ICMP - X with Dolby CP950A - silence when switching from 5.1 to Atmos CPL

[Michal Matys]

Hi everyone,

I'd like to ask if anyone has ever experienced this: when switching from a 5.1/7.1 DCP to an Atmos DCP — in this case a regular ADV -> Dolby Unfold — there are a few seconds of silence before the Unfold audio comes up (about 3 sec). It happens randomly, like the Atmos renderer takes a while to come up, but there are no errors.

When I was on site testing, I made an SPL with a 10 sec 5.1 ADV -> Unfold -> ADV -> Unfold and it didn't occur once in 5 minutes of repeating this — but I already had two different sites report the issue to me.

Dolby recommended switching from using the Dolby Atmos Input on the CP950A for the immersive sound to using the command input, so I made that config change, but today the cinema reported exactly the same issue.

We have a couple more locations with this setup that have never reported it — but since it happens randomly, they might have just not noticed.

Has anyone experienced something like this?

Thanks for any ideas!

[Leo Enticknap]

Have you changed the server compatibility mode to mode 1?
image.png

You're supposed to do this when the server is a Barco ICMP (default = Dolby and everything else; 1 = Barco; 2 = Sony; 3 = Qube). Most of the Atmos installs I do use IMS3000s or SR-1000s (also default) and very rarely an ICMP. As a result, I always forget this step and only remember it when Atmos audio doesn't play reliably.

[Michal Matys]

Have you changed the server compatibility mode to mode 1?
image.png

You're supposed to do this when the server is a Barco ICMP (default = Dolby and everything else; 1 = Barco; 2 = Sony; 3 = Qube). Most of the Atmos installs I do use IMS3000s or SR-1000s (also default) and very rarely an ICMP. As a result, I always forget this step and only remember it when Atmos audio doesn't play reliably.

Yes I have it in mode 1, otherwise te Atmos is not working at all I believe, there is no issue with playing the Atmos features, just this weird few seconds silence that occures during the switch from 5.1 to Atmos, randomly. When the atmos starts playing, then everything is okay in terms of the 950 showing Dolby Atmos content in green.

[Leo Enticknap]

Oh bugger; sorry it wasn't the easy fix.

When I've forgotten to put it into mode 1, the results can vary. The most common one I've experienced is it simply refusing to play the IAB audio at all, but occasionally it will start, but drop out to regular AES mid stream.

Is the Ethernet cable between the ICMP-X and the input to the Atmos card on the back of the CP950A a straight shot, or does the connection go through any switches? Your description reads like some sort of latency problem in an initial handshake, which makes me wonder if processing of some description is taking place in a switch, that is delaying things.

[Marco Giustini]

Are you sure the delay is on the sound processor end and not on what's after it? How is the sound passed to the amplifier after the CP950?
What happens on the GUI when the playlist switches from 5.1/7.1 to Atmos? Did you have a look at logs to see if anything pops out?

[Michal Matys]

@Leo - the two sites that reported the issue has different config - one is directly from ICMP-X port 2 to Dolby Atmos Input using separated network with direct connection. Other one is using the command network input, going through switch so this seems not to be related.

@Marco The sound is passed partially through the analog outs to analog amps, partially through digital to DMAs. Logs did not shwo any clear issue. When it occured, the cinema told me they saw that the dolby atmos content green light went ON after like the 3 seconds of silence but as I mentioned, its hard to simulate this for me onsite as it didnt happen when I was doing the tests myself.

We have found out that the RTC is off quite a lot on the 950s, so we will try to adjust that and see - this is what Dolby recommends, if that does not help, they will dig deeper.

[Marco Giustini]

If Atmos locks with a delay, we can talk about network and settings. But While Atmos is not active, PCM should be played back. Any Atmos track has a 5.1 or 7.1 backup track which is automatically played until Atmos kicks in.
And if you're using some of the analogue outputs, it does look like the 950 is the issue here. Adjusting the RTC is never a bad idea but it shouldn't affect unencrypted Atmos content.

I don't even know if it's possible: what happens if you play Atmos content and unplug the Atmos network cable? Does it play in 5.1/7.1?

[Caleb Williams]

I don't even know if it's possible: what happens if you play Atmos content and unplug the Atmos network cable? Does it play in 5.1/7.1?

The one time I tested this for fun on the reversion was seamless to 5.1 PCM

What version of the CP950 firmware are you on? Did a fresh hard reboot have any effect on this issue? We've ran into some weird things on CP950's with long uptimes, even with stable firmware.

