Wireless S/PDIF

[elektrinis]

So I have an optical S/PDIF output that I want to be wireless. I have searched around and could not find ANY ready to use solution. Mind you, bluetooth does not work due to latency and inability to transfer 5.1 DTS-encoded audio.
I have found a couple of posts that cheap CCTV video transmitters work, and I have tried it. Unfortunately it killed my wifi and would probably cause other problems.
So I am at square one.

Googled some more and figured S/PDIF has a boud rate of ~3Mbps. Are there any RF modules that can do this over distance of 10 meters? I don't want to dive in to engineering or programming stuff at this point, would only connect/solder some wires at most.

[Buriedcode]

Wireless digital audio is difficult if one requires both high bandwidth (= uncompressed, high quality) and low latency (<20ms).  There have been great strides in the past decade but generally you cannot have both of these features.  Many claim "lossless" audio when in fact they do use compression.  And often latency is buried in the spec because its unreasonable for "live" stuff like tv, like 100ms+.  One doesn't really care about latency if you're just playing music from your system.

I am only aware of a few solutions: 

- WIFI, can stream multichannel uncompressed, but of course has pretty high latency.

- Bluetooth. Only uses lossy compression.  Apt-X seems to be the best codec (to my ears) with Apt-X low latency achieving CD-quality (again, to my ears) with <40ms.

- Custom solutions from Texas instruments "PurePath" chip-set.  Expensive dev boards, and will only do 16-bit 48kHz stereo.  Latency I believe is around 20ms.  Uncompressed.

- Firing S/PDIF signals through 2.4GHz video senders.  I've also done this, there plenty of bandwidth on the video channel - about 5MHz - for S/PDIF.   As you've found out these things are wide band, and cover a huge portion of the spectrum.  As they are essentially just analogue FM transmitters they are also prone to interference (as well as pooing all over other video senders in the area).

If I were you I would get a bluetooth transmitter module that supports Apt-X Low latency.  Avantree have a large selection but there are plenty of no-brand ones on amazon. Along with a receiver that also supports LL you *should* be able to stream pretty good quality audio at a latency you won't notice (hopefully).

[elektrinis]

Hi Buriedcode,

I have actually purchased an Apt-X bluetooth module, but unfortunately it only supports PCM signal, not DD/DTS, so I can't get 5.1 stream out of it..

Are there any FM video transceivers with narrower band? Perhaps those used for for FPV RC stuff? I saw they have several channels, but not sure how good they are. These should have minimal latency, as there is no extra wrapper of digital data.

[Benta]

If you have line-of-sight, you could simply amplify the S/PDIF signal and have an optical link.

[elektrinis]

If you have line-of-sight, you could simply amplify the S/PDIF signal and have an optical link.

I need it wireless... Or are you suggesting laser or something? That would be certainly a nerdy thing to do. Too bad I'm too old to use this to impress girlz now.

[Benta]

Not a laser. simply IR transmitter and receiver. But like I said, you need LOS.

[Buriedcode]

Hi Buriedcode,

I have actually purchased an Apt-X bluetooth module, but unfortunately it only supports PCM signal, not DD/DTS, so I can't get 5.1 stream out of it..

Are there any FM video transceivers with narrower band? Perhaps those used for for FPV RC stuff? I saw they have several channels, but not sure how good they are. These should have minimal latency, as there is no extra wrapper of digital data.

Well the reason the bands are so wide is.. well... bandwidth! which is needed for NTSC/PAL signals.  It's also what you need.  If you're going with 5.1 DTS then your S/PDIF bitstream is going to be 1.5 mbit/s off the top of my head for DTS which is compressed. So ~3MHz bandwidth required minimum.  I really don't think you'll be able to use anything with less bandwidth, Shannon says no.

If however, the only downside to your scheme is interference with your WIFI then you could try attenuating the RF.  If this is relatively short rage you probably won't need the ~10mW (probably more as these A/V transmitters are rarely certified/regulated).  Also, the FPV RC stuff often uses 5.8GHz - as many control systems switched to 2.4Ghz.  This has shorter range but if your WIFI is 2.4Ghz only (802.11b/g/n) then it shouldn't interfere.  I would still try to find a way of reducing the RF output though.  For transporting audio from room to room, you don't need the 100mW some of these things kick out.

As you're dealing with sending what is effectively a digital stream though an analogue RF channel (albeit a band limited one) there will be no way of detecting errors.  I imagine DD/DTS must be able to mask some errors so you'll have to experiment in terms of antennas, attenuaters and placement, but I don't think S/PDIF handles errors at all, as its designed to be synchronous over copper or fiber.  Any interference in the channel could cause the PLL in the receiver to lose lock, or at the very least add jitter.

[NiHaoMike]

Nvidia has implemented HD audio over Wifi Direct, but it's a proprietary protocol. Not sure how the latency is but it was largely designed for gaming so it must be pretty low. Audio quality is way better than what Bluetooth can do.

I think your best option is to use UDP over Wifi with your own FEC scheme.

[elektrinis]

What about those 433MHz modules, are they AM? To my understanding AM doen not need wide bandwidth and could be done reasonably stable with binary data.
So far, however, I could not find a module with enough baud rate.

[Buriedcode]

the 433/434Mhz Band is an ISM band.  IIRC it only has about 2MHz for the entire band which is broken up into many channels of tens of kHz each.  ISM bands have strict restrictions on power output, channel width, side lobes, and even duty cycle (low duty being things like remote controls).  The highest baud rate I've seen in taht band is 19.2k.

Generally, the higher the datarate - the higher the bandwidth required -> the further up the spectrum you'll have to go for unlicensed links.  With 5.8Ghz being the higher ISM band, and probably the only one with enough bandwidth to carry the data.  This is of course when talking exclusively about sending S/PDIF signals "over the air".  There are other encoding methods and systems.

[BrianHG]

Does it need to pass FCC?
Does it need to be small?
Does it need to be low power?

[elektrinis]

Don't have to pass anything, it's a diy thing.
Has anyone tried those FPV video transmitters inhouse? Do they kill WiFi?

[SiliconWizard]

If it fits your requirements, you could try the CC8520: http://www.ti.com/product/CC8520
It's one of the few options that seems to allow 24-bit, 48 kHz (most solutions are 16-bit only and a lot of them are bluetooth or wifi-based).

TI has a development kit that is $149: http://www.ti.com/tool/cc85xxdk-headset
You would probably have to add an SPDIF to I2s (respectively I2S to SPDIF) converter at each end though.

(I wouldn't hack an analog video transmitter to transmit digital data.)

[borjam]

the 433/434Mhz Band is an ISM band.  IIRC it only has about 2MHz for the entire band which is broken up into many channels of tens of kHz each.  ISM bands have strict restrictions on power output, channel width, side lobes, and even duty cycle (low duty being things like remote controls).  The highest baud rate I've seen in taht band is 19.2k.

Moreover you would be in the middle of an amateur radio band.

[elektrinis]

SiliconWizard: thank you, will look in to it. Hopefully the chip can be used without developing any firmware.

Buriedcode: I'm not a RF guru, butI understand that band width is not that important to plain old primitive AM? The carrier is either on or off, it's just a matter of selecting the right frequency. Am I right?
The highest baudrate module I could find was from Linx - 300kbps, which is still 10x less than I need.

[Benta]

I understand that band width is not that important to plain old primitive AM?

AM radio bandwidth is two times baseband bandwidth. If you have 0...10 kHz audio, AM bandwidth will be 20 kHz. So yes, bandwidth is important.