How to calculate complex file offset

[Middlemarch]

I am trying to calculate the start of audio (SOA) position in an mp3 file.
I've had some luck working it out, but need some maths help.


The first header contains 10 bytes as per
Identifier "ID3"
Version $04 00
flags %abcd0000
Size 4 * $0xxxxxxx


Those last 4 bytes should point to the SOA.


e.g 49 44 33 03 00 00 00 00 08 00


Although I don't understand it, the SOA is calculated by

Dim ID3 As String * 10
Dim C As Long
    Dim d As Long
    C = 2 ^ 14
    d = 2 ^ 7
    If Left(ID3, 3) = "ID3" Then
       Taglen = Mid(ID3, 7)
       SOA = Asc(Mid(Taglen, 2, 1)) * C Or Asc(Mid(Taglen, 3, 1)) * d Or Asc(Mid(Taglen, 4, 1))
End if

This returns 1024 from the last 3 bytes of the size, it is wrong and should be 2048
It may well be if the all 4 values were included.
Can anyone tell me how to factor in all 4 values in the above ?


If I'm on the right track, 05 7F 7E 73 should = 12582781.

[June7]

Where did you find that formula?

[Middlemarch]

Gawd knows! But I remember it was so good. Without it there was no way to get the SOA.
But it was wrong, although it worked with mp3's with a low SOA
And I have now sorted it. More fluke that good management
e = 2 ^ 21
C = 2 ^ 14
d = 2 ^ 7
If Left(ID3, 3) = "ID3" Then
Taglen = Mid(ID3, 7)
SOA = Asc(Mid(Taglen, 1)) * e Or Asc(Mid(Taglen, 2)) * C Or Asc(Mid(Taglen, 3)) * d Or Asc(Mid(Taglen, 4))


I do wish my maths was better. be good to do the reverse, find the 3 values for the number

[June7]

Could you provide sample data and complete procedure?