Calculating TV Time

John works in the TV industry, where timing is done to a resolution finer than a second. Television video must take into account hours, minutes, seconds, and frames. (There are thirty frames per second.) John was wondering if there was a way to handle frames in Excel.

There is no way to handle frames as part of the native time values in Excel. There are, however, a couple of things you can do to work with frames. Perhaps the most obvious suggestion is to keep hours, minutes and seconds as a regular time value, and then put frames in a separate cell. The immediate drawback to this approach is that calculations for the "TV times" are not as easy as they would be if they were represented in a single value.

A way around this is to try to do your own calculations in a macro. Excel goes through an internal process of converting times to decimal values that can be worked with very easily. You could simulate this same conversion process, converting a time value (including frames) to a decimal value. The TV time, in the format 00:29:10:10, could be stored in a cell (where Excel will treat it as a string) and then converted to a value by the macro.

There is a problem here, of course: You cannot convert the time to a true decimal value between 0 and 1 like Excel does for times. The reason has to do with the limits on Excel's significant digits. To arrive at a value, you would divide the hours by 24, the minutes by 1440 (24 * 60), the seconds by 86400 (24 * 60 * 60) and the frames by 2592000 (24 * 60 * 60 * 30). When you start getting into values that small, it exceeds Excel's limits of maintaining everything to fifteen significant digits. Thus, you end up with unavoidable rounding errors on the frames value.

One solution to this problem is to not try to work with decimal values between 0 and 1, but instead work with integers. If you convert the string time into an integer value that represents the number of total frames in the time, then you can easily do math on the resulting value. The following macro will do the conversion of a string in the format already mentioned:

Function Time2Num(Raw) As Long
    Dim FirstColon As Integer
    Dim SecondColon As Integer
    Dim ThirdColon As Integer
    Dim NumHours As Integer
    Dim NumMinutes As Integer
    Dim NumSeconds As Integer
    Dim NumFrames As Integer
    Dim T2D As Long

    Application.Volatile

    FirstColon = InStr(Raw, ":")
    SecondColon = InStr(FirstColon + 1, Raw, ":")
    ThirdColon = InStr(SecondColon + 1, Raw, ":")

    NumHours = Val(Mid(Raw, 1, FirstColon - 1))
    NumMinutes = Val(Mid(Raw, FirstColon + 1, SecondColon - 1))
    NumSeconds = Val(Mid(Raw, SecondColon + 1, ThirdColon - 1))
    NumFrames = Val(Mid(Raw, ThirdColon + 1, Len(Raw)))

    T2D = CLng(NumHours)
    T2D = T2D * 60 + NumMinutes
    T2D = T2D * 60 + NumSeconds
    T2D = T2D * 30 + NumFrames

    Time2Num = T2D
End Function

To see how this works, if you have a string such as 37:15:42:06 in cell A4, and you use the formula =Time2Num(A4), the result is the value 4024266, which is the number of frames in 37 hours, 15 minutes, 42 second, and 6 frames. To convert such values back to an understandable time, you can use the following function:

Function Num2Time(Raw) As String
    Dim NumHours As Integer
    Dim NumMinutes As Integer
    Dim NumSeconds As Integer
    Dim NumFrames As Integer
    Dim RemainingTime As Long

    Application.Volatile

    NumHours = Raw \ (CLng(30 * 60) * 60)
    RemainingTime = Raw Mod (CLng(30 * 60) * 60)

    NumMinutes = RemainingTime \ (60 * 30)
    RemainingTime = RemainingTime Mod (60 * 30)

    NumSeconds = RemainingTime \ 30
    RemainingTime = RemainingTime Mod 30

    NumFrames = RemainingTime

    Num2Time = Format(NumHours, "00") & ":" & _
      Format(NumMinutes, "00") & ":" & _
      Format(NumSeconds, "00") & ":" & _
      Format(NumFrames, "00")
End Function

By combining the two functions, you can do some math with the times. For instance, suppose you had the time 00:29:10:10 in cell A4 and the time 00:16:12:23 in cell A5. If you put the following formula in a cell, you can find out the difference between the two times:

=Num2Time(Time2Num(A4)-Time2Num(A5))

The result is 00:12:57:17.

The examples presented here are rudimentary; they don't take into account any error handling or limit checking on the times used. You can either expand on the examples to fit your needs, or you can look to a third-party source. For instance, you can find an explanation (with a sample workbook) for NTSC and PAL times at the following URL:

http://www.kenstone.net/fcp_homepage/timecode_spreadsheet.html