I'm also curious if you're switching macros on the CP950 between non/Atmos content? Back when we first started using the CP950 there was a nasty *snat* issue that would come up if you switched the active macro in the middle of a clip while there was audio. Now we have 1 second black transitions with any macro changes on them and just haven't changed since they fixed that issue with a firmware update.

[Marcel Birgelen]

We had similar issues with the "classic" ICMP and our CP850. We're now playing essentially all our Atmos content via IMS3000 via Q-Sys, which is actually pretty stable. But it took years for our ICMP and the CP850 to "like" each other in regards to Atmos. Still, switching from standard 5.1 or 7.1 PCM to Atmos would always take a few seconds. That's why I had a silent DCP with a silent Atmos soundtrack of about 5 seconds, only showing a logo, but no sound, which I put in front of every Atmos show.

[Marco Giustini]

Marcel - late Atmos switching I can understand but does it play silence during that time? I remember the ICMP took some time to like Atmos and the 850 but I don't remember a similar issue (though it was quite some time ago).

[Marcel Birgelen]

Yes, it will be silent, as the format change has already been done, but something in the entire Rube Goldberg machine that is Atmos, is most likely hitting a timeout somewhere. It's not really consistent. It improved over the years, but this startup delay never went completely away. With the IMS3000, the delay is sufficiently small as to confidently switch over to the "feature content" directly.

Switching from 5.1/7.1 PCM audio to Atmos is far more complex than one might realize. While the first one is "nothing more" than a timed, multi-channel PCM bitstream, Atmos is a complex dance between the players involved...

First off all, your server has to talk to the audio processor and initiate the format switch. Then the processor starts pulling the relevant IAB content in the DCP from the server via custom HTTP protocol. The KDM needs to be loaded into the secure media block and encrypted content needs to flow through it. Meanwhile, the decrypted content needs to be interpret, rendered and timed exactly to the video output using PTP timestamps...

[Harold Hallikainen]

Does Atmos use PTP to sync the audio? I thought it used FSK sync ( https://pub.smpte.org/doc/st430-12/20230912-pub/ ) on AES channel 14. It is a bit like DTS with a timecode that the audio player syncs to. With rendering within the IMB/IMS, all that fetch and sync stuff is eliminated since the server knows where the content is and when to play it.

[Marcel Birgelen]

Does Atmos use PTP to sync the audio? I thought it used FSK sync ( https://pub.smpte.org/doc/st430-12/20230912-pub/ ) on AES channel 14. It is a bit like DTS with a timecode that the audio player syncs to. With rendering within the IMB/IMS, all that fetch and sync stuff is eliminated since the server knows where the content is and when to play it.

You're right for the IMB/IMS to processor part. I was confused with Atmos Connect, which is AES67 done wrong, but that's already the rendered form.

[Marco Giustini]

Marcel,

i understand the complexity when activating Atmos but getting rid of that silence shouldn’t be a complex task? Switching from 5.1/7.1 to a working Atmos should be inaudible if done properly without clicks and gaps. I’m not a developer but I’m not sure why the ‘few seconds of silence’ should be an option.

on a much simpler situation, the old DTS players used to wait until the queue of sound was depleted and then drop to the backup format on the CP, causing an audible gap in the sound.
the XD10 fixes that, the backup format is called before sound actually stops playing from the player. (Then there is a small gap caused by the format switch but that’s a different story)

I’m digressing

[Marcel Birgelen]

I do develop software from time to time... I'm not sure if I call myself a software developer though. But I'm not involved with either the ICMP nor Dolby's software, so, it's not me to blame.

The thing is, there is no graceful cutover here, like the processor waiting with switching from PCM audio to Atmos audio until the Atmos engine is "idling". I guess that's where the primary design flaw is. But given the complexities involved, the different systems that need to handshake, communicate, allocate and fill buffers, sync up with the master clock, etc., I CAN understand why it takes a second or two to get from zero to "full Atmos". But even with a "smooth transitioning mechanism", cutting over HARD from PCM audio to Atmos in-action will almost certainly also introduce some noticeable artifacts. I guess the best thing you can do is simply wait and play nothing until the engine is fully initialized or do some kind of soft-crossover when switching between sound formats, to minimize the impact.

We were one of the first users of the original ICMP, I remember it on day one as being so buggy, just playing any random junk left in its frame or audio buffer even when just switching DCPs in the playlist, which had even a slightly different format than the previous one. I remember me putting in a pause and mute/unmute, dowser close, dowser open in between every playlist item.