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fix SMPTE ST12-1 bits interpretation #937

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fix SMPTE ST12-1 bits interpretation #937

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2005bf2
only document the time address bits of SMPTE ST12
robUx4 Dec 22, 2024
0dc65dc
mention we don't keep the 16-bit ST12 synchronization word
robUx4 Dec 22, 2024
5b51f0e
break down the ST12-1 timecode example interpretation
robUx4 Dec 22, 2024
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59 changes: 41 additions & 18 deletions block_additional_mappings/smpte-st12-1-timecode.md
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Expand Up @@ -6,38 +6,61 @@ If the Block uses Lacing, the timecode value is associated with the first frame

The `Block Additional Mapping` contains a full binary representation of a 64-bit SMPTE timecode
value stored in big-endian format and expressed exactly as defined in Section 8 and 9
of SMPTE 12M [@!SMPTE.ST12-1]. For convenience, here are the bit assignments for a
SMPTE ST 12-1 binary representation as described in [@?RFC5484, section 6.2]:
of SMPTE 12M [@!SMPTE.ST12-1], without the 16-bit synchronization word.
For convenience, here are the time address bit assignments as described in [@?RFC5484, section 6.2]:

| Bit Positions | Label |
|:-------------:|:-----------------------|
| 0--3 | Units of frames |
| 4--7 | First binary group |
| 8--9 | Tens of frames |
| 10 | Drop frame flag |
| 11 | Color frame flag |
| 12--15 | Second binary group |
| 16--19 | Units of seconds |
| 20--23 | Third binary group |
| 24--26 | Tens of seconds |
| 27 | Polarity correction |
| 28--31 | Fourth binary group |
| 32--35 | Units of minutes |
| 36--39 | Fifth binary group |
| 40--42 | Tens of minutes |
| 43 | Binary group flag BGF0 |
| 44--47 | Sixth binary group |
| 48--51 | Units of hours |
| 52--55 | Seventh binary group |
| 56--57 | Tens of hours |
| 58 | Binary group flag BGF1 |
| 59 | Binary group flag BGF2 |
| 60--63 | Eighth binary group |
Table: SMPTE ST 12-1 Bits Assignment{#ST12Bits}
Table: SMPTE ST 12-1 Time Address Bit Positions{#ST12Bits}

For example, a timecode value of "07:32:54;18" can be expressed as a 64-bit SMPTE 12M value as:
For example, a timecode value of "07:12:26;18" can be expressed as a 64-bit SMPTE 12M value as:

```
10000000 01100000 01100000 01010000
00100000 00110000 01110000 00000000
```

Or with the irrelevant bits marked with an "x" which gives 26 usable bits:

```
1000xxxx 01xxxxxx 0110xxxx 010xxxxx
0010xxxx 001xxxxx 0111xxxx 00xxxxxx
```

This is interpreted in hexadecimal:

- 0x8 units of frames
- 0x1 tens of frames
- 0x6 units of seconds
- 0x2 tens of seconds
- 0x2 units of minutes
- 0x1 tens of minutes
- 0x7 units of hours
- 0x0 tens of hours

Given no value is above 9, the BCD coding correspond to the actual values:

- 8 units of frames
- 1 tens of frames
- 6 units of seconds
- 2 tens of seconds
- 2 units of minutes
- 1 tens of minutes
- 7 units of hours
- 0 tens of hours

Or:

- 18 frames
- 26 seconds
- 12 minutes
- 07 hours