SDS100/SDS200: SDS100/200 slight delay in audio compared to BCD325P2 (in relation to a Unication G5 pager)

[n1chu]

I’ve noticed the audio coming from a SDS100/200 and a BCD325P2 are slightly out of sync when receiving the same transmissions and when I inquired as to why this happens I was told the SDS series scanners are software defined radios (SDR) and the BCD325P2 is not. From that I took it to mean SDR is how Uniden combats simulcast issues… but…

I just purchased a Unication G5 pager/scanner and noticed no slight delay when receiving the same transmissions regardless of what mode the G5 is receiving when compared to the G5. (Currently the G5 is programmed with analogue conventional and digital P2 trunked.) Both the SDS series and the G5 are hearing everything without any difference-both are in sync.

Initially I was under the impression the SDS series incorporated SDR as a way to combat simulcast issues. But another explanation as to how the SDS series combats simulcast issues was by using filters, software defining has nothing to do with correcting the simulcast issues… instead, it was the filters that did this. Could it be Uniden found it necessary to use SDR in order to include the filters? …where a statement that says SDR was incorporated to combat simulcast issues is partially correct? …that it does allow for simulcast corrections? Because without it filters would not be available? Clarification on this would be helpful. Because the G5 handles simulcast issues and it’s not a SDR.

 

[nessnet]

The filters are to mitigate RFI, because the 'front end' of the SDS units are so wide and prone to near signal interference..

The superior simulcast performance of the SDS is because the multipath interference from simulcast is being delt with in the software.
Remember, simulcast/multipath problems are because of the different timing of signals - even at light speed, signals are arriving at the radio at (very slightly) different times because of their different RF paths. Doppler effect comes into play also.

 

[n1chu]

Ok. My understanding of simulcast issues and why they happen are in agreement with yours.

What got me wondering about it was when I compared the G5 to the SDS radios… there’s no difference (timewise) in the audio. (The previous models Uniden has such as the 996P2 & 325P2, which are not SDR’s, may receive the same signals without simulcast distortion but the audio is slightly offset. It’s either lagging or ahead of the audio emanating from the SDS200 or SDS100. But the G5 is completely in sync, time-wise, with the SDS models.) I’m just wondering what it is that the G5 (also a good radio when dealing with simulcast distortion) is doing as compared to the SDS… one final question… You say the simulcast distortion is dealt with in the SDS100/200 SOFTWARE. Since the SDS100/200 are SOFTWARE defined radios, would I be correct in stating Uniden is using a SDR to manage the simulcast problems inherent on their previous models such as the BCDx36HP’s?

 

[doctordialtone]

may receive the same signals without simulcast distortion but the audio is slightly offset.

This may be because the two radios are receiving different sites broadcasting the same content from different locations.

You'd need to verify which site you are receiving on both receivers.

I experience this all the time between different scanners monitoring CLMRN.

YMMV

 

[GTR8000]

A slight difference in audio timing between different receivers at the same location is almost always attributed to the power of the DSP, not to "receiving different sites". P25 simulcast timing is very tight, and so you're not going to have two receivers side by side receiving a different signal. If a receiver has a more powerful DSP, the audio will decode slightly faster than a receiver that has a less powerful DSP.

 

[sparklehorse]

Ok. My understanding of simulcast issues and why they happen are in agreement with yours.

What got me wondering about it was when I compared the G5 to the SDS radios… there’s no difference (timewise) in the audio. (The previous models Uniden has such as the 996P2 & 325P2, which are not SDR’s, may receive the same signals without simulcast distortion but the audio is slightly offset. It’s either lagging or ahead of the audio emanating from the SDS200 or SDS100. But the G5 is completely in sync, time-wise, with the SDS models.) I’m just wondering what it is that the G5 (also a good radio when dealing with simulcast distortion) is doing as compared to the SDS… one final question… You say the simulcast distortion is dealt with in the SDS100/200 SOFTWARE. Since the SDS100/200 are SOFTWARE defined radios, would I be correct in stating Uniden is using a SDR to manage the simulcast problems inherent on their previous models such as the BCDx36HP’s?

With the G5, which as I understand it IS an SDR, you have the option in the programming software to enable or disable ‘Voice Buffering’. I always have that enabled as I’ve read that transmissions can be clipped at the beginning if it’s disabled. So my BCD325P2 is ALWAYS ahead of my G5 when parked on the same channel. As I recall my G5 and SDS100 are much closer, but I think the SDS also runs a bit ahead of the G5.
.

 

[Icanhearit]

A slight difference in audio timing between different receivers at the same location is almost always attributed to the power of the DSP, not to "receiving different sites". P25 simulcast timing is very tight, and so you're not going to have two receivers side by side receiving a different signal. If a receiver has a more powerful DSP, the audio will decode slightly faster than a receiver that has a less powerful DSP.

