Review and Measurements of REL Wireless Subwoofer Transmitter

[amirm]

Just to add to above the input to this device is analog, not digital PCM. It could digitize it if it wanted but it may very well not.

 

[GrimSurfer]

Just to add to above the input to this device is analog, not digital PCM. It could digitize it if it wanted but it may very well not.

OK. That makes sense. If it were a PCM digital transmission, interference would be minimized even if another wifi transmitter was operating on the same channel. There might be latency issues due to error control etc but distortion would be unlikely because foreign packets would be ignored.

But with wifi operating on the same (or even proximate) frequency as an analog transmitter, the analog receiver couldn't tell the difference between a digital and analog signal within the carrier. All it would "see" is waveform. Distortion in the intended analog signal in this scenario is quite plausible.

The selection of 2.4 GHz seems strange until one considers what Amir said -- it is an open frequency for civilian use.

Interesting!

 

[amirm]

But with wifi operating on the same (or even proximate) frequency as an analog transmitter, the analog receiver couldn't tell the difference between a digital and analog signal within the carrier. All it would "see" is waveform. Distortion in the intended analog signal in this scenario is quite plausible.

I suspect spread-spectrum modulation is used which makes it very immune to interference/noise. Older analog schemes using AM/FM modulation were horrible years back.

 

[amirm]

The selection of 2.4 GHz seems strange until one considers what Amir said -- it is an open frequency for civilian use.

There is no other choice as any buyer would have to apply and get an FCC license to operate the device!

 

[GrimSurfer]

There is no other choice as any buyer would have to apply and get an FCC license to operate the device!

Oh, I agree. They couldn't even use 5 GHz because, in some countries outside the US, that frequency is for indoor devices only... and might preclude the REL being used on an enclosed deck etc. outside the family structure. And you'd still have the same problem (though there is a sharper atmospheric attenuation at 5 gigs).

They could go lower in the spectrum (800 MHz, IIRC) but the interference there would likely be worse because that's where older baby monitors, cordless phones etc. operate. Very noisy part of the spectrum.

So I think the only logical thing to do would be to keep things digital and have a DAC at the end of the transmission chain... but that would almost certainly drive up the price.

It's a difficult product decision but one whose problems you may see more of in the future, if you expand your testing to wireless speakers/subs.

 

[gene_stl]

Did you notice, or measure any latency with the unit?

 

[amirm]

Did you notice, or measure any latency with the unit?

As noted, you can compute the latency from the phase graph. It is about 22 milliseconds which is in the upper range of their spec.

I have not run any type of calibration on the system to see how it acts with the sub with wireless vs wired connection. Just casually listening, it seemed to have low enough delay. Shotguns and such were right on the money.

 

[rajapruk]

Is the latency constant/stable over time, with various other crap in the air?

 

[Willem]

I looked at the manual and it says it uses the 5.8 Giga Herz band.

 

[RayDunzl]

Why would it have so much latency if it were transceiving an analog signal?

 

[audio_tony]

I found this datasheet for "Audio / Video Modules 5.8 GHz" and I wonder if REL are using similar tech, but transmitting audio only.

It's very similar to TV video senders which used a 6MHz carrier for audio transmission.

This would make most sense, as the audio bandwidth would be good too.

 

[amirm]

I looked at the manual and it says it uses the 5.8 Giga Herz band.

The hard copy manual I have doesn't say that. 5.8 Gigahertz units I have seen are for audio/video, not just audio.

 

[amirm]

Why would it have so much latency if it were transceiving an analog signal?

Good question. Seems like it is digital then using a chip like this from Nordic: https://www.sparkfun.com/datasheets/Components/nRF24L01_prelim_prod_spec_1_2.pdf

This one operates at 2.4 Gigahertz.

 

[vkvedam]

What's the latency for an FM signal? If there's anything like that as it's not digital

 

[GrimSurfer]

Radio signals travel at the speed of light. At the distances we're talking about in home audio, there would be essentially zero time delay in the signal leaving the transmitting antenna and it being picked up by the receiving antenna.

Any latency that occurs is due to the circuitry. This is usually negligible because, unlike digital communications, there is no requirement for error control, buffering, etc. for an unencrypted frequency modulated signal.

https://www.rfvenue.com/blog/2015/02/27/the-trouble-with-analog-fm

 

[restorer-john]

Open it up.

It will be using an A/D, the Nordic NRF-24L01 at T/X end and the same chip plus a D/A at the receiving end. Easy, reliable, and pretty decent performance for a slugwooger.

