Re: [payload] Addressing comments on 2nd WGLC of draft-ietf-payload-rtp-ancillary-08

On the C bit, I'm not disagreeing with signalling of the data channel where appropriate, just improving the wording.  In particular "if set to '0' the ANC data  corresponds to the luma (Y) data channel" is not correct because '0' would also correspond to case where there are no luma/color difference data channels or there is no association with SDI (and perhaps no requirement as to which data channel to use, even if there were an association with SDI).

On data stream number, the numbering will be different from the interface standards because they have link number and data stream number but we are combining them into a single data stream number.  I think some (normative) explanation is needed.

John
________________________________________
From: payload [payload-bounces@ietf.org] on behalf of Thomas Edwards [Thomas.Edwards@fox.com]
Sent: 06 May 2017 00:01
To: payload@ietf.org
Subject: Re: [payload] Addressing comments on 2nd WGLC of draft-ietf-payload-rtp-ancillary-08

>On 5/5/17, 10:38 AM, "John Fletcher" <John.Fletcher@bbc.co.uk> wrote:

> Regarding the C bit, I think something like my originally suggested wording is better:

> If the data corresponds to the color-difference data channel of an associated SDI interface, the C flag shall be set to "1".  If the data corresponds to the luma data channel of an associated SDI
>interface, or the interface does not have separate luma and color-difference data channels, or the data is not associated with an SDI interface, the C flag shall be set to "0".

> Reasons being (a) you should mention that luma/color-difference channels are things in an SDI interface; (b) your suggested wording says  that if it is set to "0" the ANC data  corresponds to the >luma (Y) data channel, but that is not necessarily true, it might be because there are no luma/color-difference channels or because  there is no associated SDI interface.

I think we’ve discussed this before (see: https://mailarchive.ietf.org/arch/msg/payload/cd-_KMT18N7x4qJx--3BgBYbWp0).  There are ANC data packet standards that require the use of particular data channels.  Even if the ANC data packet is not sourced from an SDI interface, it is best if it is marked in the proper data channel so that downstream devices re-embedding in SDI can do their job properly.  And even if there is no SDI period, I don’t think it hurts much to mark the ANC data packets in the data channel required by their defining standard.

>Regarding link number, you probably need to explain that if there are 2 links with 4 data channels each, you number the data channels across all links so that link 2, data channel 1 is the 5th data > channel etc.

The link a particular stream uses would be specified by the interface standard.  I can add that as a note.

>You might also want to allow another bit so you could go up to 24 Gbit/s interfaces.

I’m OK with throwing one more bit in to be extensible.  However, to be clear, there are no current SMPTE SDI standards or projects that I am aware of that need more than 32 streams.  I also hope that if SMPTE does standardize a 24 Gbps interface that they do it in a less arcane fashion than having 64 streams.

 -Thomas

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