Synced Audio Stream on multiple HD-200 extenders

[Tikker]

Originally Posted by MrFusion

Assuming all the HD200s are the same hardware revision and firmware (or Sage implement a strict scheduling algorithm), everything should be more-or-less ok. Whilst there might, maybe, be some slight drift between units, I wouldn't expect this to be as significant as, say, the simple delay experienced when moving between two perfectly synced sound sources which are some distance apart...

you'd be surprised at how much the drift is

we get IPTV here delivered via multicast

identical set top boxes and indentical tv's have a fair bit of drift, simply due to joining the stream at slightly different times, and processings, luck, etc

[SilentBob]

Originally Posted by Fuzzy

that still wouldnt' get it synced up.

Really? I currently only have one extender, so I have no experience with this, but if you tune to the same OTA tv channel on multiple extenders is it not synced? If it is synced, then why would the same not be true using the Boxee "tuner"?

[Fuzzy]

correct, it will not be synced. Basically, each extender only really cares about it's own playback. It will play from the recorded file, at the speed it determines. This speed, in theory, should be the same, but it will drift over time. Also, there is no guaranteed way to ensure they start playback at teh same time either. If boxee truly does synced playback, it is because it actually has some sort of synching mechanism built in... sage has no such feature.

[MrFusion]

Originally Posted by Tikker

you'd be surprised at how much the drift is

we get IPTV here delivered via multicast

identical set top boxes and indentical tv's have a fair bit of drift, simply due to joining the stream at slightly different times, and processings, luck, etc

It'd be interesting to find out why this is happening. My guess is that, since the STBs and TVs aren't designed to remain in synch, they don't enforce the right discipline to do so.

Multicast on a LAN should ensure that all devices receive the same packet at "essentially" (ie only a few milliseconds difference - which you won't hear) the same time.

A simple scheduling algorithm (ie buffer x thousands of packets & keep the queue at exactly that depth +/- a couple of packets) coupled with a static offset (ie +/- x milliseconds) to account for different processing chain delays (ie caused by amp / receiver / tv) should be just about spot on.

The audio stream would essentially act as a synchronising clock, and device drift (ie caused by slightly different crystal speeds, sunspots, whatever) would never be a real issue -- yes, you might get a bit wobbly every now and then if one device misses a handful of packets, but that would reconverge within seconds. On a clean LAN you'd probably never notice...

[Protoman]

With some creativity and some planning one could have synced A/V across multiple tvs. The solution is a HDMI Matrix switch. I have one I use at home and it has the ability to take 4 inputs and output any of them to 2 outputs to tvs. I don't have it configured that way, but it could be an easy change.

I have an older version of one of these. http://www.monoprice.com/products/pr...seq=1&format=2

I can have Output (TV) A and B both use Input 1 or I could have Output (TV) A use Input 1 and Output B use Input 2. For those that are confused basically any Output (TV) can use any Input including the same Input the other Output (TV) is currently using.

For this example of syncing audio and video across multiple TVs you would have Output (TV) A and B use the same Input (HD-200).

The reason it would take a little planning is that you could have 2 HD-200s connected to this HDMI Matrix switch and using IR repeaters could select which Input (HD200) each Output (TV) can use at that moment. Instead of having an HD-200 at each TV and running Cat5 to each TV, you could have the HD-200s centrally located and run HDMI to each TV. Personally I would run both HDMI and Cat5 but thats just me.

Would this solution work? It sure can. Is it an ideal solution? Probably not, but if you want synced A/V across multiple TVs now, then this would be a possible way to do it.


Hope this has been helpful and informative,
Protoman

[Fuzzy]

Sync'd playback is not technically out of possibility with sage, it just isn't something that is currently implemented. It wouldn't take much to have the server broadcast a timestamp to all clients, and have the clients skip to that timestamp. It's just a matter that most have not really needed this feature.

[gplasky]

Originally Posted by MrFusion

Multicast on a LAN should ensure that all devices receive the same packet at "essentially" (ie only a few milliseconds difference - which you won't hear) the same time.

Multicast will just broadcast to multiple network devices. There is no check or guarantee that anything is kept in sync. And there is no retry for any dropped packets. Even if it is all kept on the same network sync could vary anywhere from milliseconds to multiple seconds.

If you are truly looking for synchronized broadcast of audio you would need to synchronize the audio decoder clock at each receiving station with the source station. You would also need to figure out how to synchronize the audio and video data streams received at each receiving station if you were looking at whole home audio/video.

Gerry

[SilentBob]

IMO there are 2 perceptions of "synced" in this thread.

The purist version is that you can play the stream to 2 extenders that are sitting side by side and the 2 shows will play *perfectly* in sync. It *might* be possible to get this to work, but I'm personally skeptical. At my previous house, I was running comcast to multiple TVs...when watching the same channel I noticed that the audio on my 2 tvs in separate rooms would often be slightly out of sync....perhaps this is all attributable to the different tuners.