Just like this:

I have these on my desk pulled from a wireless amplifier for rear speakers I gutted the other day. Very cool little boards. 5V supply, line-in and line out.

Backside with clock,reg and mostly unused pins from what looks like a possible USB interface?

 

[GrimSurfer]

So it's a likely a digital signal hopping device? That would explain the latency at the very least.

 

[vkvedam]

It is ADC to DAC after all

 

[pkane]

This is a review and detailed measurements of the REL Acoustics Wireless Subwoofer and Transmitter. I purchased it from Amazon and it costs US $98 including Prime shipping.

As the name indicates, this is a transmitter and associated receiver to allow you to locate the subwoofer for your home theater anywhere you wish. The pair of boxes are rather high quality:

View attachment 34569​

The external power supplies are small and provide 5 volt at 500 milliamps. As such, it could have used a USB charger as some of its competitors use (e.g. SVS). That way, you could power the transmitted by the USB port in your AVR.

There are a ton of these products in the market but from what I can tell, many are using Bluetooth. The REL does not and hence advertises low latency. This is good as while you can delay your main channels to match any delay, you may lose lip sync.

The two units paired automatically on power up although there is also a manual process for pairing which I believe could be used if you have more than one pair.

Audio Measurements
As usual, let's look at our dashboard but with the lower frequency tone of 30 Hz :

View attachment 34570

There is some gain which was surprising to me.

SINAD is computed using the full audio bandwidth which doesn't apply here as your sub won't go that high. So in reality, performance is even better than 74 dB. This should be more than transparent for a sub as it is likely to have much worse distortions.

Frequency response shows that there is a built-in filter:
View attachment 34571

This helps the response of the system as it is the higher frequencies which get noisier. The available bandwidth is plenty for any type of subwoofer you may use with it.

Let's see if the distortion varies during the typical subwoofer spectrum (and then some):

View attachment 34572

Not an issue. Yes, there is some rise in distortion but that is well above 100 Hz where the sub is likely rolling off anyway. And even there, we are talking -60 dB.

Testing level versus distortion we get:

View attachment 34573

Other than clipping near 2 volts, the rest is pretty good actually given the fact that this is a wireless transmission.

I then ran a sweep of phase shift relative to frequency:

View attachment 34574

Don't mind the sudden drops. The graph basically says the phase shift keeps increasing with frequency linearly which means it has a constant, frequency independent delay which is good.

So far so good.

RF Interference
I ran a real-time capture of THD+N:
View attachment 34575

At around 42 seconds, my son turned on Wifi on his phone and ran a speedtest. As you see, that caused fairly high levels of THD+N, caused by sudden shifts in noise level and glitches. This was with him being 6 feet away from the transmitter. I then had him move 12 or so feet away and do the same thing. Strangely, the interference was worse there. RF works in magical ways! What the eye sees, is not in sync with how RF energy couples into something.

It is clear then that Wifi interference can cause serious issues with this device. I will be testing this in a scenario similar to this as my AVR is just a few feet away from my Wifi Access Point. Will report back on what the audible considerations might be later.

Conclusions
It took us three years to remodel our current home. I ran miles and miles of cables "just in case." And conduits. Alas, didn't think one day we may want to have more than 2 channels in our living room (thought we would use our dedicated theater a lot more than we are). So now I need a way to connect a subwoofer which would be sitting under a table in the middle of our living room. Hence the REL wireless subwoofer transmitter.

I am relieved to see the transparency that the REL achieves for this intended application. Interference with Wifi though is a concern. With just one spectrum (2.4 Gigahertz) available to every device known to man, this is kind of unavoidable. A more sophisticated spread spectrum system could have dealt with this better but likely not cheap to build. Bluetooth may do better given its error correction but without testing, it is hard to know.

Anyway, if you can keep this device away from Wifi, the REL will likely do the job. Recommendation pending actual listening tests.

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As always, questions, comments, recommendations, etc. are welcome.

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Thanks for the review, Amir. I have a similar problem with my video system: I put a single wire under the carpet when first installed, so I have a wired subwoofer. But, no wiring for the back/surround speakers. Would love to hear about a reasonable wireless solution with low latency and good SQ. I have active speakers I can throw in for this, but haven’t found a good transmitter/receiver pair.

 

[amirm]

My pleasure. I have a way to wire surround speakers but requires drilling holes so I may test the full bandwidth solutions there. Anyone wants to suggest one? I saw one from SVS I think.