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Author Topic: Causes of high latency in Digital Radios and Comms  (Read 1129 times)

Andrew Broughton

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Causes of high latency in Digital Radios and Comms
« on: October 27, 2019, 02:49:41 pm »

So, with modern circuitry, I would think that the ADC/DAC part of the process would add negligible latency to the signal, especially since the signal is not particularly "hi fi", but just voice i.e. low bandwidth.
I'll assume the reason for the 100's+ ms of latency in digital walkies and coms has something to do with to the limited bandwidth of the RF signal? Or is it something else?
Can someone ETMLI5 how this latency happens and what the causes are for it? Feel free to throw in some math if you know how it all works "under the hood"...


Always been curious...
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Scott Holtzman

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Re: Causes of high latency in Digital Radios and Comms
« Reply #1 on: October 27, 2019, 07:14:02 pm »

So, with modern circuitry, I would think that the ADC/DAC part of the process would add negligible latency to the signal, especially since the signal is not particularly "hi fi", but just voice i.e. low bandwidth.
I'll assume the reason for the 100's+ ms of latency in digital walkies and coms has something to do with to the limited bandwidth of the RF signal? Or is it something else?
Can someone ETMLI5 how this latency happens and what the causes are for it? Feel free to throw in some math if you know how it all works "under the hood"...


Always been curious...

I think it's just low CPU power.  Takes a bit of juice to encode and decode low bit rate voice sessions.

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Scott AKA "Skyking" Holtzman

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Henry Cohen

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Re: Causes of high latency in Digital Radios and Comms
« Reply #2 on: October 27, 2019, 07:29:12 pm »

So, with modern circuitry, I would think that the ADC/DAC part of the process would add negligible latency to the signal, especially since the signal is not particularly "hi fi", but just voice i.e. low bandwidth.
I'll assume the reason for the 100's+ ms of latency in digital walkies and coms has something to do with to the limited bandwidth of the RF signal? Or is it something else?
Can someone ETMLI5 how this latency happens and what the causes are for it? Feel free to throw in some math if you know how it all works "under the hood"...

The reasons for the amounts of latency in coms versus two-way radios are somewhat different (beyond the basic concept of a digital audio architecture) due as much to operational requirements as customer expectations.

Modern digital intercom systems are typically provisioning hundreds if not thousands of talk paths and matrixing as well as other data commands. So in addition to the small latency for A-D/D-A, there's a fair amount of signal processing going on. I think you'll find newer IP based digital coms are almost always sub-100mS as long as multiple network, and the internet, are not involved. OEM's are always looking to reduce the latencies, but not to the point it will diminish other features users have come to expect.

As for two-way radios, latency at about 300-400 mS is simply not a high priority factor. It's rare to have more than one radio within the same acoustical range as a talker. But, as you surmised, limited bandwidth - spectral efficiency - along with encryption ability and low power draw are higher priorities. Virtually all DMR, NXDN and P25 radios use the same DVSI vocoder chipset.
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Pete Erskine

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Re: Causes of high latency in Digital Radios and Comms
« Reply #3 on: October 27, 2019, 10:44:01 pm »

Can someone ETMLI5 how this latency happens and what the causes are for it? Feel free to throw in some math if you know how it all works "under the hood"...

I think the real problem is HEARING the latency.  If you call a cell phone and listen to both phones you will hear the latency.

Most modern digital full duplex intercom systems (Bolero, Freespeak and Crewcom) have low latency better than your typical cell phone call.  Where comms users begin to complain is when they are forced to listen to their own voice, double delayed, because of problems in the routing of the signal or leaky Hybrids and it sounds like an echo.

Some older digital comms like Tempest had much greater latency because of the Phoneme based codec which was slow and the 2-wire hybrid which could never be nulled completely.  this echo was the beginning of comms echo complaints.

In BTR days the audio was transmitted to the base, summed with the audio from other packs and re-transmitted back for everyone to hear...analog added no perceptible latency so it was actually a feature... if you heard yourself, the message went through and you could verify that the comm was working.

New digital system can't do this...all 3 systems, above, make the sidetone internally, in the pack, so that this issue is not caused by hearing the double delay.  As a result, if you want to walk test the newer comms systems you must talk into one and listen to a second beltpack to confirm the quality.
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Scott Holtzman

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Re: Causes of high latency in Digital Radios and Comms
« Reply #4 on: October 28, 2019, 01:09:47 am »

I think the real problem is HEARING the latency.  If you call a cell phone and listen to both phones you will hear the latency.

Most modern digital full duplex intercom systems (Bolero, Freespeak and Crewcom) have low latency better than your typical cell phone call.  Where comms users begin to complain is when they are forced to listen to their own voice, double delayed, because of problems in the routing of the signal or leaky Hybrids and it sounds like an echo.

Some older digital comms like Tempest had much greater latency because of the Phoneme based codec which was slow and the 2-wire hybrid which could never be nulled completely.  this echo was the beginning of comms echo complaints.

