downbeat, quavers upbeat).
The same pattern will fit in a medium tempo like a shuffle rhythm or swing.
In a faster tempo (steady beat, March) the conductor will give just 1 4 like a 2/4 time with triplets. The strumming pattern for the guitar could be the same as 2/4 (2 downbeats 1 2 respectively 1 4 in 6/8 time, where the second beat will be played less strong or tapped (stopped).
If pieces had no time signature, players wouldn't have much of a clue how they were to be played, without laboriously going through and analysing first.
So, more often than not, a time signature is written at the beginning. It can be an exact feature, or can give a rough guide to the mapping of the rhythm of a piece.
Theory puts these time signatures into several different types, 4/4 being the most used.
4/4 is based on 4 beats in each bar, so there will be an underlying feel of a count of 1 2 3 4. Here, as in most mainstream music, the first beat of each bar is deemed to be more emphasised than any other. That's the main pulse. Some music in 4/4 doesn't even play on that beat 1, but it's felt (by most) nevertheless.
Often in 4/4 there is another 'sub-pulse' found on beat 3 - hemce 1 2 3 4.
6/8 is compound time. It's that because there are two distinct feels in it. It can be counted 1 2 3 4 5 6 using the quavers, or, 1 2 splitting it into larger parts. In slower pieces, the former works better. In faster 6/8s, it's easier to count/feel the latter.
It's not an exact science - it's not even a science at all - and all we try to do is make it clearer for people to understand. (And 6/8 seems to be one that is often misunderstood!).
At the end of the day, it's simpler to compartmentalise music into smaller sections so it's easier to read or even listen to - often yu may tap your foot or clap in time, and where that happens gives a clue as to how we split music into bars. They are simply put there by humans to simplify it. Sometimes it doesn't appear to have worked!
I'm sure at least some of this has already been covered in previous q/a.
Certain time signatures don't allow you to guess where the pulse is, and require you to figure it out from the music. 5/4 5/8, 7/4, 7/8 all have variable pulse locations which can potentially change from measure to measure--but hopefully not often because that can be confusing.
Sometimes the composer will mention how the pulse should be felt but otherwise you have to look at how the general flow of the music is written to determine it.