Interesting thoughts Tim... perhaps worth documenting on the wiki?
Even though it was just "in theory", what kind of sync error was it?
Were we talking microseconds or even more?
I have a receiver on a yagi pointed at a distant simulcasted site. I
have noticed on occasion, I can hear the distant simulcasted site
start to send audio underneath the carrier of the site that is closest
(and strongest to me). The distant site is also the master site, and
has the lowest latency, but I'm talking two words worth of audio is
send before my other site starts sending audio. Once they are both
transmitting, I have not noticed any timing issues (i.e. distortion
etc). I'm not sure if this issue was related to what I posted
previously about the variable audio delay on unkey.
I'm running app_rpt on a Raspberry Pi using a cut down image we use here in VK.
"When simulcasting the audio from all (non captured) transmitters needs to
arrive at the receiver with in an acceptable time frame (80 us). The DAC
theory was that it didn't start sending audio at correct clock cycle every
time. That would cause the audio to be out of sync between RTCMs causing
simulcast distortion.
As I said, that was the theory. The new theory is that the external clock
source was causing the DAC to trigger inappropriately. I spoke with someone
I met here on the list (Kevin I think its was) who is using a different
external clock and reports perfect simulcast operations"
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Interesting thoughts Tim... perhaps worth documenting on the wiki? Even though it was just "in theory", what kind of sync error was it? Were we talking microseconds or even more?
I have a receiver on a yagi pointed at a distant simulcasted site. I have noticed on occasion, I can hear the distant simulcasted site start to send audio underneath the carrier of the site that is closest (and strongest to me). The distant site is also the master site, and has the lowest latency, but I'm talking two words worth of audio is send before my other site starts sending audio. Once they are both transmitting, I have not noticed any timing issues (i.e. distortion etc). I'm not sure if this issue was related to what I posted previously about the variable audio delay on unkey.
I'm running app_rpt on a Raspberry Pi using a cut down image we use here in VK.
[http://vklink.com.au](http://vklink.com.au)
"When simulcasting the audio from all (non captured) transmitters needs to
arrive at the receiver with in an acceptable time frame (80 us). The DAC
theory was that it didn't start sending audio at correct clock cycle every
time. That would cause the audio to be out of sync between RTCMs causing
simulcast distortion.
As I said, that was the theory. The new theory is that the external clock
source was causing the DAC to trigger inappropriately. I spoke with someone
I met here on the list (Kevin I think its was) who is using a different
external clock and reports perfect simulcast operations"
In response to Jeff's issue - make sure your RTCM's 9.6MHz clock is GPS
locked. Both Joe KC2IRV and myself have confirmed that, although using
OCXO's by themselves work, they aren't perfect and move around. If you
still have the little crystal in there, that will be the major source of
your issues, it is nowhere near stable enough.
Fix the overlap issues to where a majority of your users are, don't worry
about mobiles, it will just sound mostly like multipathing to them, if done
right. Fixed stations really suffer the most in bad overlap areas.
Jesse - I use a radio base called Unilab/Stanillite KL series. They come in
KL70 (can be modded to 50MHz), KL150 (for 2m), KL450 (for 70cm) and KL850.
I'm pretty sure they were exclusive to the Australian market.
I've got all my audios set up the same, straight into the modulator diode.
All freq response is exactly the same, and levels set the same.
The issue doesn't appear to be audio related but software related as per
Tim's post earlier. My Pi does run at 100mbps though, perhaps I should be
forcing that down to 10mbps as a start. I'm just not sure why changing the
simulcast launch delay causes the issue, at least on the surface to be
worse when that's on a per board basis back at the TX sites.
Attached are my simulcast Radio Mobile plots for the two site system I have.
Sam - I'm currently using the BG7TBL GPSDO's that are readily available on
eBay. They give both a 10MHz output and a 1PPS. I then use a board that Joe
KC2IRV developed that uses a 10MHz signal from the GPS to control the VTune
pin on a 9.6MHz OCXO.
Back to the one of the topics of the thread -
I was driving home within the overlap area of the sites again, and noticed
that extra "audio delay" to my secondary site. The audio starts first on
the main site, few words later, the audio starts on the secondary site...
it's quite weird...
Can I attach MP3's to the list here? I might try to obtain an example.
The variable delay on unkeying is still a problem too.
Very good, Hayden. Yes, I’ve been in touch with Joe about his project from when he ordered the RTCMs. I was inquiring as to what Jeff was running. Didn’t mean to veer off topic.
