You would never have a time signature with 7 on the bottom.
Time signatures: a quick recap
A time signature is simply an instruction to divide the bar into the beat (the number on the bottom) and to fill the bar with so many beats (the number on the top).
N
M | where N is any number
where M is any of the numbers in the set {1,2,4,8,16,32,64,128,256} |
We usually only see the numbers 2, 3, 4, 5, 6, 9, 12 on the top of most pop music time signatures.
We usually see only the numbers 2, 4 or 8 on the bottom of most pop music time signatures.
To further complicate matters we have:
- Simple time signatures where the beat is divided into groups of 2 and each of these divisions are further divided into groups of two; and
- Compound time signatures where the beat is divided into groups of 3 and each of these divisions are further divided into groups of two
Common Time Signatures
| Simple | Compound | | | |
|---|
| Duple | 2
2 | 2
4 | 2
8 | 6
4 | 6
8 |
|---|
| Triple | 3
2 | 3
4 | 3
8 | 9
4 | 9
8 |
|---|
| Quadruple | 4
2 | 4
4 | 4
8 | 12
4 | 12
8 |
|---|
We can have irregular time signatures such as 5/8 or 7/4, but the beat is generally divided into groups of 2 and so fall into the simple time signature category. Such irregular time signatures are often internally grouped in groups of 2 or 3 for counting purposes.
We can even have compounded irregular time signatures such as (1-1/3)/2 (i.e. 2 triplet half notes (minims)) - the top half of the time signature is a fraction used to indicate how much of the beat should be used within a bar. Most daws do not bend themselves to this concept....
However, some daws allow numbers like 3 or 6 or 12 on the bottom of their time signatures is that the sw was deigned by people who do not understand how time signatures are supposed to work and so created something to match their incomplete or non-existent knowledge.
These type of pseudo-time signatures represent a bar of triplet half-notes (minims) or quarters (crotchets) or 8ths (quavers); a bar of triplet 16ths (semi-quavers) in this framework would have a time signature with 24 on the bottom.
All of these obtuse interpretations would be better represented by the compound time signatures given above, shifting the number from the bottom to the top and replacing the bottom number with either 2 or 4 or 8 or 16.
The meat of your questions
Now to your main question: what time signatures are used in advertising, music for television series and music for films?
Simple answer: any time signature that takes the composers fancy.
Even bigger answer: there is no need to remain in any one time signature long - shifting meter is a very easy way to create interesting, unique music.
and your last question "so 4/4, your only allowed 4 notes per bar?"
No, you are only allowed 4 quarter notes (crotchets) per bar, these 4 notes can be divided into pairs or triplets or any other permitted divisions
Go read this extended tutorial on durations, rhythm and time signatures:
Bandcoach - Time: Sounds, Silences, Signatures and Rhythm