Now the other version of "synced", the version *I* am envisioning, is one where the same show is playing on multiple TVs in the house at "pretty much" the same spot.

To address the more lenient interpretation of syncing I think multicast would be a wonderful solution. Here are a couple of personal scenarios where multicast would be perfect:

1) moving between multiple rooms. Often when I'm working on something I may be going back and forth between two different locations. It is nice to be able to tune both TVs to the same feed and let them run. That way as I move back and forth, the same show is running in both locations.

2) Watching tv with another person in separate rooms. My wife and I may start watching the same show in the living room, but then she goes into the kitchen to make dinner, or I go in my office to do some work. We still want to continue watching the same show.

This is what I was attempting to accomplish by using a separate device running Boxee with a PVR150 attached to it....My intent was to simply tune any extender to that particular tuner and then I could watch the same thing on multiple TVs.

If implementing multi-cast is a possibility, that would be even more ideal. I would love to be able to join multiple extenders to 1 multicast "group". Any of those extenders should be able to control (pause, FF, RW, pick another show, etc).

I would think that by using multicast it should be more efficient for broadcast networks like MoCa and wireless that might have more of a problem playing multiple HD streams. Want to watch the same BluRay movie in 4 different rooms? I doubt wireless or MoCa can handle 4 simultaneous streams, but 1 multicast stream should be possible.

[paulbeers]

Protoman is right. To truly guarantee perfect sync playback you need to use a matrix of some sort. Heck even my tv's with QAM tuners do not have perfect sync playback simply because they both have their own amount of delay in the broadcast. For my whole house audio I am using a matrix in my basement that allows me to use multiple cheap amplifiers in order to keep perfect sync (or you can use an expensive multi-zonal amplifier). Even if you use multicast, you will run into minor "skips" periodically as the units try to stay in sync.

[MrFusion]

Originally Posted by SilentBob

Now the other version of "synced", the version *I* am envisioning, is one where the same show is playing on multiple TVs in the house at "pretty much" the same spot.

Spot on. Exactly my point in previous post. The idea is not to have two HD200s / amps / speakers next to each other (and be listening to them), the idea is to have multiple units around the house.

Once you're in that situation, "good enough" is good enough. That is, depending on distance between units, and your relative position, you will generally notice more impact due to physics than network. Sound travelling through air is much slower than multicast packets!

Say you had two sets of speakers 20 metres apart. Worst-case delay is when you are standing at one set of speakers (sound needs to travel 20m to reach you from the other set). Best case is when you are smack in the middle (equidistant, 0 difference in delay between the sound sources).

Speed of sound is approximately 343 metres per second (varies based on air pressure, humidity, and frequency, but close enough).

This means, worst case scenario (ignoring the greater delays caused by multipath issues), you will experience a 58ms delay ("out of sync") when you're standing at one set of speakers, dropping linearly to 0 when you are at the equidistant point.

A reasonable quality wired 100Mb/s LAN, in a reasonably simple home network, will introduce no more than 10ms delay between endpoints.

In other words, the effect of just walking around will be much much greater than anything introduced by the network.

Originally Posted by gplasky

Multicast will just broadcast to multiple network devices.

That's the point. Broadcast. On a LAN a given packet will appear at all appropriate devices at more-or-less the same time (ie only a handful of milliseconds apart at worst). There is no retry, resend, or other mechanism. This is multicast, not TCP, not unicast. Note: I am assuming a wired LAN which is reasonably clean, in a reasonably simple 'home use' configuration.

There is no check or guarantee that anything is kept in sync

I disagree, the discipline I discussed above will cause the very packets themselves to act as the synchronisation mechanism.

And there is no retry for any dropped packets. Even if it is all kept on the same network sync could vary anywhere from milliseconds to multiple seconds.

I disagree.

The odd packet loss will never be noticed, and drops of large numbers of packets is extremely rare (again, assuming wired LAN of reasonable quality).

By implementing the discipline I mentioned above, any small packet loss will hardly ever be noticed, and will reconverge faster than you'd notice. Consider, 44kHz per channel, 16 bit samples = 176,000 bytes per second. Add a handful for control, packet overhead, etc. Call it 180,000 bytes per second. Given this is your very own LAN & not the interwebs, you don't need a super efficient payload ratio, and it's multicast so it's inherently efficient anyway, you could happily peg it at -say- 500 packets per second (360 bytes per packet) which wouldn't even make things run warm (max packet rate at 100Mbps being about 148,800 pps -- see ftp://ftp.cs.princeton.edu/techreports/2002/645.pdf). Each lost packet represents a 2 millisecond skip, which you wouldn't ever notice.

Even losing 10 or 20 packets wouldn't be much of an issue -- the slightest of 'ticks' that most people would never notice.

And, on a clean LAN, this level of packet loss is pretty well unprecedented.

Couple the above with the aforementioned discipline (ie keep buffer at a given number of packets plus allowance for jitter which you average out), any drift would quickly disappear.