···
Starting to drift off topic… however…
Attached are my simulcast Radio Mobile plots for the two site system I have.
Sam - I’m currently using the BG7TBL GPSDO’s that are readily available on eBay. They give both a 10MHz output and a 1PPS. I then use a board that Joe KC2IRV developed that uses a 10MHz signal from the GPS to control the VTune pin on a 9.6MHz OCXO.
I was driving home within the overlap area of the sites again, and noticed that extra “audio delay” to my secondary site. The audio starts first on the main site, few words later, the audio starts on the secondary site… it’s quite weird…
Can I attach MP3’s to the list here? I might try to obtain an example.
The variable delay on unkeying is still a problem too.
I have a few PCGM's that I never used, if anyone needs a few please send me a
direct email.
Thank you,
···
On 2/14/18 5:09 PM, Hayden Honeywood wrote:
In response to Jeff's issue - make sure your RTCM's 9.6MHz clock is GPS
locked. Both Joe KC2IRV and myself have confirmed that, although using OCXO's
by themselves work, they aren't perfect and move around. If you still have the
little crystal in there, that will be the major source of your issues, it is
nowhere near stable enough.
It would be great to get a forum type discussion happening for simulcast.
The mailing list I guess is still sufficient.
What are you using to lock your RTCM's to 9.6MHz? If it's just the internal
crystal still in there, then this is nowhere near sufficient, and will
cause the audio to drift around like crazy.
You need some form of stable frequency reference that is the same between
sites (i.e. GPS, OCXO etc)
I think from a previous posting the PCGM's had issues with creating a whole
lot of noise and crud, could be wrong. Although mounting in a shielded
enclosure probably helps. The board Joe came up with only requires a
handful of parts and an OCXO.... he was also experimenting with the Leo
Bondar programmable GPS unit, although this doesn't give out the 1PPS, so
you'd effectively need two GPS units at the site.
The network I run on my simulcast system is via the internet, not via
direct links as we don't have a path between sites. The internet is a
business grade connection with low (<35mS) latency to the slowest site.
I’ve never heard one transmitter come up before another. I seen long delays but on all RTCMs at the same. Are your RTCMs connected to the same node number?
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–
Starting to drift off topic… however…
Attached are my simulcast Radio Mobile plots for the two site system I have.
Sam - I’m currently using the BG7TBL GPSDO’s that are readily available on eBay. They give both a 10MHz output and a 1PPS. I then use a board that Joe KC2IRV developed that uses a 10MHz signal from the GPS to control the VTune pin on a 9.6MHz OCXO.
I was driving home within the overlap area of the sites again, and noticed that extra “audio delay” to my secondary site. The audio starts first on the main site, few words later, the audio starts on the secondary site… it’s quite weird…
Can I attach MP3’s to the list here? I might try to obtain an example.
The variable delay on unkeying is still a problem too.
Sorry to say this, but those are the bad external clock source I mentioned in another post.They are also very dirty. I know one person who’s equipment was turned off by the site manager due to PCGM interferance.
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Hi Tim,
Yes, they are all connected to the same node number.
I'll get recording of what it sounds like and post here. That is issue 1)
Issue 2) is the variable delay on ALL the RTCM's as you describe. I might
start talking on the repeater with a little echo delay on unkey, then a few
overs later the delay might have ballooned out to 1-2 seconds.
Well for setting the 9.6 MHz clock onthe RTCM, it's likely ok. Using it for
the PLL reference would not be advised unless you look at it and it's clean.
I've never used them for that.
···
On 2/15/18 6:30 PM, Tim Sawyer wrote:
Sorry to say this, but those are the bad external clock source I mentioned in
another post.They are also very dirty. I know one person who's equipment was
turned off by the site manager due to PCGM interferance.
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–
Where in Radio Mobile online is the ability to select the delay? It might be right in front of my eyes, but I can’t see it.
It was only being used for clocking the RTCM. I believe the problem is power circuitry radiates bunches of hash and trash. It was interfering with something on VHF high band, like 173.050… somewhere near there.
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I've made a YouTube video with clips from a single conversation I had this
morning on the repeater. The audio was recorded on my dashcam.
As can be heard in the video, when I start the conversation, the audio
delay is nil, or very little when unkeying. However, by the time I'm
finished (I think the conversation lasted about 20 minutes) the audio delay
is out to 4 or 5 words!
We were both using the same repeater. We have a repeater linked in from
another location. I've noticed when talking to someone via that method, the
audio delay issue does not build up so quickly.
Hopefully this explains a bit better the issue I'm having.
