Re: [LAD] 802.11n sound card

On 12/24/2009 11:31 AM, Arnold Krille wrote:
> Hi,
>
> On Wednesday 23 December 2009 23:52:46 Patrick Shirkey wrote:
>
>> On 12/23/2009 08:08 PM, Arnold Krille wrote:
>>
>>> There is no way to make it work with guaranteed low latency. You need
>>> somewhat low latencies to play the keyboard to other music. And you need
>>> it guaranteed unless you always record xruns and gaps as part of your
>>> musical experience...
>>> And even then its wireless and using a frequency-range used by almost all
>>> other people around you. Just count the wireless networks in your flat
>>> provided you live in a flat with other people living
>>> upstairs/downstairs/next to you. And then try to get a fast,
>>> non-interrupted stream (latency about 20ms or shorter for real playing).
>>>
>> So the issue is with other streams being picked up by the receiver which
>> affects latency by increasing collisions?
>> Would this still be a problem on a secured connection? Surely the
>> receiver would ignore all data that is not being transmitted over the
>> secured access point?
>>
> No, the problem is one or two layers deeper in the stack. We are talking wifi
> here. No matter if there is only two devices on that network or a secure
> connection, its still wireless transmission over radio frequencies in the 2GHz
> range. Which is per se much more affected by any disturbance then a dedicated
> cable is. Every mobile phone, every blue-tooth device, every neighbours
> network, every iron in your ceiling will influence this. And not only with a
> constant background-noise in your frequency range, but also with momentary
> scrambling and such stuff. So in the layers you can not (easily) control by
> software there is already lots of resending and rescheduling of packets. And
> all these introduce uncertainties and latencies you don't want in your audio
> transmission. Unless you can do with 100ms latency and more...
>
>

This is good analysis. Thanks for taking the time.

Is that number 100ms a real number or just an estimate?

Also in terms of audio quality it sounds like an issue similar to the
difference between vinyl and mp3 yet we all know which format is winning
the sales battle.

>> Does anyone have an idea of how to work out the actual latency for a
>> wifi packet at 300Mb/s?
>>
> The data-rate is not important. What is important is the struggle you do to
> get a payload packet out and the reception acknowledged. Wifi is very versatile
> and allows usage (almost) everywhere. But you pay a high price in
> predictability and latency.
>
> Heck, even the wireless microphones struggle with problems and these "just"
> send some analogue data. While digital is simpler in that its only zero or
> one, its actually lots more complex on getting data out reliably.
>
>

While in Korea I was on a cdma network and the voice quality and signal
quality/coverage was a factor of 10 times better than my experience with
gsm networks in other countries.

>> If we are talking about a studio setup with devices no further than 2
>> meters distance from each other that should aloow sub 20ms send/receive.
>> The bottle neck appears to be the chip and the code that manages the
>> transmit/receive process.
>>
> No, the bottle-neck is your neighbours accessing google and facebook via wifi
> disturbing your signal momentarily producing gaps and xruns. And the only way
> around it is using an area where there are no neighbours.
>
> Once the stream is stable and reliable, it will easily go below 20ms latency.
>

This part is what I was expecting.

> But getting the stream to be reliable is already complicated with things like
> firewire (which has isochronous channels for exactly this purpose) and its more
> complicated with usb (which has a master telling the clients when to send
> bigger payloads to not disturb the other bus-members). Ask the jack-over-udp
> guys how difficult it is to create such stream reliably over tcp-ip network. And
> they take lost packets into account (which means an xrun) and advise you to
> use it on dedicated networks if you want more the the immediate experience...
>
>

In this case are we comparing a network located over large distances
with internet nodes in between or a couple of computers sitting within a
couple of meters of each other?

> If jacknet over wifi fills your needs, use it. But if it doesn't don't blame it
> on its developers, blame it on the underlying network (-technology) used.
>
>

Absolutely.

Patrick Shirkey
Boost Hardware Ltd

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Received on Thu Dec 24 12:15:03 2009