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Author Topic: Causes of high latency in Digital Radios and Comms  (Read 1206 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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Ike Zimbel

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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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~Ike Zimbel~
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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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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 »


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