In BTR days the audio was transmitted to the base, summed with the audio from other packs and re-transmitted back for everyone to hear...analog added no perceptible latency so it was actually a feature... if you heard yourself, the message went through and you could verify that the comm was working.

New digital system can't do this...all 3 systems, above, make the sidetone internally, in the pack, so that this issue is not caused by hearing the double delay.  As a result, if you want to walk test the newer comms systems you must talk into one and listen to a second beltpack to confirm the quality.

Peter/Henry do you know of anyone using the SILK CODEC for any production intercoms?  Since Skype released it for free it has become ubiquitous in IP phone systems.  It has a very low CPU footprint. 

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Ike Zimbel

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Re: Causes of high latency in Digital Radios and Comms
« Reply #5 on: October 28, 2019, 09:34:46 am »


In BTR days the audio was transmitted to the base, summed with the audio from other packs and re-transmitted back for everyone to hear...analog added no perceptible latency so it was actually a feature... if you heard yourself, the message went through and you could verify that the comm was working.

...and these days, RAD is the only system to still offer this feature. Just sayin'...
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Russell Ault

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Re: Causes of high latency in Digital Radios and Comms
« Reply #6 on: October 28, 2019, 02:32:23 pm »

...and these days, RAD is the only system to still offer this feature. Just sayin'...

Isn't the BTR-800 E88 band still legal in North America? :)

-Russ
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Re: Causes of high latency in Digital Radios and Comms
« Reply #7 on: October 28, 2019, 06:27:08 pm »

Isn't the BTR-800 E88 band still legal in North America? :)

-Russ
Yes. I meant to say the only new system. And, keep in mind that in some markets, the chances of having open TV channels in both "E", 590.100 to 607.900 MHz, and "88", 470.100 to 487.900 MHz are pretty much zero.
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John P. Farrell

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Re: Causes of high latency in Digital Radios and Comms
« Reply #8 on: October 29, 2019, 10:10:41 am »

As for two-way radios, latency at about 300-400 mS is simply not a high priority factor. It's rare to have more than one radio within the same acoustical range as a talker.

I disagree.  On a tour there are often many radios in the same physical space.  Were replaced all our analog radios with CP200D a few years ago and the delay is maddening when someone walks into the production office or the crew is talking on the deck. 

JF
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Pete Erskine

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Re: Causes of high latency in Digital Radios and Comms
« Reply #9 on: October 29, 2019, 10:51:07 am »

I disagree.  On a tour there are often many radios in the same physical space.  Were replaced all our analog radios with CP200D a few years ago and the delay is maddening when someone walks into the production office or the crew is talking on the deck. 

JF

I agree...Digital radios should NEVER be used in a production environment.
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Ike Zimbel

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Re: Causes of high latency in Digital Radios and Comms
« Reply #10 on: October 29, 2019, 11:29:29 am »

I agree...Digital radios should NEVER be used in a production environment.
Hear! Hear! Or, I wish I could hear. They sound like crap, the intelligibility is terrible, and turning them up does not make them more intelligible. I've also noticed that in venues like arenas, stadiums, ballrooms etc. that most of the venue staff (cooks, cleaners etc.) often have their radios on their belt and turned up to full volume. Lots of fun when you happen to find yourself in an elevator with these folks... :'(
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Pete Erskine

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Re: Causes of high latency in Digital Radios and Comms
« Reply #11 on: October 29, 2019, 12:13:45 pm »

Isn't the BTR-800 E88 band still legal in North America? :)

-Russ

Yes however it is soon to be discontinued...and it's a terrible band anyway.
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Henry Cohen

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Re: Causes of high latency in Digital Radios and Comms
« Reply #12 on: October 29, 2019, 01:09:11 pm »

I disagree.  On a tour there are often many radios in the same physical space.  Were replaced all our analog radios with CP200D a few years ago and the delay is maddening when someone walks into the production office or the crew is talking on the deck. 

I was referring to the real world of LMR deployments and workflows; not the micro-niche market that is entertainment production.
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Henry Cohen

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Henry Cohen

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Re: Causes of high latency in Digital Radios and Comms
« Reply #13 on: October 29, 2019, 01:12:09 pm »

I agree...Digital radios should NEVER be used in a production environment.

When ordering radios from a vendor (for either rental or purchase), order all simplex channels programmed in analog mode. Additionally, if there's only one repeater channel on the radio, that too can be ordered as analog. The only time you must run in digital mode is when there are multiple repeater channels.
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Henry Cohen

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brian maddox

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Re: Causes of high latency in Digital Radios and Comms
« Reply #14 on: October 29, 2019, 03:44:23 pm »

I agree...Digital radios should NEVER be used in a production environment.

The "push the button and then wait for the chirp to talk" aspect is enough to make me want to throw the things across the room.

