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S/PDIF over 5.8GHz radio link
S/PDIF over 5.8GHz radio link
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Old 13th September 2018, 02:00 PM   #1
studio45 is offline studio45
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Default S/PDIF over 5.8GHz radio link

Hi guys,
I'm looking for some help with a radio transmission problem, but it pertains to digital audio.

Basically my use case is that I have a pair of mobile soundsystems on wheels, which need to be able to play the same music at the same time, whilst in motion, within a range of up to 20 metres. My budget precludes using any of the industry-standard wireless audio equipment from eg Sennheiser or Shure.

I've had moderate success using domestic 5.8GHz "AV Sender" transmitters and receivers to create a radio link that transports an S/PDIF stream over the analogue video channel (yellow RCA connector). Reception is very clear and the range is up to 50 metres, but only when the two stations are not moving. As soon as there is significant relative movement between TX and RX, the audio signal starts to break up unacceptably. This situation is improved somewhat by using circularly polarised antennas, but remains an issue. It is not improved by using a higher-powered transmitter - the original system radiates 25mW, but even moving to an 800mW TX does not make a lot of difference to the movement problem.

I've been analysing the signals in the system on my 50MHz Tektronix scope and have a couple of questions (pictures included below).

1) The first picture is the S/PDIF waveform as measured at the input of the TX. It is reasonably square, but should there be so much ringing at the transitions?

2) The second picture is the signal as measured at the output of the RX, terminated into 75 ohms. It is obviously quite badly degraded, although the DAC can still lock onto it with surprising accuracy and reconstruct a good audio signal....most of the time. Should I expect to see a better-looking received signal, given the bandwidth of a composite video channel?

3)Having observed these signals, it is easy to see on the scope that physical movement of either station causes a large timing deviation in the received signal, which I suppose would constitute a massive amount of jitter in the digital bitstream. Given that, is this system doomed to failure, or could I re-clock at the receiver and gain an improvement?

Thanks in advance for any help you can offer
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Old 13th September 2018, 02:50 PM   #2
aboos is offline aboos  Germany
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Using S/PDIF as RF transmission signal with a standard (analog) AV RF link is not a good idea for several reasons!
1. The S/PDIF fundamental clock frequency is around 1.7 MHz - thus the video bandwidth will definitely degrade the signal as a square wave signal is composed of the fundamental and many many higher harmonics which will be 'cut' by the AV bandwidth. Your scope images clearly show this
2. The S/PDIF signal must be highly stable and have low jitter as the signal is directly used to sync the local clock oscillator at the receiving end to reclock the signal. Any increased jitter will lead to degraded sound quality or to a total loss
3. The wavelength at 5.8 GHz is a mere 6.5 cm - this means that especially in cases where you may have signal reflections like in a room or some other objects around- you will see strong signal strengths modulation by slight movements of the receiver. Small shifts may already lead to a total signal null independent of the transmission power - this was also one of your observations as increasing transmission power did not improved signal quality a lot.
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Old 13th September 2018, 04:32 PM   #3
MAAC0 is offline MAAC0  Portugal
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Do You have a satellite dish ? What happens when it is out of alignment a few centimeters (By few I mean only 3-4cm). There You have Your answer. It operates also in the GHertz range.
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Old 13th September 2018, 05:22 PM   #4
Pano is online now Pano  United States
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S/PDIF over 5.8GHz radio link
Would wifi streaming work better? It should have the range.
OR
20 meter is a stretch for Bluetooth, but I've done it with good antennas.
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Old 13th September 2018, 06:26 PM   #5
studio45 is offline studio45
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Thanks for all your replies

Yes I am rapidly appreciating that I know very little about GHz radios; about as much as i know about any other type of radio, come to that.

I am using circularly polarised antennas, which are omnidirectional, and reject multipath interference (in theory). So there shouldn't be any issue with reception of the carrier over my intended range, but the symbols encoded on it are arriving at fairly random times when either station is moving. Hence my question about re-clocking (from a local clock, presumably, the timing information in the original signal having been largely compromised....)

Wifi streaming has been mentioned - eg using a Chromecast system; it's probably what I'll look into next. Disadvantage is that it requires the establishment of a Wifi network to stream the data over, which in some environments eg busy music festivals may be difficult. And the latency may be difficult to predict, and certainly non-zero - which when one system is the left channel and the other is the right, will certainly be a problem.

I've been told that applying signal conditioning at the receiver (eg a Schmitt trigger to square up the waveform) would be pointless as the DAC has such circuitry at its input anyway. Is that correct?
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Old 14th September 2018, 05:02 AM   #6
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by studio45 View Post
I've been told that applying signal conditioning at the receiver (eg a Schmitt trigger to square up the waveform) would be pointless as the DAC has such circuitry at its input anyway. Is that correct?
Dac circuits typically do recondition the SPDIF waveforms before they go into the actual dac chip. However, if the signal is jittery what happens inside the dac depends on the type of dac. Most dacs try to sync a PLL to the incoming SPDIF stream and so it may go hunting around if there is excess jitter. ESS Sabre dacs use a state machine instead of the typical PLL to figure out what the waveform should look like and effectively reconstruct it as it should be. There may be dac chip register settings that affect PLL bandwidth and could affect tolerance for jittery signals. Firmware in dacs may leave such settings at the defaults, but even if you could make some change at that level it could only help so much.