Agree, the SDS[12]00 have delay when comparing with BCT125 or BCD436HP over airband etc analogue source, nothing to to with simulcast sites.

 

[MStep]

The slight delay when listening at the same time to the same traffic on an SDS and non-SDS radio creates quite an interesting audio "echo" effect which is reminiscent of the old shortwave SAC broadcasts. Sounds very impressive to visitors at the shack.

 

[n1chu]

Sparklehorse, Thank you. Apparently I was not as clear in my questioning the different times the audio arrived at the different radio’s speakers (when making my comparison). It’s understandable what I wrote could be misunderstood to relate to the time differences when receiving the same transmissions but from different sites (simulcast). But, as you correctly gathered my meaning, it was not the simulcast timings, (the slight difference in time it takes to receive the same transmission, at the …almost… same time, from two or more sites, all located at different distances from my receivers.) I was referring to how the different receivers I mention processed what was received AFTER it was received, namely the time it takes a Software Defined Radio to get the audio to its speaker as compared to the time a non-SDR does. But, you mentioned you are under the impression the G5 is in fact, also a SDR… which makes my question moot. Both the Uniden SDS models and the G5 are SDR’s and the time it takes both to process signals and pass them on to the speaker are virtually the same. I was working under the impression the G5 was not a SDR. Thanks for clearing this up for me. And thanks to all who replied. ALL replies were greatly appreciated.

 

[jtwalker]

I’d guess it’s the nature of an SDR receiver. Executing instructions in software will be slower than the same instructions in hardware. The SDR’s analyze and decode using software/firmware so makes sense to me they would lag slightly behind a non-SDR receiver.

 

[freqhopping]

I'll bet the reason for the delay is because you have recording/replay enabled. The audio needs to get saved to a buffer first. Disable that and compare. I know this happens with the 436HP. Disable recording and the audio between the 436HP and any of my other scanners is in-sync.

 

[TeeJayZee]

I'll bet the reason for the delay is because you have recording/replay enabled. The audio needs to get saved to a buffer first. Disable that and compare. I know this happens with the 436HP. Disable recording and the audio between the 436HP and any of my other scanners is in-sync.

Delay still there even when recording/replay is disabled.

And the delay is on both digital and analog signals.

I also have both the BCD325P2 and the SDS100 units.

 

[n1chu]

I'll bet the reason for the delay is because you have recording/replay enabled. The audio needs to get saved to a buffer first. Disable that and compare. I know this happens with the 436HP. Disable recording and the audio between the 436HP and any of my other scanners is in-sync.

Yea, that’s not it. I hear the same delay when listening on ham portables that are not SDR. And there’s no recording function on them. But thanks for your help. A lot of guys put their thinking caps on for this one!

 

[crippledchicken]

I found not running anything but my sds200 seems to satisfy me. If I run another one at the same time,I get irritated.

 

[DVINTHEHOUSEMAN]

The delay is because all signals are processed through the DSP, even analog, and so it takes longer to get out the other side than a straight analog receiver. This is even evident with professional radios, my ASTRO Saber and XTS radios are always ever so slightly behind my analog Sabers when receiving an analog transmission. It's barely noticeable, nowhere near the SDS's lag time.

 

[Icanhearit]

I'll bet the reason for the delay is because you have recording/replay enabled. The audio needs to get saved to a buffer first. Disable that and compare. I know this happens with the 436HP. Disable recording and the audio between the 436HP and any of my other scanners is in-sync.

Nah, if you read my early reply, I said 436HP has NO delay, the SDR based SDS100& 200 both have delay (which all not enable recording, in fact never ever enabled)

 

[JoeBearcat]

It takes time for the SDR in the unit to sample the spectrum, convert it to data, and process that data.

None of the above exists in the 436.

 

[CanesFan95]

I think voice buffering is only meant to delay the audio for an alert tone, not so much for avoiding clipping. If anything, enabling it would make the Unication slower, not faster than the scanner.

 

[gary123]

You will find that even two receivers of the same make may have noticeable delays compared to each other. This is 100% normal. Its because of several factors including the internal clocking, process speed of the chips and the point where the P25(DMR as well) detection circuitry local on to the signal and starts decoding. If you have some more sophisticated equipment you can eave measure that the "out of sync" audio veired from transmission to transmission and even service to service.

There is nothing wrong with your equipment.

 

[MStep]

Here is the answer:

As Gary123 points out, "there is nothing wrong with your equipment".

But kidding aside, there is a LOT of internal processing that went into the genius of the design of the SDS series, mostly due to the hard work of the late Paul Opitz. The fact that there is but a micro-second delay while all the information contained in the signal is being processed so that the radio knows what to do and how and when to decode continues to amaze me. JoeBearcat put that into very concise form in his post above.

The SDS series were way ahead of their time when they were first introduced.