But then Half-Duplex makes me crazy in and of itself.  I remember when the Nextel phones first came out and everyone was raving about the whole PTT feature and i was like "we've got Full Duplex PHONES - Why would we EVER go back to glorified radios????"
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Scott Holtzman

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Re: Causes of high latency in Digital Radios and Comms
« Reply #15 on: October 29, 2019, 09:50:06 pm »

The "push the button and then wait for the chirp to talk" aspect is enough to make me want to throw the things across the room.

But then Half-Duplex makes me crazy in and of itself.  I remember when the Nextel phones first came out and everyone was raving about the whole PTT feature and i was like "we've got Full Duplex PHONES - Why would we EVER go back to glorified radios????"

Interesting note about PTT emulation.  I worked for Winphoria networks out of Boston (who was later purchased by Motorola, what a win for us who came to work at the little startup) they were a bunch of Bell Labs guys who had designed the first softswitch for land mobile use.  They were having a hard time convincing the cellular folks that a few racks of servers could replace 100 cabinets of switching gear and the VC money was burning.  Verizon was absolutely up in arms over how to compete with Sprint who had purchased Nextel and was marketing the crap out of PTT services. 

This is actually even more interesting because the whole reason the IDEN PTT mode was on the Nextel phones is they purchased 800Mhz land mobile licenses where interconnect is a secondary server to dispatch (2 way radio) so the PTT mode was added for legal reasons.  It was immensely popular and Verizon took notice of this. 

Guru Pi the CTO of Winphoria was a brilliant dude and he re-purposed our softswitch to provide a PTT expereince over 1xRTT cellular data networks.  Our switching nodes were located in the Verizon CO's right next to the data frame for no latency.  While latency surely was an issue so was call setup time so they had to keep the data connection up that trashed battery life.  The first PTT phones were less than stellar.  Side note I bet the coming low latency 5G will finally making emulating a 2 way radio on a WAN tolerable.

Finishing the story, I had gotten the job with Winphoria because they were installing one of these nodes and software geniuses don't know how to read Bell Telephone installation guidelines.  Between almost blowing up a -48V BDFB, not isolating the rack from the floor etc. they did not get a warm reception.  I never even saw our offices the first 2 months of my job.  I was hired and flew straight to Verizon Engineering HQ in Bedminster NJ. 

That project was never commercially successful however I earned tremendous respect for the Verizon Wireless engineering team and their quest for excellence.   I spend 6 more years with Motorola until the Galvin's lost control of the company and I left to pursue my ISP full time.  Never would have gotten back into the production world if it wasn't for this odd chain of events.

A little history on IDEN, Cellular and PTT. 

What I need to do is obtain an itinerant part 90 license and build a little repeater for production.  This conversation has reminded me that I wanted to upgrade our crappy FRS UHF radios. 

 
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Pete Erskine

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Re: Causes of high latency in Digital Radios and Comms
« Reply #16 on: October 29, 2019, 09:52:58 pm »

The "push the button and then wait for the chirp to talk" aspect is enough to make me want to throw the things across the room.

But then Half-Duplex makes me crazy in and of itself.  I remember when the Nextel phones first came out and everyone was raving about the whole PTT feature and i was like "we've got Full Duplex PHONES - Why would we EVER go back to glorified radios????"

Depends on what you are doing with them.  Traffic movement, logistics OK.  Calling cues and trouble shooting no.  I was on a gig with intense IT.  $ people were setting it up and making adjustments...they got so frustrated with radios that the show was beginning to suffer.  I had 4 Crewcom full duplex beltpacks extra and gave them to that crew and the difference was amazing.
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Andrew Broughton

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Re: Causes of high latency in Digital Radios and Comms
« Reply #17 on: October 30, 2019, 07:13:35 pm »

Thanks, everyone.

So is the consensus that digital walkie-talkies COULD be made to have low latency and instant-PTT but it's just not a priority? After all, there's digital wireless microphones with high quality audio and low latency, so you'd think it would be a breeze to do even better with the (relatively) low fidelity/bandwidth requirements of walkie-talkies. It's not like "quality" digital radios are cheap or anything!
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Jason Glass

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Re: Causes of high latency in Digital Radios and Comms
« Reply #18 on: October 30, 2019, 08:54:44 pm »

Thanks, everyone.

So is the consensus that digital walkie-talkies COULD be made to have low latency and instant-PTT but it's just not a priority? After all, there's digital wireless microphones with high quality audio and low latency, so you'd think it would be a breeze to do even better with the (relatively) low fidelity/bandwidth requirements of walkie-talkies. It's not like "quality" digital radios are cheap or anything!

No. Cramming intelligible vocal audio into 6.25 kHz of mandated RF channel bandwidth requires extreme data compression, and that compression takes processing time.

Microphones have 200 kHz of channel bandwidth to work with.  It still takes time, but less time.  With hardware a fraction of the size of walkies.

Both scenarios are also within the constraints of power consumption vs. battery size and mass vs. TX power.

The math works out, until the next RF tech breakthrough.

ProSoundWeb Community

Re: Causes of high latency in Digital Radios and Comms
« Reply #18 on: October 30, 2019, 08:54:44 pm »


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