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Atlantic
2011-11-07, 12:05
Our WiFi system became overloaded while playing flac files to two sync'd players - SB Receivers, in this case. I'd added to the problem (I think) by recording the flac files, from analogue sources, using a 24 bit depth.

The topic has been much visited in recent weeks/months, in the context of looking for solutions, but the threads where it's been raised have produced some interesting 'numbers'. It's the numbers I'd like to think about (the solutions have been well-aired).

The 24 bit flac files (44k sample rate) are showing bit rates which vary between 1.2Mbps to 1.4Mbps. These seem high compared with, for example, a posting in the (excellent) 'big party' thread

http://forums.slimdevices.com/showpost.php?p=657983&postcount=15

suggesting that 16 bit 44k PCM is 1.4Mbps (presumably that's stereo, is it?). It seemed to me that flac isn't compressing PCM by much. (Well, maybe saving 8 bits per 24 bits isn't too bad, really?)

Our flac files at 1.4 Mbps are taking 2 x 1.4 Mbps (ie, 2.8 Mbps) for transmission to two sync'd players and, in our case where we use a repeated signal because of line of 'sight' issues, actually 2 x 2.8 over the WiFi channel, roughly 5.6 Mbps. Adding 10% or so for IP overhead such as framing, error checking, acknowledgment, whatever, we're over 6 Mbps. Even using Wireless-G, this seems too demanding for our installation.

There is one question I'd like to ask. I used Audacity to capture these analogue sources and save them to flac. Audacity has 'options' under its flac filesave menu, where you can select the 'quality' (best to worst), which confuses me a little. Since flac is lossless, I assume there isn't a 'good' or 'bad' in a fidelity or perfection sense; I assume the 'best' or 'worst' is a measure of the aggression of the space saving (with an implication for the ultimate decoder, the SBR in our case). Does anyone know if that is the case? Does anyone suffer a fidelity loss, after all, with these Audacity settings?

And one query about SBS helping us out. We're getting some work done to install a wired network - it'll take a little time, though. Meanwhile, I looked at SBS settings to se if it would temporarily transcode flac down to, say, MP3, but MP3 is greyed out in settings. Can anyone think of any way to transcode flac in SBS so that a lossy format is transmitted to our SBRs, for the time being?

regards, Atlantic

Mnyb
2011-11-07, 12:27
Mp3 greyd out is lame installed ? What kind of server , bitrate limiting may be an option if you can make that work ? If the server has the cpu to run lame .

The settings in flac is compression ratios and does not impare sq , but could make it harder for the player to decode -5 is ok i think for hirez content -8 for CD quality .

i think you actually get rather big files if converting some analog sources to 24 bit SN for vinyl for example is not great the majority of the signal will be random noise and random noise does not compress well, the file will not be that much smaller . 24 bit gives you 144dB Some vintage sources can be noisy so most of the signal content is noise. even using a perfect source at least the 3 - 4 of the bottom bits will be noise there are not much practical circutry that has more SN than 20 bits aproximately .

You can run some good resampling and make 16/44.1 files a good samplerate converter with dither ? I'm a bit clueless on audacitys samplerate converting.

pski
2011-11-07, 12:32
You would have to be using the wifi version of cups and string for it to have trouble with the bandwidth you mention. What are the players' signal strength?

Have you considered the possibility that the server's trans coding may be the bottle neck?

P

Atlantic
2011-11-07, 13:00
Mp3 greyd out is lame installed ? What kind of server , bitrate limiting may be an option if you can make that work ? If the server has the cpu to run lame .



Mnyb

Oh, well done; I don't think LAME is installed. It's a new server - Debian 6 (Squeeze?) - and I haven't added the extra repositories (forgot). Thanks.




i think you actually get rather big files if converting some analog sources to 24 bit SN for vinyl for example is not great the majority of the signal will be random noise and random noise does not compress well, the file will not be that much smaller . 24 bit gives you 144dB Some vintage sources can be noisy so most of the signal content is noise. even using a perfect source at least the 3 - 4 of the bottom bits will be noise there are not much practical circutry that has more SN than 20 bits aproximately .


Some seriously good points there, especially the inherent noise level to be expected on older sources, and on consumer-grade equipment, and the likelihood that the noise will occupy the lower bits and not be compressible. Good post.



You would have to be using the wifi version of cups and string for it to have trouble with the bandwidth you mention. What are the players' signal strength?


Hehe. I think we are using wet string, because of the solid and thick internal walls (very severe attenuation), and the multipath (I suspect we're surviving on reflections).

The players are quite close to the repeater and showing 90% and 70% reception (ie, from the repeater) but I'll check again when it next happens. The repeater doesn't report its own performance, though, sadly.




Have you considered the possibility that the server's trans coding may be the bottle neck?



P

I will check this; it's a new D6 linux build doing nothing else than SBS (at the moment) so shouldn't be stressed too much. It is very much worth checking, though, in case something is set up wrongly. Do you happen to know which log to check for this type of symptom? I can do a search, anyway.

Handy posts, both. Regards, Atlantic

pski
2011-11-07, 17:17
With wifi dat hi you should have clear sailing.

I don't think this stuff sends a separate stream to each synced player.

If that were the case, I'd been out of gas earlier. I often lame the boom while I'm at the dock and four other synced players have no issues.

This in addition to most of the music being m4a which means the server is trans-coding to flac.

p

Of course, you are talking about some format you need. It could be that as well.

Mnyb
2011-11-07, 20:26
i hope the server computer is wired, otherwise you can muliply your figures by 2 again.

brookheather
2011-11-10, 08:03
You can try and copy a file between two PCs either side of the wireless bridge to check your maximum throughput. You could look at using 200Mbps HomePlugs instead of a wireless bridge or get some cheap Wireless N routers (e.g. D-Link DIR-615) and flashing them with DD-WRT for the wireless bridging.

- Simon.