Looking at other potential possibilities, there is software and or hardware to send audio over ethernet by various means, which may include wireless networks. A streaming audio software system may have enough buffering to be able to handle timeouts due receiver motion. The issue in that case might be that receivers can get out of sync with one another if they are operating in the same open space.

It looks like if you want to get into figuring out what might be done, it really depends on your application. Are people walking around playing music on cellphones, are there moving robots playing streaming audio to entertain guests, are they COW systems (computer on wheels) which are desktops or laptops on mobile carts, etc?

Other than the foregoing, you might want to look at some of the options here to see if any might suit your needs: Audio Networking Explained | Sweetwater
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Old 14th September 2018, 12:42 PM   #7
Neil Davis is offline Neil Davis  United States
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S/PDIF over 5.8GHz radio link
I think you can make this work with the right TX/RX combination, but I don't know enough about the available products to steer you in the right direction. Charlie Laub did a lot of experimenting with transmitting the SPDIF signal, and I believe he found a good combination for unobstructed transmissions. You might want to email him for some advice.

Here is the problem: SPDIF audio from a device like a CD player is formatted as 2 32-bit streams (left and right) at 44.1KHz. That's an information rate of 2.822MHz. However, the signal is then Manchester encoded to embed the clock, and that results in an effective bit rate of 5.6448MHz. If the signal is getting sent as NTSC-M, channel spacing is 6MHz and there is usually a filter in the "AV sender" receiver to limit the signal to 4.5MHz. So, sure, the signal is going to look crappy at the input to the SPDIF receiver. Any SPDIF receiver is going to provide good conditioning, but as you might guess, trying to send a 5.6MHz signal through a 4.5MHz pipe is going to be a problem.

Charlie found some RX/TX modules from the radio-control world that were capable of transmitting higher quality video. I know that the channel spacing for 5.6GHz RC systems is 20MHz, so there should be a working combination that isn't limited to 4.5MHz. Also, you might be able to use multiple channels to get more bandwidth. You need to find one that allows more than 525-line NTSC. These systems are designed to be used with drones and other RC vehicles, so they should have the right antennas for sources that are moving. Sorry, I don't know specifically which ones to look at, but I'm guessing that there will be a low-cost solution that will work.

Another possibility is to use a pair of WiFi modules like the ones from Parts Express (WFA02/WFA28). These are mesh routers that provide audio synchronization. That is, they can be paired, with the main module receiving packetized audio, and the "slave" unit gets audio directly from the master without using the TCP/IP protocols. So you would send packetized WiFi audio to the master, and then the master sends low-latency digital audio to the slave. If that connection between the master and slave can work while the units are moving, it might solve your problem. The WFA28 has two antennas vs one for the WFA02, so that might be the better option. You can open up these units and connect a different antenna, as they have a standard U-FL connector. (see picture).

You might find this discussion useful: Open source Active Wifi speaker project

Click the image to open in full size.

Last edited by Neil Davis; 14th September 2018 at 12:58 PM.
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Old 14th September 2018, 12:52 PM   #8
Pano is online now Pano  United States
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S/PDIF over 5.8GHz radio link
Latency could be a problem, yeah. Unless it's less than the delay in the air.

There are some long distance Bluetooth TX/RX units out there that ought to do the trick. Not sure about splitting left and right, tho. Bluetooth usually only pairs one to one, but there are some units that will do more.

Is sending high quality analog an option? That might be easier. I've done that a lot for shows and it works pretty well.
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Old 14th September 2018, 03:50 PM   #9
studio45 is offline studio45
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Thanks guys this is all marvellously helpful. @Neil that is exactly the information I have been trying to find - what the bandwidth requirements of S/PDIF actually are. It totally makes sense that the video channel isn't quite wide enough.

I am actually using an "FPV" transmitter designed to relay a composite video signal from a flying RC model to a ground based receiver. It's an Eachine TX5828 (Eachine TX5258 5.8G 72CH 25/200/500/800mW Switchable FPV Transmitter Support OSD Configuring) and I have the recommended RHCP antenna attached.

It can transmit on 9 different bands, some of which are divided into 20MHz channels and some into 40MHz. I guess it's not going to be using the *whole* channel, but even so I would have thought half the available bandwidth would be a reasonable assumption? So 10MHz at least. It's at moments like this that I wish I had access to a GHz band spectrum analyser, because I could answer these questions in about five minutes.....

My current receivers seem to be band A only. I've no idea if they have any bandwidth limitations, or will just demodulate everything they can from a given carrier - I guess the latter is what I need.

@Pano - I haven't found any one-to-many Bluetooth systems, do you have any links?

And yes I've tried the analogue audio channels - range is better and there are fewer dropouts, BUT the level of broadband noise behind the received signal is really, REALLY bad. Plus, when signal is briefly lost a very loud PHUT! transient is generated, which tends to activate the loudspeaker protection circuit on my amplifiers, requiring a power cycle to bring the sound back....far from ideal!
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Old 14th September 2018, 04:50 PM   #10
CharlieLaub is offline CharlieLaub  United States
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I suggest you try to use the lowest feasible sample rate for your application. When I was trying to send SPDIF over the video channel of an AV sender as a way to do whole-home distributed audio back in 2013/14, I found that 48kHz was barely doable, and 44.1kHz slightly more reliable. If you can drop to 32kHz, which some DACs will still accept, you might have better luck. The waveform shape should be better preserved. The lower you can go the better it will be, since the time that the spdif signal spends at either the high or low states will increase as sample frequency decreases.
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