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Ancillary Time Code (ATC) according to SMPTE 12M-2 |
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Ancillary Time Code (ATC) according to SMPTE 12M-2
Overview
The SMPTE standard SMPTE 12M-2-2008 defines the “Transmission of Time Code in the Ancillary Data Space” and replaces the RP 188. In the past there were different methods of implementations regarding ATC. This note describes the implementation of ATC according to SMPTE 12M-2 and what are the changes regarding Alpermann+Velte time code modules.
The following specifications are concerned with ancillary time code:
| Standard | DID | DBN/SDID | DC | Time Code Type | Remark |
| RP 196 | 64h 64h |
64h 7Fh |
8h 9h |
HANC LTC HANC VITC |
No longer listed! |
| SMPTE 291M | F5h | 00h | 8h | LTC | Specification unknown! |
| RP 188/SMPTE 12M-2 | 60h | 60h | 10h | ATC | RP 188 led to different im-plementations! |
It is strongly recommended to implement ATC according to SMPTE 12M-2!
Introduction
There are four transport methods of time code known (regarding only “12M” time codes, so not dealing with MPEG, film, digital audio, midi etc.):
| LTC | Transported as a serial (analogue audio) signal independent of any video signal. |
| VITC | Inserted in an active line within the VBI of an analogue video signal. |
| D-VITC | Similar to the VITC inserted in an active line within the VBI of a digital video signal. |
| ATC | Ancillary data packets carried in the ancillary data space of a digital data stream. |
Remarks regarding the D-VITC: There is no specification of D-VITC for HD digital video! ATC can be used for SD digital video as well.
For HD digital video ATC should be used.
ATC can be of type ATC_LTC and ATC_VITC.
| The ancillary data spaces of a digital television data stream are H-ANC in the horizon-tal blanking area and V-ANC in the vertical blanking interval.
H-ANC: All lines after EAV but before SAV. V-ANC: In the vertical blanking region after SAV but before EAV. |
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Ancillary data packets should not be located in the lines identified as a switch point – defined by SMPTE RP 168.
Format of Ancillary Time Code Packets
SMPTE 291M defines the format for a type 2 ANC packet.
16 User Data Words consist of time code data and the type of time code, i.e. LTC or VITC or fur-ther user defined time codes.
For HD digital video the ANC packets shall be carried in the Y stream.
ATC according to SMPTE 12M-2 is applicable to 8-, 10-, or 12-bit television systems.
Recommendations of SMPTE 12M-2
| Location for ATC in HD video: | ATC_VITC in H-ANC, line 9, ATC_LTC in H-ANC, line 10. |
| Location for ATC in SD video: | in V-ANC as early as possible from line 9 of 625/50/I and line 13 of 525/59.94/I. |
Counting frames up to 50 or 60 is not provided, so for progressive systems running at greater than 30 frames-per-second each time code count references a frame pair. The first frame of the frame pair receives the new time address which then will be repeated at the second frame. With LTC it is not possible to distinguish between the frames of a pair, but the VITC marks the individual frames using the field flag. Evaluating this flag enables a time code reader or inserter to identify the first or second frame.
| Interlaced: |  |
| Progr. 25/30 frames: |  |
| Progr. 50/60 frames: |  |
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Something New and Special
ATC_VITC:
In detail the ATC_VITC consists of two types, VITC1 and VITC2. Apart from this type definition the payload of both types is identical. Implementations of VITC1 and VITC2 should be used to distin-guish between the fields of interlaced formats and the frames of a frame pair of progressive for-mats, together with the field mark flag. Now only ATC_VITC will be presented, and the internal encoder or decoder handles the VITC1 and VITC2 type.
Older versions of the RUBIDIUM modules DT/DV and HT/HV as well as our PC boards PCIHD, PCIeD and PCIeHD have treated VITC1 and VITC2 as separate time codes, this now has changed:
| DT/DV and HT/HV | from version 2.0.168, November 2009, |
| PCIHD/PCIeD/PCIeHD | from version 7.19, October 2009. |
After an update please take care using only the latest configuration tools and drivers.
ATC_LTC:
The SMPTE 12M-2 gives not a clear recommendation about the transmission rate of the ATC_LTC: „... at least once a frame ...“. In interlaced video formats the transmission rate may be once a frame or every field. We decided to offer the once-a-frame method as a default, inserting the ATC_LTC in the first field. The user may select the every-field method by activating the „All Fields“ checkbox. This concerns our RUBIDIUM modules DT/DV and HT/HV.
ATC_LTC -> ATC_VITC:
The HT and HV modules are our time code converters in HD digital video systems. If only ATC_LTC is present in a progressive video system with frame rate greater than 30 frames per second or in interlaced systems, these converter modules are able to convert the ATC_LTC to an ATC_VITC. This method will keep the time addresses and add the advantage of the field flags, thus making the individual frames/fields distinguishable.
Others:
You can download the latest versions from: Software
Please don’t hesitate to contact Alpermann+Velte if you have any ATC problems. We would like to help analyzing this problem.
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