Re: [LAD] AVB applications

On Fri, 04 Mar 2011 23:15:30 -0600
Duncan Gray <duncang@email-addr-hidden> wrote:

> This is to bring a discussion from the Jack Dev list to this more
> appropriate forum as suggested by Arnold Krille.
>
> First, I hear lots of people seemingly thinking that AVB (IEEE 1722) and
> the IEEE 1588 version of Precision Timing Protocol can be done in the
> kernel.
>
> It cannot and it must not. They both need hardware assist. Period. A
> timestamp is specified to be inserted based on the leading edge of the
> header immediately after the preamble. If anyone ever makes some
> neighboring equipment that has done these with the required precision,
> then you will kill that clock network and there will be yet another good
> reason not to bring Linux to the workplace.
> The ONLY exception is that if the listener is a stream-to-disk system,
> then the timestamp system can simply be ignored. Such a listener will
> never turn on PTP, but that won't hurt, because it will just ask for the
> 1722 stream and the talker will spit it out without knowing that that
> node doesn't play PTP.
>
> The version of PTP that is used in AVB is from the 802.1AS
> specification. The acronym PTP is now an ambiguous one that has at
> least these two uses, and I have heard some other hardware-assisted
> networked timing schemes called PTP.
>
> IEEE 1588 specifies an epoch-based struct with 48 bits of seconds (This
> gives 8.9 million years before a "y2k" hits IEEE 1588) and a 32-bit
> number that specifies nano-seconds. the 802.1AS sub-spec also uses this
> epoch-based timestamp.
>
> PTP maintains one suite of transactions to keep itself timed. This is
> blind to AVB.
>
> AVB creates Word Clock timeframes using the PTP wall-clock that MUST be
> made available to the 1722 layer. IF YOU HAVE PTP, then you can
> synthesize predictive wallclocks using a buffer-full scheme in a
> PTP-capable NIC. That NIC has to be configured to pay out the frames
> per the 802.1Qav forwarding and scheduling spec. This is how streamers
> will deliver streams that are well-timed, low-jitter streams. There are
> fruit companies doing this as we speak with new NICs that have been
> enabled from Broadcom and Marvell (and any host of others.)
>
> It is possible to fake a GrandMaster clock using kernel-timed
> calculations. The Best Master Clock Algorithm (BMCA) of a two-node
> system will be forced to accept such a sloppy clock and the slave will
> achieve lock, but with jitter that will fail a normally specified PTP
> system. Noisy environment listeners will not hear this, but clean
> listening will reveal the various artifacts of such jitter.
>
> You can just make a leaky-bucket PLL at a receiver and use the DPLL
> frequency to inform SRC. This hack will be un-noticed by the average
> media-player person, but not by the critical listener.
>
> When the 1722 timestamp is constructed, it is a complex assembly from
> the 802.1AS timestamp. The 802.1AS timestamp is a two-part thing, as
> specified above with its first part being simply seconds. This will not
> roll over in the lifetime of Linux, our species or even our continents,
> let alone a recording session. The second part is specified to roll
> over at decimal one billion-1 = 999999999 = 0x 3B9ACBFF. The timestamp
> in IEEE 1722 rolls over at unsigned long = 4294967295 = 0xFFFFFFFF,
> which is 4.294967296 seconds. I apologize for quoting "weeks between
> rollover" in the previous thread.
>
> IEEE 1588 and IEEE 1722 are Ethernet-Only protocols, do not shoe-horn
> them into IP.
>
> I have heard lots of people say that AVB is just some thing for consumers.
> go to http://grouper.ieee.org/groups/1722/ and hover over some of the
> names to find where they work. It was, in fact, designed FIRST to very
> easily accommodate Pro Audio:
>
> Reliability.
> Multiple-Node Synchronization without the need for Sample Rate Conversion.
> Low-Jitter.
> Unlimited (at least not limited by the protocol, only the bandwidth)
> channel count.
>
>
> And then it would be a trivial subset to get two - or 5.1, 7.1 any
> surround count - channels to go from my CD player to any media player
> over some LAN. (However, as of two autumns ago, they were still
> kvetching over Wi-Fi.)
>
> Finally, Yes, the CLOCK_REALTIME can be very simply pasted from a good
> PTP instance.

Thanks for this. Very informative, although *way* above my abilities :(

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
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